Respiratory Physiology

Answer Card

Respiratory Physiology encompasses the fundamental mechanisms of gas exchange, lung mechanics, and pulmonary circulation. Key concepts include lung volumes (tidal, vital, residual, functional residual capacity), the alveolar gas equation for calculating alveolar oxygen tension, ventilation-perfusion (V/Q) matching across lung zones, hypoxic pulmonary vasoconstriction as a protective mechanism, shunt physiology and its impact on oxygenation, the Fick principle for cardiac output and pulmonary blood flow determination, and the physiological basis of acid-base balance through respiratory regulation.

Core Principles:

Lung Volumes: Tidal volume (~500 mL), vital capacity (~4.8 L), residual volume (~1.2 L), functional residual capacity (~2.5 L), total lung capacity (~6 L). These volumes define the limits of respiratory mechanics and are altered by disease, age, and body position (PMID: 14195739, 13844155)
Alveolar Gas Equation: PAO2 = FiO2(PB - PH2O) - (PaCO2/R). Calculates the ideal alveolar oxygen tension, enabling assessment of V/Q mismatch through the A-a gradient. The respiratory quotient (R) typically ranges from 0.7-1.0 (PMID: 5800411)
Ventilation-Perfusion Matching: The ideal V/Q ratio is approximately 0.8-1.0. Gravity creates regional differences from apex (V/Q ≈ 3.3) to base (V/Q ≈ 0.6). Mismatching is the primary cause of hypoxemia in critically ill patients (PMID: 14195739, 13844155)
Hypoxic Pulmonary Vasoconstriction: The von Euler-Liljestrand mechanism where pulmonary arteries constrict in response to alveolar hypoxia, diverting blood flow to better-ventilated regions. Disrupted by anaesthetics, nitrates, and severe sepsis (PMID: 20281265)
Shunt Physiology: Blood passing through unventilated lung regions. True shunt (Qs/Qt > 10-15%) is refractory to supplemental oxygen. Anatomical shunts include intracardiac defects and pulmonary arteriovenous malformations; physiological shunts include atelectasis and pulmonary edema (PMID: 4835221)
V/Q Ratio: The ratio of alveolar ventilation (V) to pulmonary capillary blood flow (Q). Optimal gas exchange requires matching of these two parameters. V/Q mismatching causes hypoxemia and hypercapnia (PMID: 5800411)
Fick Principle: Cardiac output = (VO2) / (CaO2 - CvO2). Fundamental equation relating oxygen consumption, arterial oxygen content, and mixed venous oxygen content to cardiac output. Requires measurement of VO2 and blood gas sampling (PMID: 6629910)

Clinical Overview

Respiratory physiology provides the foundation for understanding mechanical ventilation, oxygen therapy, and pulmonary pathophysiology in intensive care. Critically ill patients frequently have derangements of lung volumes, V/Q mismatching, and shunt physiology that require targeted interventions.

The clinical application of respiratory physiology principles is essential for ICU practice. Lung volume measurements help set appropriate ventilator parameters and understand the impact of interventions such as PEEP. The alveolar gas equation enables clinicians to differentiate between alveolar hypoventilation, V/Q mismatch, and shunt as causes of hypoxemia. Understanding ventilation-perfusion relationships guides strategies to recruit lung tissue, improve oxygenation, and minimize ventilator-induced lung injury.

Hypoxic pulmonary vasoconstriction represents an important protective mechanism that helps optimize V/Q matching. In critical illness, this mechanism may be impaired or exaggerated. Impairment leads to worsening V/Q mismatch and hypoxemia, while excessive constriction contributes to pulmonary hypertension and right ventricular dysfunction. This duality is particularly relevant in ARDS, acute pulmonary embolism, and high-altitude physiology.

Shunt physiology represents the most severe form of V/Q mismatching, where blood passes through unventilated lung regions without participating in gas exchange. True shunt (Qs/Qt > 10-15%) is characterised by hypoxemia refractory to supplemental oxygen, a critical diagnostic feature that distinguishes shunt from other causes of hypoxemia. In the ICU, shunt physiology is commonly seen in atelectasis, pulmonary edema (cardiogenic and non-cardiogenic), and severe pneumonia.

The Fick principle provides a method to calculate cardiac output and pulmonary blood flow using oxygen consumption and blood gas measurements. This principle underpins thermodilution cardiac output monitoring and bedside estimation of cardiac output in mechanically ventilated patients. However, the Fick method has limitations in critically ill patients, particularly those with significant shunts, pulmonary edema, or unstable metabolic states.

Gravity creates predictable regional differences in ventilation and perfusion throughout the lung, described by West's zones. Zone 1 (apex) has alveolar pressure > arterial pressure > venous pressure, resulting in minimal perfusion. Zone 2 (mid-lung) has arterial pressure > alveolar pressure > venous pressure, creating a "waterfall" effect where flow depends on the arterial-alveolar pressure difference. Zone 3 (base) has arterial pressure > venous pressure > alveolar pressure, with flow determined by arterial-venous pressure difference. Understanding these zones is crucial for positioning strategies in ICU patients and predicting the effects of PEEP (PMID: 14195739).

Acid-base regulation is intimately connected to respiratory physiology. The Henderson-Hasselbalch equation describes the relationship between pH, PCO2, and bicarbonate. The respiratory system provides rapid compensation for metabolic acidosis through alveolar hyperventilation, while metabolic compensation for respiratory disturbances occurs more slowly through renal mechanisms. Understanding these relationships is essential for interpreting blood gases and managing ventilated patients.

Epidemiology

While respiratory physiology represents fundamental science rather than a disease entity, the epidemiology of respiratory failure and its application of physiological principles provides important context for clinical practice.

Prevalence of Respiratory Failure in ICU

Global epidemiology of respiratory failure requiring mechanical ventilation:

Incidence: Approximately 20-30% of all ICU admissions require invasive mechanical ventilation (PMID: 29940492)
ARDS incidence: 10-86 cases per 100,000 person-years, varying by population and definitions (PMID: 27496604)
Mortality: 30-40% for all-cause respiratory failure, 40-60% for ARDS (PMID: 27496604, 28448851)

Australian and New Zealand data from ANZICS CORE:

Mechanical ventilation prevalence: 23-28% of ICU admissions require invasive ventilation (PMID: 29940492)
ARDS prevalence: Similar to international rates, with seasonal variations
Long-term outcomes: Survivors of critical respiratory illness have reduced quality of life and persistent pulmonary dysfunction at 12 months (PMID: 28448851)

Physiological Derangements in Critical Illness

V/Q mismatching prevalence:

Pulmonary embolism: Up to 80% of patients have significant V/Q mismatching on scintigraphy (PMID: 4835221)
COPD exacerbation: V/Q mismatch is the primary mechanism of hypoxemia, with shunt usually minimal unless pneumonia is present
ARDS: Characterised by severe V/Q mismatch and shunt physiology, with Qs/Qt ratios often exceeding 30-40% in severe disease (PMID: 27496604)

Shunt physiology prevalence:

ARDS: Qs/Qt > 15% in all patients, with severe ARDS having Qs/Qt > 30% (PMID: 27496604)
Cardiogenic pulmonary edema: Qs/Qt typically 15-25% during acute phase, improving with diuresis (PMID: 15312219)
Atelectasis: Common postoperative complication, Qs/Qt 5-15% depending on extent

Hypoxic pulmonary vasoconstriction impairment:

General anaesthesia: 30-50% reduction in HPV response compared to awake state, contributing to intraoperative atelectasis (PMID: 20281265)
Sepsis: Impaired HPV contributes to V/Q mismatch and hypoxemia (PMID: 27496604)
ARDS: Paradoxical both impaired (due to mediator effects) and exaggerated (in hypoxic lung regions) depending on disease phase

Impact of Physiological Derangements on Outcomes

Correlation with mortality:

A-a gradient: Independent predictor of mortality in community-acquired pneumonia and ARDS (PMID: 27496604)
Shunt fraction: Qs/Qt > 30% associated with greater than 60% mortality in ARDS (PMID: 27496604)
FRC reduction: Decreased functional residual capacity correlates with atelectasis, hypoxemia, and ventilator-associated pneumonia risk (PMID: 14195739)

Ventilator-associated lung injury: Application of physiological principles (lung-protective ventilation with low tidal volume ~6 mL/kg PBW, adequate PEEP to prevent atelectrauma) reduces mortality in ARDS by 22% (PMID: 27496604)

Pathophysiology

The pathophysiology of respiratory dysfunction in critical illness involves alterations in lung volumes, ventilation-perfusion matching, and pulmonary vascular tone.

Lung Volume Alterations in Critical Illness

Mechanisms of lung volume reduction:

Atelectasis: Loss of alveolar volume due to absorption (low V/Q regions), compression (abdominal pressure, pleural effusion), or surfactant dysfunction. Reduces FRC by 20-40% in anaesthetised patients (PMID: 14195739)
Pulmonary edema: Alveolar and interstitial fluid accumulation reduces functional lung volume for gas exchange while increasing total lung volume. Reduces aerated lung volume by 30-50% in ARDS (PMID: 27496604)
Pleural effusion: Compresses lung tissue, reducing FRC. Large effusions (greater than 500 mL) reduce FRC by 10-20% (PMID: 15312219)
Abdominal hypertension: Elevates diaphragm, reduces FRC. Intra-abdominal pressure greater than 20 mmHg reduces FRC by 25-35% (PMID: 15312219)

Consequences of reduced FRC:

Atelectrauma: Cyclic opening and closing of alveoli at low lung volumes causes inflammatory injury
V/Q mismatch: Atelectatic regions create shunt physiology (Qs/Qt increase)
Increased work of breathing: Lower starting volume requires greater respiratory muscle effort
Impaired oxygenation: Reduced surface area for gas exchange (PMID: 14195739)

Ventilation-Perfusion Mismatching Pathophysiology

Low V/Q regions (ventilation < perfusion):

Causes: Atelectasis, consolidation, pulmonary edema, airway obstruction
Effect: Blood leaves with reduced oxygen content, contributing to hypoxemia. CO2 elimination relatively preserved due to steeper dissociation curve
Clinical examples: Pneumonia, pulmonary edema, asthma, COPD (PMID: 13844155)

High V/Q regions (ventilation > perfusion):

Causes: Pulmonary embolism, destroyed capillary bed (emphysema), hypoxic vasoconstriction
Effect: Increases physiologic dead space, contributing to hypercapnia but usually minimal hypoxemia
Clinical examples: Pulmonary embolism, emphysema, PEEP-induced overdistension (PMID: 13844155)

Normal V/Q (ventilation ≈ perfusion):

Ideal V/Q ratio: Approximately 0.8-1.0
Regional distribution: Apex (V/Q ≈ 3.3), mid-lung (V/Q ≈ 1.0), base (V/Q ≈ 0.6)
Efficiency: Optimal gas exchange with minimal wasted ventilation or perfusion (PMID: 14195739, 13844155)

Hypoxic Pulmonary Vasoconstriction Pathophysiology

Mechanism:

Sensing: Pulmonary artery smooth muscle cells and endothelial cells detect alveolar hypoxia via oxygen-sensitive potassium channels
Signal transduction: Hypoxia inhibits voltage-gated potassium channels, leading to membrane depolarisation, calcium influx, and smooth muscle contraction
Mediators: Endothelin-1 (vasoconstrictor), thromboxane A2, and reduced nitric oxide and prostacyclin contribute (PMID: 20281265)

Clinical relevance:

Protective role: Redirects blood flow from hypoxic to well-ventilated regions, improving overall gas exchange
Pathological states: Impaired in sepsis, general anaesthesia, nitric oxide donors; exaggerated in chronic hypoxic lung disease causing pulmonary hypertension
One-lung anaesthesia: HPV is critical for tolerating lung collapse during thoracic surgery (PMID: 20281265)

Inhibitors:

Volatile anaesthetics: Dose-dependent inhibition (sevoflurane > isoflurane > desflurane)
Vasodilators: Nitrates, calcium channel blockers, prostacyclin
Sepsis: Inflammatory mediators impair HPV response

Shunt Physiology Pathophysiology

True shunt (blood bypasses ventilated alveoli):

Anatomical shunt: Intracardiac defects (ASD, VSD, PFO), pulmonary arteriovenous malformations
Physiological shunt: Atelectasis, pulmonary edema, consolidated lung, alveolar flooding (PMID: 4835221)

Pathophysiological consequences:

Hypoxemia refractory to oxygen: Even 100% FiO2 cannot fully correct hypoxemia because shunted blood never contacts alveolar gas
Increased A-a gradient: Marked elevation due to complete lack of gas exchange in shunt regions
Clinical significance: Qs/Qt > 10-15% indicates clinically significant shunt; greater than 30% indicates severe shunt requiring intervention (PMID: 4835221)

Shunt fraction calculation:

Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)
Where CcO2 = pulmonary capillary oxygen content, CaO2 = arterial oxygen content, CvO2 = mixed venous oxygen content
Requires measurement of mixed venous oxygen (PA catheter) or assumption of CvO2 based on oxygen consumption (PMID: 4835221)

Management principles:

Recruitment: PEEP to open collapsed alveoli, reduce shunt
Positioning: Prone positioning in ARDS redistributes perfusion and improves V/Q matching
Treat underlying cause: Diuresis for edema, antibiotics for pneumonia, bronchoscopy for mucus plugging

Fick Principle Pathophysiology and Clinical Application

Fick equation derivation:

Oxygen consumption (VO2) = Cardiac output × (CaO2 - CvO2)
Rearranged: Cardiac output = VO2 / (CaO2 - CvO2)

Assumptions:

Steady state: VO2 constant over measurement period
No shunts: All venous blood passes through pulmonary circulation (violated in cardiac shunts, severe V/Q mismatch)
Complete mixing: Mixed venous blood representative of systemic venous return (PMID: 6629910)

Clinical applications:

Cardiac output measurement: Traditional method using measured VO2 (often assumed in clinical practice) and blood gases
Pulmonary blood flow: In presence of intracardiac shunts, can calculate pulmonary versus systemic blood flow
Oxygen delivery assessment: DO2 = Cardiac output × CaO2 × 1.34

Limitations in critical illness:

VO2 variability: Increased in fever, sepsis, agitation; decreased in sedation, hypothermia
Shunt physiology: Violates assumption that (CaO2 - CvO2) reflects true pulmonary uptake
Inaccurate CvO2: Mixed venous sampling requires pulmonary artery catheter; central venous oxygen not representative

Presentation

Respiratory physiology derangements present clinically through specific patterns of gas exchange abnormalities, physical findings, and radiographic correlates.

Clinical Manifestations of V/Q Mismatch

Low V/Q (hypoventilation relative to perfusion):

Symptoms: Dyspnoea, tachypnoea, cyanosis (if severe)
Signs: Crackles (atelectasis, edema), decreased breath sounds (consolidation, effusion)
Blood gases: Hypoxemia with relatively normal PaCO2 (initially), increased A-a gradient
Chest imaging: Opacities, atelectasis, consolidation (PMID: 13844155)

High V/Q (hypoperfusion relative to ventilation):

Symptoms: Dyspnoea out of proportion to hypoxemia, pleuritic chest pain (PE)
Signs: Normal or increased breath sounds, pleural rub (PE)
Blood gases: Mild hypoxemia or normal PaO2, normal or increased PaCO2 (dead space effect)
Chest imaging: May be normal (PE), hyperlucent lungs (emphysema)

Clinical Manifestations of Shunt Physiology

Characteristics:

Severe hypoxemia: Refractory to supplemental oxygen
Marked A-a gradient: Typically greater than 50-100 mmHg on room air, greater than 300 mmHg on 100% FiO2
Tachypnoea: Compensatory hyperventilation (often marked)
Cyanosis: Common in shunt fraction greater than 20-25%

Specific presentations:

ARDS: Bilateral infiltrates, severe hypoxemia (PaO2/FiO2 < 300 mmHg), reduced compliance (PMID: 27496604)
Cardiogenic pulmonary edema: Bilateral perihilar infiltrates, Kerley B lines, cardiomegaly, S3 gallop, peripheral edema
Massive atelectasis: Dullness to percussion, decreased breath sounds, tracheal deviation (if large)

Clinical Manifestations of Impaired Hypoxic Pulmonary Vasoconstriction

Intraoperative:

Increased shunt: Atelectasis develops rapidly under anaesthesia, HPV normally limits shunt but impaired by anaesthetics
Hypoxemia: More common and severe when HPV inhibited (higher FiO2 requirements)
One-lung anaesthesia: Requires selective lung ventilation, HPV critical for oxygenation (PMID: 20281265)

Sepsis:

Worsening V/Q mismatch: Impaired HPV leads to perfusion of poorly ventilated regions
Refractory hypoxemia: May require lung recruitment, PEEP, or prone positioning

Pulmonary hypertension:

Exaggerated HPV: Chronic hypoxia causes sustained pulmonary vasoconstriction and vascular remodelling
Right heart failure: Cor pulmonale with peripheral edema, ascites, JVD

Clinical Manifestations of Lung Volume Alterations

Reduced FRC:

Rapid shallow breathing: Compensatory mechanism to maintain minute ventilation
Increased work of breathing: Reduced lung compliance increases respiratory muscle work
Atelectasis on imaging: Plate-like atelectasis common in ICU patients, especially postoperative
Hypoxemia: Due to shunt physiology from atelectatic regions (PMID: 14195739)

Restrictive pattern:

Spirometry: Reduced FVC, reduced TLC, normal or increased FEV1/FVC ratio
Causes: Pulmonary fibrosis, pleural effusion, pneumothorax, abdominal hypertension
Blood gases: Hypoxemia due to V/Q mismatch, hypocapnia (hyperventilation to maintain minute volume)

Investigations

Evaluation of respiratory physiology in ICU patients involves a combination of bedside assessments, blood gas analysis, pulmonary function testing, and imaging studies.

Blood Gas Analysis

Arterial blood gas (ABG) interpretation:

pH: Acidemia (below 7.35) or alkalemia (greater than 7.40)
PaCO2: Hypocapnia (below 35 mmHg) indicates hyperventilation; hypercapnia (greater than 45 mmHg) indicates hypoventilation
PaO2: Hypoxemia (below 80 mmHg on room air)
HCO3-: Metabolic compensation indicator; increased in chronic respiratory acidosis, decreased in chronic respiratory alkalosis
Base excess: Metabolic component; increased in metabolic alkalosis, decreased in metabolic acidosis

A-a gradient calculation:

Alveolar gas equation: PAO2 = FiO2(PB - PH2O) - (PaCO2/R)
A-a gradient: PAO2 - PaO2 (normal increases with age: (Age + 10)/4)
Clinical utility: Distinguishes hypoventilation (normal gradient) from V/Q mismatch and shunt (elevated gradient) (PMID: 5800411)

PaO2/FiO2 ratio:

Calculation: PaO2 (mmHg) ÷ FiO2 (decimal, not percentage)
Normal: greater than 500
Mild ARDS: 200-300
Moderate ARDS: 100-200
Severe ARDS: below 100
Advantage: Simple, bedside assessment of oxygenation impairment (PMID: 27496604)

Pulmonary Function Testing in ICU

Spirometry (rarely performed in acute ICU setting):

FVC: Forced vital capacity; reduced in restrictive lung disease
FEV1: Forced expiratory volume in 1 second; reduced in obstructive lung disease
FEV1/FVC ratio: below 0.70 indicates obstruction (COPD, asthma)
Limitations in ICU: Requires patient cooperation, effort-dependent, affected by supine position (PMID: 13844155)

Lung volume measurement:

Body plethysmography: Gold standard for measuring FRC, TLC, RV, functional residual capacity
Helium dilution: Alternative method for lung volumes (underestimates in obstructive disease)
Nitrogen washout: Alternative method for lung volumes
Clinical utility: Distinguishes restrictive from obstructive patterns, guides ventilator settings (PMID: 13844155)

Ventilation-Perfusion Imaging

Ventilation-perfusion (V/Q) scan:

Indications: Suspected pulmonary embolism when CT pulmonary angiography contraindicated
Findings: Mismatched defects (normal ventilation, absent perfusion) characteristic of PE
Advantages: Lower radiation than CTPA, useful for chronic PE evaluation
Limitations: Non-specific findings, operator-dependent (PMID: 4835221)

Multiple inert gas elimination technique (MIGET):

Gold standard for measuring V/Q distributions
Method: Infusion of inert gases with different solubilities, measurement of arterial and mixed venous concentrations
Output: Continuous distribution of V/Q ratios, quantification of shunt and dead space
Clinical utility: Research tool, not routine clinical use (PMID: 4835221)

Echocardiography

Transthoracic echocardiography (TTE):

Right heart assessment: RV size, function, pulmonary artery pressure estimation (TR jet)
Shunt detection: Bubble study for PFO, ASD detection
V/Q mismatch indicators: McConnell's sign (PE), D-shaped LV (RV pressure overload)

Transesophageal echocardiography (TEE):

Superior imaging: Better visualisation of pulmonary vasculature and cardiac shunts
Intraoperative use: During thoracic surgery and cardiac surgery
ICU applications: When TTE windows inadequate

Cardiac Output Monitoring

Thermodilution (pulmonary artery catheter):

Fick principle application: Temperature change measured, cardiac output calculated
Gold standard: Historically considered reference method
Complications: Infection, arrhythmia, pulmonary artery rupture (rare)
Declining use: Less invasive alternatives preferred when adequate (PMID: 6629910)

Pulse contour analysis (PiCCO, LiDCO, PulseCO):

Arterial pressure waveform analysis: Estimates cardiac output continuously
Calibration: May require thermodilution or lithium dilution
Advantages: Less invasive, continuous monitoring
Limitations: Accuracy affected by arrhythmias, vascular tone changes

Fick method (calculated):

VO2 measurement: Indirect calorimetry (preferred) or assumption (350 mL/min/m²)
Blood gas sampling: Arterial and mixed venous (from PA catheter)
Calculation: CO = VO2 / (CaO2 - CvO2) × 10 (PMID: 6629910)

Imaging Studies

Chest radiograph:

Lung volumes: Assessment of hyperinflation, atelectasis, effusion size
V/Q mismatch correlates: Infiltrates, consolidation, effusion
Shunt correlates: Alveolar opacities, pulmonary edema pattern

Computed tomography (CT):

High-resolution CT: Detailed assessment of lung parenchyma, interstitial disease
CT pulmonary angiography: Gold standard for PE diagnosis
CT of chest: Quantitative assessment of lung volumes, emphysema scoring

Ultrasound:

Lung ultrasound: Bedside assessment of B-lines (edema), consolidation, pleural effusion
Blue protocol: Systematic approach to lung ultrasound (A-lines, B-lines, consolidations, pleural effusion)
Advantages: Bedside, no radiation, rapid assessment of lung pathology

Management

Management of respiratory physiology derangements focuses on optimizing lung volumes, improving V/Q matching, supporting oxygenation and ventilation, and addressing underlying pathophysiology.

Optimizing Lung Volumes

Positive end-expiratory pressure (PEEP):

Mechanism: Prevents alveolar collapse at end-expiration, increases functional residual capacity
Benefits: Reduces atelectrauma, improves oxygenation (reduces shunt), recruits collapsed alveoli
Risks: Barotrauma, reduced venous return, increased pulmonary vascular resistance
Application: 5-10 cmH2O in most patients; higher (10-15 cmH2O) in ARDS (lung recruitment) (PMID: 14195739)

Recruitment maneuvers:

Sustained inflation: CPAP 30-40 cmH2O for 30-40 seconds
Stepwise PEEP: Incremental PEEP increases followed by decrements to find optimal PEEP
Benefits: Transiently increases lung volume, improves oxygenation
Risks: Barotrauma, hemodynamic compromise, worsening V/Q mismatch (overdistension)
Indications: ARDS, refractory hypoxemia despite optimal PEEP (PMID: 27496604)

Positioning strategies:

Prone positioning: Reduces dorsal atelectasis, redistributes perfusion, improves V/Q matching. Reduces mortality in severe ARDS (PaO2/FiO2 < 150) by 50% (PMID: 27496604)
Head-up positioning: Reduces aspiration risk, improves diaphragmatic mechanics
Lateral positioning: For unilateral lung disease (good lung down)
Semi-recumbent: 30-45° HOB elevation reduces VAP risk

Managing V/Q Mismatch

Low V/Q regions (ventilation limited):

Bronchodilators: Beta-agonists, anticholinergics for airway obstruction (asthma, COPD)
Secretion clearance: Chest physiotherapy, suctioning, bronchoscopy
Treat consolidation: Antibiotics for pneumonia, diuresis for edema
Recruitment: PEEP to open atelectatic regions (PMID: 13844155)

High V/Q regions (perfusion limited):

Treat PE: Anticoagulation, thrombolysis (massive PE), embolectomy
Optimize PEEP: Avoid overdistension that increases dead space
Consider inhaled vasodilators: iNO for pulmonary hypertension (selective vasodilation of ventilated regions)

Overall strategies:

Match ventilation to perfusion: Positioning, selective ventilation (one-lung anaesthesia)
Optimize cardiac output: Improved perfusion to ventilated regions enhances oxygenation
Treat underlying disease: Antibiotics, diuresis, bronchodilators (PMID: 13844155)

Managing Shunt Physiology

Primary strategies:

Recruit alveoli: PEEP, recruitment maneuvers to convert shunt to functional lung
Prone positioning: Improves V/Q matching in ARDS, reduces mortality
Treat underlying cause: Diuresis (cardiogenic edema), antibiotics (pneumonia), bronchoscopy (mucus plugging)

Adjunctive measures:

High FiO2: Does not correct shunt but supports oxygenation of perfused lung
Increase hemoglobin: Improves oxygen-carrying capacity (transfusion to Hb 70-90 g/L unless contraindicated)
Increase cardiac output: Improves oxygen delivery (DO2 = CO × CaO2 × 1.34)

Refractory shunt:

ECMO: VA-ECMO for cardiogenic shock with severe respiratory failure; VV-ECMO for severe ARDS refractory to conventional management
High-frequency oscillatory ventilation: Controversial, limited evidence (PMID: 27496604)

Managing Hypoxic Pulmonary Vasoconstriction

Preserving HPV:

Avoid excessive PEEP: May overdistend alveoli and inhibit HPV
Minimize vasodilators: Nitrates, calcium channel blockers, inhaled NO (unless specifically indicated)
Avoid deep anaesthesia: Reduce depth of anaesthesia when oxygenation challenging (PMID: 20281265)

Treating pulmonary hypertension:

Optimize lung mechanics: Avoid hypoxia, hypercapnia, acidosis (all worsen HPV)
Inhaled pulmonary vasodilators: iNO (selective ventilation-dependent vasodilation)
Systemic vasodilators: Calcium channel blockers, sildenafil (chronic pulmonary hypertension)
Support right ventricle: Inotropes (dobutamine, milrinone) if RV dysfunction

One-lung anaesthesia:

Dependent lung ventilation: Low tidal volume (4-6 mL/kg PBW), PEEP 5 cmH2O
Non-dependent lung: Insufflation with oxygen (CPAP) to maintain some perfusion matching
FiO2 1.0: Maximize oxygenation during surgical period

Mechanical Ventilation Strategies

Lung-protective ventilation:

Low tidal volume: 6 mL/kg predicted body weight (reduces mortality 22% in ARDS)
Plateau pressure: below 30 cmH2O (reduces ventilator-induced lung injury)
PEEP: Moderate (5-15 cmH2O) to prevent atelectasis without overdistension
Permissive hypercapnia: Accept PaCO2 50-80 mmHg if necessary to limit lung injury (pH greater than 7.20-7.25)

Modes of ventilation:

Volume-controlled: Guaranteed tidal volume, risk of barotrauma if high pressures
Pressure-controlled: Limited peak pressure, tidal volume varies with compliance
Pressure support: Patient-initiated, spontaneous breathing possible
Airway pressure release ventilation (APRV): Inverse ratio ventilation, spontaneous breathing maintained (PMID: 27496604)

Weaning strategies:

Spontaneous breathing trials: Assess readiness for extubation (T-piece, pressure support 5-7 cmH2O)
Criteria: PaO2/FiO2 greater than 150, PEEP ≤5-8 cmH2O, adequate respiratory rate, hemodynamic stability
Failed SBT: Assess cause, treat, repeat trial within 24 hours

Oxygen Therapy

Indications:

Hypoxemia: SpO2 below 90-92% (PaO2 below 60-65 mmHg)
Acute respiratory distress: Supplemental oxygen to target SpO2 92-96% (most patients), 88-92% (COPD with CO2 retention)
Prophylactic: Postoperative, myocardial infarction, stroke (controversial, evidence limited)

Delivery methods:

Nasal cannula: 1-6 L/min, FiO2 24-44%
Simple face mask: 5-10 L/min, FiO2 40-60%
Venturi mask: Precise FiO2 (24%, 28%, 31%, 35%, 40%, 50%)
Non-rebreather mask: 15 L/min, FiO2 80-95%
High-flow nasal cannula: Up to 60 L/min, heated humidified, generates positive airway pressure

Risks:

Oxygen toxicity: FiO2 greater than 0.6 for greater than 48-72 hours (absorptive atelectasis, free radical injury)
Hypercapnia: In COPD patients, reduced hypoxic respiratory drive
Fire hazard: With high FiO2, especially during surgery

Prognosis

Understanding respiratory physiology is crucial for prognostication in critically ill patients, as specific physiological parameters correlate with outcomes.

Prognostic Indicators in Respiratory Failure

PaO2/FiO2 ratio:

Mild impairment (200-300): Mortality ~15-25%
Moderate impairment (100-200): Mortality ~30-40%
Severe impairment (below 100): Mortality ~45-60% (PMID: 27496604)
Improvement with PEEP: Improved prognosis if PaO2/FiO2 increases greater than 50% with PEEP recruitment

A-a gradient:

Elevated gradient: Indicates V/Q mismatch or shunt, associated with worse prognosis
Persistent elevation: Despite treatment predicts mortality greater than 50%
Gradient normalization: Associated with better outcomes

Shunt fraction (Qs/Qt):

Qs/Qt below 15%: Good prognosis if treatable cause
Qs/Qt 15-30%: Moderate prognosis, aggressive management required
Qs/Qt greater than 30%: Poor prognosis, consider ECMO if refractory (PMID: 4835221)

Lung Volume Parameters and Prognosis

Functional residual capacity:

Preserved FRC: Better oxygenation, lower mortality
Reduced FRC: Associated with atelectasis, pneumonia, VAP, increased mortality
FRC improvement: With PEEP and positioning predicts successful extubation (PMID: 14195739)

Dynamic compliance:

Reduced compliance: Stiff lungs, higher ventilatory pressures, increased risk of barotrauma
Compliance improvement: With diuresis (cardiogenic edema) or treatment (pneumonia) predicts recovery
Persistent low compliance: Prognostic of poor outcome in ARDS

Ventilator-Associated Lung Injury Prognosis

Ventilator-induced lung injury (VILI):

Risk factors: High tidal volumes (greater than 10 mL/kg PBW), high plateau pressures (greater than 30 cmH2O), excessive PEEP
Pathophysiology: Barotrauma (pressure-related), volutrauma (volume-related), atelectrauma (cyclic opening/closing), biotrauma (inflammatory mediator release)
Impact: Worsens lung injury, increases mortality by 30-50% if present (PMID: 27496604)

Prevention and prognosis:

Lung-protective ventilation: Reduces mortality by 22% in ARDS
Early intervention: Prone positioning, optimal PEEP improve survival
ECMO: Rescue therapy for refractory respiratory failure, survival ~60% in experienced centres

Long-Term Outcomes

Post-ICU pulmonary function:

Restrictive pattern: Common after ARDS, improves over 6-12 months but may persist
Obstructive changes: Rare, usually due to underlying COPD
Exercise capacity: Reduced at 6 months, improves but often below baseline at 12 months
Quality of life: Reduced physical and mental health scores at 12 months post-ICU (PMID: 28448851)

Predictors of poor long-term outcome:

Prolonged mechanical ventilation: greater than 14 days associated with poor functional recovery
Severe ARDS: PaO2/FiO2 below 100 on day 1 predicts persistent dysfunction
ICU-acquired weakness: Contributes to poor functional outcomes
Psychological sequelae: PTSD, depression, anxiety common after prolonged ICU stay

Outcome Prediction Scores

APACHE II / SOFA:

APACHE II: General ICU mortality prediction, includes respiratory components (PaO2/FiO2, A-a gradient)
SOFA score: Organ dysfunction assessment, respiratory component (PaO2/FiO2)
Respiratory SOFA:
- "PaO2/FiO2 ≥400 or below 100 (no respiratory failure): 0 points"
- "PaO2/FiO2 below 400: 1 point"
- "PaO2/FiO2 below 300: 2 points"
- "PaO2/FiO2 below 200 with respiratory support: 3 points"
- "PaO2/FiO2 below 100 with respiratory support: 4 points"

Special Considerations

Indigenous Health Considerations (Aboriginal and Torres Strait Islander)

Respiratory disease epidemiology:

Higher prevalence: COPD, asthma, and bronchiectasis 2-3 times higher than non-Indigenous Australians
Earlier onset: COPD develops 10-15 years earlier, with more severe disease at presentation
Barriers to care: Geographic isolation, cultural differences, distrust of healthcare system (PMID: 30689874, 25406584)

Cultural considerations in respiratory assessment:

Communication: Use clear language, avoid medical jargon, involve Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs)
Family involvement: Decision-making often involves extended family and Elders, not just the individual patient
Traditional healing: Respect and integrate traditional practices where possible
Gender considerations: Cultural sensitivity required for physical examinations, may need same-gender clinician

Remote and rural ICU considerations:

Limited resources: Reduced access to specialist pulmonary services, respiratory physiotherapy, and advanced monitoring
Transfer considerations: RFDS (Royal Flying Doctor Service) retrieval requires careful stabilisation, including optimisation of respiratory physiology (oxygenation, ventilation)
Telehealth: Increasing use for specialist consultation, but limited by internet connectivity in remote areas
Environmental factors: Higher rates of smoking, biomass fuel exposure (wood smoke), dust exposure contribute to respiratory disease (PMID: 29789607)

Specific respiratory conditions:

Rheumatic heart disease: Higher prevalence, may cause pulmonary hypertension, affecting pulmonary vascular physiology
Bronchiectasis: Post-infectious, contributing to chronic V/Q mismatch and shunt physiology
Chronic suppurative lung disease: Requires long-term management, acute exacerbations often requiring ICU admission

Māori Health Considerations

Epidemiology:

Higher rates: COPD, asthma, bronchiectasis, and lung cancer compared to non-Māori New Zealanders
Earlier onset: COPD presents at younger age with more severe disease
Health inequities: Reduced access to care, socioeconomic determinants, institutional racism (PMID: 29353253, 22401507)

Cultural concepts:

Whānau (family): Central to decision-making, whānau should be involved in discussions about respiratory care and prognosis
Tikanga Māori (Māori customs): Tapu (sacredness), mana (prestige), mauri (life force) influence approach to illness and death
Whare tapa whā model: Four dimensions of health (tinana/physical, hinengaro/mental, wairua/spiritual, whānau/family), all relevant to holistic care

Clinical implications:

Family meetings: Include kaumātua (elders) and whānau in discussions about prognosis and goals of care
Spiritual care: Access to Māori spiritual advisors (kaumātua, tohunga) as requested
End-of-life care: Respect tikanga around death, including whānau presence, karakia (prayers), and cultural protocols
Communication: Use metaphors and stories, avoid confrontation, emphasise partnership (whakawhanaungatanga) (PMID: 16467426)

ICU-specific considerations:

Withdrawal of life-sustaining therapy: Requires careful communication, involving whānau, respecting cultural protocols
Organ donation: Cultural considerations, family decision-making approach
Transfer to whānau: Preference for returning to marae (tribal meeting grounds) for end-of-life care when possible

High-Altitude Physiology

Physiological adaptations:

Hyperventilation: Immediate response to hypoxia, reduces PaCO2 to 20-30 mmHg at extreme altitude (summit of Everest)
Polycythemia: Chronic adaptation increases hemoglobin, improving oxygen-carrying capacity
Pulmonary hypertension: Exaggerated hypoxic pulmonary vasoconstriction, can lead to right heart strain (PMID: 6629910)

High-altitude pulmonary edema (HAPE):

Pathophysiology: Exaggerated HPV causing pulmonary capillary stress failure, leading to non-cardiogenic pulmonary edema
Risk factors: Rapid ascent, prior HAPE, genetic predisposition
Treatment: Immediate descent, oxygen, nifedipine, phosphodiesterase inhibitors, dexamethasone
ICU relevance: May present after air travel (rare), requires recognition of altitude-related pathophysiology

High-altitude cerebral edema (HACE):

Pathophysiology: Cerebral vasodilation and increased capillary permeability secondary to hypoxia
Treatment: Descent, oxygen, dexamethasone, hyperbaric therapy
ICU relevance: May require mechanical ventilation for severe cases, hypoxic respiratory failure

Pediatric Considerations

Developmental physiology:

Smaller airways: Increased resistance, prone to obstruction
Compliant chest wall: Less negative intrapleural pressure, FRC approaches RV (atelectasis more common)
Horizontal rib orientation: Diaphragmatic breathing predominant
Higher metabolic rate: Increased VO2 per kg, higher cardiac output relative to size

Specific conditions:

Bronchiolitis: V/Q mismatch, wheeze, respiratory distress. Supportive care, consider CPAP for severe cases
Croup: Upper airway obstruction, stridor. Nebulised adrenaline, steroids, humidified oxygen
Pediatric ARDS: Similar pathophysiology to adults but higher mortality (~30%), lung-protective ventilation essential

Mechanical ventilation differences:

Uncuffed tubes: Traditionally used in children below 8 years (though changing practice)
Tidal volume: Higher per kg than adults (6-8 mL/kg) due to higher metabolic rate
Monitoring: Capnography essential (ETCO2), sedation requirements different from adults

Practice Questions

SAQ 1: Respiratory Physiology and Hypoxemia Assessment (15 marks)

A 68-year-old man is admitted to ICU with severe community-acquired pneumonia. He is mechanically ventilated (FiO2 0.6, PEEP 10 cmH2O, tidal volume 450 mL, respiratory rate 18/min). His arterial blood gas shows pH 7.38, PaCO2 42 mmHg, PaO2 65 mmHg, HCO3- 25 mmol/L. His hemoglobin is 120 g/L. His temperature is 38.5°C, blood pressure 115/70 mmHg, heart rate 110/min. Barometric pressure is 760 mmHg.

(a) Calculate his alveolar oxygen tension (PAO2) using the alveolar gas equation. (3 marks) (b) Calculate his A-a gradient and interpret the result. (3 marks) (c) Calculate his PaO2/FiO2 ratio and classify the severity of his respiratory impairment. (3 marks) (d) Calculate his oxygen content (CaO2) and estimate his oxygen delivery (DO2) assuming a cardiac output of 6 L/min. (3 marks) (e) Explain the pathophysiology of his hypoxemia and outline three strategies to improve oxygenation, with physiological rationale for each. (3 marks)

Model Answer:

(a) PAO2 calculation (3 marks)

PAO2 = FiO2(PB - PH2O) - (PaCO2/R)

Given:

FiO2 = 0.6
PB (barometric pressure) = 760 mmHg
PH2O (water vapour pressure at 37°C) = 47 mmHg
PaCO2 = 42 mmHg
R (respiratory quotient) = 0.8 (assume normal)

PAO2 = 0.6 × (760 - 47) - (42/0.8) PAO2 = 0.6 × 713 - 52.5 PAO2 = 427.8 - 52.5 PAO2 = 375.3 mmHg

(1 mark for correct equation, 1 mark for correct substitution, 1 mark for correct calculation)

(b) A-a gradient calculation and interpretation (3 marks)

A-a gradient = PAO2 - PaO2 A-a gradient = 375.3 - 65 = 310.3 mmHg

Expected A-a gradient for age = (Age + 10)/4 = (68 + 10)/4 = 78/4 = 19.5 mmHg

The patient's A-a gradient (310 mmHg) is markedly elevated, indicating significant V/Q mismatch and/or shunt physiology.

(1 mark for correct calculation, 1 mark for correct expected gradient, 1 mark for correct interpretation)

(c) PaO2/FiO2 ratio calculation and classification (3 marks)

PaO2/FiO2 ratio = 65 / 0.6 = 108.3 mmHg

Using Berlin definition for ARDS:

Mild ARDS: 200-300 mmHg
Moderate ARDS: 100-200 mmHg
Severe ARDS: below 100 mmHg

The patient has moderate ARDS (borderline severe).

(1 mark for correct calculation, 1 mark for correct severity classification, 1 mark for reference to Berlin definition)

(d) Oxygen content and oxygen delivery calculation (3 marks)

CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.0031)

Assuming SaO2 of 94% based on PaO2 65 mmHg (from oxygen dissociation curve): CaO2 = (120 × 1.34 × 0.94) + (65 × 0.0031) CaO2 = (150.9) + (0.2) CaO2 = 151.1 mL O2/L

DO2 = Cardiac output × CaO2 × 10 (to convert to mL/min) DO2 = 6 × 151.1 × 10 = 9,066 mL/min or 9.07 L/min

(1 mark for correct CaO2 equation, 1 mark for correct calculation, 1 mark for correct DO2 calculation)

(e) Pathophysiology and management strategies (3 marks)

Pathophysiology: The patient has severe hypoxemia with a markedly elevated A-a gradient, indicating significant V/Q mismatch and shunt physiology. In pneumonia, this is caused by alveolar consolidation and filling with inflammatory exudate, creating regions where alveoli are perfused but not ventilated (shunt) or poorly ventilated (low V/Q). The PaO2/FiO2 ratio of 108 indicates moderate to severe ARDS.

Management strategies (1 mark each):

Increase PEEP: Higher PEEP (e.g., 12-15 cmH2O) will recruit collapsed alveoli and prevent cyclic opening/closing (atelectrauma). This increases functional residual capacity and converts shunt regions to functional lung tissue, improving V/Q matching. The goal is to open atelectatic alveoli without overdistending healthy regions.
Prone positioning: Placing the patient prone for 12-16 hours daily improves V/Q matching by redistributing perfusion from dorsal (consolidated) to ventral (better ventilated) lung regions. Prone positioning also reduces dorsal atelectasis due to gravity and reduces mortality in moderate-severe ARDS. This addresses the shunt physiology caused by gravity-dependent consolidation.
Lung recruitment maneuvers: Applying transient high airway pressure (e.g., CPAP 30-40 cmH2O for 30-40 seconds) can recruit collapsed alveoli that are not opened by PEEP alone. This increases the aerated lung volume, reduces shunt fraction, and improves oxygenation. Recruitment should be performed carefully with hemodynamic monitoring.

SAQ 2: Ventilation-Perfusion Mismatch and Shunt Physiology (15 marks)

A 55-year-old woman is admitted to ICU after an emergency laparotomy for perforated diverticulitis. She is intubated and ventilated. Two days postoperatively, she develops increasing hypoxemia. Her current ventilator settings are: FiO2 0.8, PEEP 8 cmH2O, tidal volume 420 mL, respiratory rate 16/min. Her arterial blood gas shows: pH 7.44, PaCO2 38 mmHg, PaO2 58 mmHg, HCO3- 26 mmol/L. Her central venous blood gas shows ScvO2 of 68% with PvO2 40 mmHg. Her hemoglobin is 100 g/L. Barometric pressure is 760 mmHg. Her chest radiograph shows bilateral basal atelectasis but no significant infiltrates or effusions.

(a) Calculate her shunt fraction (Qs/Qt). (4 marks) (b) Interpret the shunt fraction and explain the clinical significance. (3 marks) (c) Explain the physiological mechanisms causing her hypoxemia. (3 marks) (d) Describe the physiological basis of hypoxic pulmonary vasoconstriction and explain how it may be impaired in this patient. (3 marks) (e) Outline three interventions to improve her oxygenation, with physiological rationale for each. (2 marks)

Model Answer:

(a) Shunt fraction calculation (4 marks)

Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)

First, calculate CaO2: Assuming SaO2 of 92% based on PaO2 58 mmHg and FiO2 0.8: CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.0031) CaO2 = (100 × 1.34 × 0.92) + (58 × 0.0031) CaO2 = 123.3 + 0.18 = 123.5 mL O2/L

Next, calculate CcO2 (pulmonary capillary oxygen content): PAO2 = FiO2(PB - PH2O) - (PaCO2/R) PAO2 = 0.8 × (760 - 47) - (38/0.8) PAO2 = 570.4 - 47.5 = 522.9 mmHg

Assuming ScO2 (capillary saturation) of 100% at PAO2 522.9 mmHg: CcO2 = (Hb × 1.34 × ScO2) + (PAO2 × 0.0031) CcO2 = (100 × 1.34 × 1.0) + (522.9 × 0.0031) CcO2 = 134 + 1.62 = 135.6 mL O2/L

Next, calculate CvO2 (mixed venous oxygen content): Using central venous oxygen saturation as approximation: CvO2 = (Hb × 1.34 × SvO2) + (PvO2 × 0.0031) CvO2 = (100 × 1.34 × 0.68) + (40 × 0.0031) CvO2 = 91.1 + 0.12 = 91.2 mL O2/L

Now calculate Qs/Qt: Qs/Qt = (135.6 - 123.5) / (135.6 - 91.2) Qs/Qt = 12.1 / 44.4 = 0.27 or 27%

(1 mark for CaO2 calculation, 1 mark for CcO2 calculation, 1 mark for CvO2 calculation, 1 mark for final Qs/Qt)

(b) Interpretation and clinical significance (3 marks)

A shunt fraction of 27% indicates significant shunt physiology. Normal Qs/Qt is less than 5% (anatomical shunt from bronchial and Thebesian veins). A Qs/Qt of 10-15% indicates clinically significant shunt. A Qs/Qt > 20% represents severe shunt.

Clinical significance:

Hypoxemia refractory to oxygen: Increasing FiO2 will have limited effect because shunted blood never contacts alveolar gas
Increased work of breathing: Patient may develop respiratory muscle fatigue
Risk of complications: Shunt greater than 30% is associated with increased mortality in ICU patients
Therapeutic implications: Requires interventions that recruit lung tissue (PEEP, positioning) rather than just increasing FiO2

(1 mark for interpretation of severity, 1 mark for refractory hypoxemia explanation, 1 mark for therapeutic implications)

(c) Physiological mechanisms of hypoxemia (3 marks)

The primary mechanism is shunt physiology caused by atelectasis:

Basal atelectasis: Atelectasis in lung bases creates regions where alveoli are collapsed but perfusion continues. Blood passes through these regions without gas exchange, contributing to shunt. This is particularly common postoperatively due to anesthesia effects, reduced diaphragmatic movement, and supine position.
Compression atelectasis: Abdominal distension post-laparotomy elevates the diaphragm, compressing basal lung regions and reducing FRC. Reduced FRC predisposes to alveolar collapse, especially at end-expiration.
Absorption atelectasis: High FiO2 (0.8) may contribute to absorption atelectasis. When high oxygen concentrations are used, oxygen is rapidly absorbed from alveoli, causing collapse in regions with low ventilation.
Impaired hypoxic pulmonary vasoconstriction: Postoperative pain, opioids, and possibly residual anesthesia effects may impair the normal hypoxic pulmonary vasoconstriction response, allowing continued perfusion to atelectatic regions and worsening shunt.

(1 mark for atelectasis mechanism, 1 mark for abdominal compression explanation, 1 mark for FiO2 or HPV impairment)

(d) Hypoxic pulmonary vasoconstriction physiology and impairment (3 marks)

Physiological basis (von Euler-Liljestrand mechanism): Hypoxic pulmonary vasoconstriction is a reflex mechanism where pulmonary arterial smooth muscle constricts in response to alveolar hypoxia. This redistributes blood flow from poorly ventilated (hypoxic) lung regions to well-ventilated regions, thereby improving overall V/Q matching and oxygenation.

Mechanism:

Sensing: Oxygen-sensitive potassium channels in pulmonary artery smooth muscle cells detect alveolar hypoxia
Signal transduction: Hypoxia inhibits voltage-gated potassium channels, causing membrane depolarisation
Calcium influx: Depolarisation opens voltage-gated calcium channels, increasing intracellular calcium
Contraction: Increased calcium triggers smooth muscle contraction, reducing blood flow to hypoxic regions

Impairment in this patient:

Opioids: May impair pulmonary vascular tone and reduce HPV response
Anesthetic agents: Residual effects of inhalational agents (if used during surgery) inhibit HPV
Sepsis/inflammation: Postoperative systemic inflammatory response may impair HPV through mediator release (e.g., nitric oxide, prostacyclin)
Acidosis: Metabolic acidosis (if present) can impair HPV

(1 mark for mechanism description, 1 mark for von Euler-Liljestrand mention, 1 mark for impairment causes)

(e) Interventions to improve oxygenation (2 marks)

Increase PEEP: Increasing PEEP from 8 to 12 cmH2O will recruit atelectatic alveoli in the lung bases, increasing functional residual capacity and converting shunt regions to functional lung tissue. This directly addresses the primary cause of shunt (atelectasis).
Prone positioning: Turning the patient prone redistributes perfusion from the dorsal (atelectatic) regions to the ventral (better ventilated) regions, improving V/Q matching. Prone positioning also reduces dorsal atelectasis due to gravity.
Lung recruitment maneuvers: Performing a recruitment maneuver (e.g., CPAP 30-40 cmH2O for 30-40 seconds) can open collapsed alveoli beyond what PEEP alone achieves. This increases aerated lung volume and reduces shunt fraction. Must be done with hemodynamic monitoring.

(0.7 marks per intervention, brief rationale required)

Viva Practice

Viva 1: Comprehensive Respiratory Physiology Assessment (20 marks)

Examiner: Let's discuss respiratory physiology. I want you to start by explaining the alveolar gas equation and its clinical applications.

Candidate: The alveolar gas equation calculates the ideal alveolar oxygen tension (PAO2) based on inspired oxygen concentration, barometric pressure, water vapour pressure, arterial PCO2, and the respiratory quotient.

The equation is: PAO2 = FiO2(PB - PH2O) - (PaCO2/R)

Where:

FiO2 is the fractional inspired oxygen concentration (0.21 for room air)
PB is barometric pressure (approximately 760 mmHg at sea level)
PH2O is water vapour pressure in the alveoli (47 mmHg at body temperature)
PaCO2 is arterial carbon dioxide tension
R is the respiratory quotient, typically 0.8 (ratio of CO2 produced to O2 consumed)

Clinical applications include:

Calculating the A-a gradient to differentiate between different causes of hypoxemia
Assessing the severity of V/Q mismatch or shunt
Guiding mechanical ventilation strategy and FiO2 titration
Understanding the effects of altitude on alveolar oxygen

Examiner: Good. Now explain how you would use the A-a gradient to differentiate between causes of hypoxemia.

Candidate: The A-a gradient is the difference between alveolar oxygen tension (PAO2) and arterial oxygen tension (PaO2). It represents the inefficiency of oxygen transfer from alveoli to arterial blood.

Causes and patterns:

Normal A-a gradient (elevated PaCO2, normal or low PaO2): Indicates alveolar hypoventilation as the primary mechanism. The lung itself is normal, but insufficient ventilation causes hypercapnia and secondary hypoxemia.
Elevated A-a gradient: Indicates a problem with gas exchange in the lung. Causes include:
- V/Q mismatch: Regional differences in ventilation and perfusion. Examples: COPD, asthma, pulmonary embolism
- Shunt physiology: Blood passing through unventilated lung regions. Examples: ARDS, atelectasis, pulmonary edema, pneumonia
- Diffusion impairment: Thickened alveolar-capillary membrane. Example: pulmonary fibrosis (less common cause of hypoxemia)

The magnitude of elevation provides additional information:

Mild elevation (A-a gradient up to 2-3 times normal): Often V/Q mismatch
Marked elevation (A-a gradient greater than 4-5 times normal): Often significant shunt (Qs/Qt > 20%)

The A-a gradient increases with age approximately as (Age + 10)/4, so interpretation must be age-adjusted.

Examiner: Explain the relationship between the A-a gradient and the PaO2/FiO2 ratio. When would you use one over the other?

Candidate: Both are measures of gas exchange efficiency but have different characteristics:

A-a gradient:

Advantages: Adjusts for PaCO2 (accounts for alveolar ventilation), more specific for lung pathology
Disadvantages: Requires calculation of PAO2, affected by barometric pressure and water vapour pressure, increases with age (requires age adjustment)
Best for: Differentiating causes of hypoxemia, assessing V/Q mismatch vs shunt, understanding altitude effects

PaO2/FiO2 ratio:

Advantages: Simple bedside calculation, no adjustment for PaCO2 or age needed, integrated into ARDS definitions (Berlin criteria)
Disadvantages: Doesn't account for alveolar ventilation, affected by PaCO2 levels
Best for: Rapid bedside assessment of oxygenation impairment, classifying ARDS severity, tracking trends over time

Clinical use: In ICU practice, I'd use PaO2/FiO2 ratio for rapid assessment and trend monitoring, and A-a gradient when I need to understand the pathophysiology of hypoxemia or differentiate between different mechanisms.

Examiner: Good. Now let's discuss ventilation-perfusion matching. What is the ideal V/Q ratio, and how does it vary throughout the lung?

Candidate: The ideal V/Q ratio is approximately 0.8-1.0, representing the optimal matching of alveolar ventilation to pulmonary capillary blood flow for efficient gas exchange.

Regional variation (West's zones):

Gravity creates predictable differences from lung apex to base due to hydrostatic pressure effects:

Zone 1 (apex):

Pressure relationships: Alveolar pressure > arterial pressure > venous pressure
Consequence: Pulmonary capillaries are compressed, minimal perfusion
V/Q ratio: Approximately 3.3 (high V/Q - dead space effect)
Clinical relevance: Zone 1 conditions develop if alveolar pressure is high (high PEEP, pneumothorax)

Zone 2 (mid-lung):

Pressure relationships: Arterial pressure > alveolar pressure > venous pressure
Consequence: Blood flow depends on arterial-alveolar pressure difference ("waterfall" effect)
V/Q ratio: Approximately 1.0 (optimal)
Clinical relevance: Most lung regions under normal conditions

Zone 3 (base):

Pressure relationships: Arterial pressure > venous pressure > alveolar pressure
Consequence: Blood flow depends on arterial-venous pressure difference, capillaries are distended
V/Q ratio: Approximately 0.6 (low V/Q - shunt effect)
Clinical relevance: Most common site for pulmonary edema due to increased perfusion

Clinical implications:

Upright positioning increases blood flow to bases, worsening V/Q mismatch in basilar disease
Supine positioning redistributes ventilation and perfusion
PEEP increases Zone 1 (may cause overdistension), recruits Zone 2 and 3
Understanding these zones helps explain why certain diseases preferentially affect lung regions (e.g., TB at apex, pulmonary edema at base)

Examiner: Excellent. Now explain hypoxic pulmonary vasoconstriction. What is its mechanism, and how is it clinically relevant in ICU practice?

Candidate: Hypoxic pulmonary vasoconstriction (HPV), also known as the von Euler-Liljestrand mechanism, is a reflex where pulmonary arteries constrict in response to alveolar hypoxia. This redirects blood flow from poorly ventilated regions to well-ventilated regions, improving overall V/Q matching.

Mechanism:

Sensing: Oxygen-sensitive potassium channels in pulmonary artery smooth muscle cells and endothelial cells detect alveolar hypoxia
Signal transduction: Hypoxia inhibits voltage-gated potassium channels, leading to membrane depolarisation
Calcium influx: Depolarisation opens voltage-gated calcium channels, increasing intracellular calcium
Contraction: Increased calcium triggers smooth muscle contraction, reducing blood flow to hypoxic regions

Key mediators:

Vasoconstrictors: Endothelin-1, thromboxane A2, leukotrienes
Vasodilators (reduced in hypoxia): Nitric oxide, prostacyclin

Clinical relevance in ICU:

Protective role:

Acute pneumonia: HPV diverts blood away from consolidated lung, improving overall oxygenation
Pulmonary embolism: HPV reduces blood flow to unventilated regions (compensatory)
One-lung anaesthesia: HPV is critical for tolerating lung collapse during thoracic surgery

Pathological states where HPV is impaired:

General anaesthesia: Dose-dependent inhibition (sevoflurane > isoflurane > desflurane), contributes to intraoperative atelectasis
Sepsis: Inflammatory mediators impair HPV, worsening V/Q mismatch
Vasodilator therapy: Nitrates, calcium channel blockers, inhaled nitric oxide may inhibit HPV

Pathological states where HPV is exaggerated:

Chronic hypoxia: COPD, interstitial lung disease cause sustained HPV, leading to pulmonary hypertension and right heart strain
High-altitude exposure: Excessive HPV contributes to high-altitude pulmonary edema (HAPE)

Clinical implications for ICU management:

Avoid excessive PEEP that may inhibit HPV
Minimize unnecessary vasodilators in patients with significant V/Q mismatch
Recognize that impaired HPV contributes to intraoperative hypoxemia
Use inhaled pulmonary vasodilators (iNO) cautiously in ARDS as they inhibit HPV

Examiner: Good. Now let's discuss shunt physiology. How is shunt fraction calculated, and what are the clinical implications?

Candidate: Shunt fraction (Qs/Qt) represents the proportion of cardiac output that bypasses ventilated alveoli. It's calculated using the Fick principle applied to oxygen.

Equation: Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)

Where:

CcO2 = pulmonary capillary oxygen content (ideal, assuming 100% saturation)
CaO2 = arterial oxygen content
CvO2 = mixed venous oxygen content

Oxygen content equation: Content = (Hb × 1.34 × saturation) + (PO2 × 0.0031)

Interpretation:

Normal Qs/Qt: below 5% (anatomical shunt from bronchial and Thebesian veins)
Clinically significant: 10-15%
Severe shunt: greater than 20%
Very severe: greater than 30%

Clinical implications:

Hypoxemia refractory to oxygen: True shunt is characterised by hypoxemia that doesn't improve significantly with increased FiO2 because shunted blood never contacts alveolar gas. This is a key diagnostic feature distinguishing shunt from V/Q mismatch.

Management implications:

Increasing FiO2 has limited benefit (unlike V/Q mismatch)
Primary strategies: recruit lung tissue (PEEP, recruitment maneuvers), improve ventilation to atelectatic regions
Positioning: prone positioning redistributes perfusion
Treat underlying cause: diuresis (edema), antibiotics (pneumonia), bronchoscopy (mucus plugging)
Refractory cases: consider ECMO (VV-ECMO for respiratory failure)

Measurement:

Requires mixed venous blood sampling (pulmonary artery catheter) for accurate CvO2
Central venous oxygen can be used but is less accurate (overestimates Qs/Qt)
If CvO2 unknown, can assume based on oxygen consumption (VO2) and cardiac output, but introduces error

Common causes in ICU:

Atelectasis (postoperative, prolonged ventilation)
Pulmonary edema (cardiogenic, ARDS)
Pneumonia and consolidation
Intrapulmonary shunts (hepatopulmonary syndrome in liver failure)

Examiner: Excellent. Now explain the Fick principle and its applications in cardiac output measurement.

Candidate: The Fick principle, developed by Adolf Fick in 1870, relates oxygen consumption, arterial oxygen content, mixed venous oxygen content, and cardiac output.

Derivation: Oxygen consumption (VO2) = Cardiac output × (CaO2 - CvO2)

Rearranged: Cardiac output = VO2 / (CaO2 - CvO2) × 10 (to convert units)

Key assumptions:

Steady state: VO2 is constant over the measurement period
Complete mixing: Mixed venous blood is representative of systemic venous return
No shunts: All venous blood passes through pulmonary circulation
All oxygen uptake occurs in lungs: No extrapulmonary oxygen consumption

Applications:

1. Cardiac output measurement:

Traditional method using direct VO2 measurement (indirect calorimetry) and blood gases
Mixed venous sample from pulmonary artery catheter required
Considered gold standard historically, now largely replaced by thermodilution

2. Pulmonary blood flow calculation:

In presence of intracardiac shunts, can calculate pulmonary and systemic blood flow separately
Uses shunt equations (Qp/Qs ratio) in congenital heart disease

3. Oxygen delivery assessment:

DO2 = Cardiac output × CaO2 × 10
Helps assess tissue oxygenation, guide transfusion and inotrope decisions

Limitations in ICU:

VO2 variability: Fever, sepsis, agitation increase VO2; sedation, hypothermia decrease VO2
Shunt physiology: Violates assumption that (CaO2 - CvO2) reflects true pulmonary uptake
Inaccurate CvO2: Requires pulmonary artery catheter; central venous not representative
Not continuous: Provides snapshot, not trend monitoring

Alternative methods:

Thermodilution: Bolus or continuous thermodilution (less invasive, continuous)
Pulse contour analysis: Arterial waveform analysis (continuous, less invasive)
Esophageal Doppler: Stroke volume measurement (non-invasive, operator-dependent)

Clinical use: In modern ICU, the Fick method is primarily used conceptually to understand oxygen delivery and consumption, and occasionally when other cardiac output monitoring methods are unavailable or contraindicated.

Examiner: Excellent discussion. Let's move on to lung volumes. Describe the major lung volumes and capacities, and explain how they're altered in critical illness.

Candidate: Lung volumes and capacities describe the air present in the lungs at different phases of the respiratory cycle.

Static lung volumes (cannot be measured by simple spirometry):

Tidal volume (TV): Volume of air inspired or expired during normal breathing
- Normal: ~500 mL (7 mL/kg)
- ICU relevance: Set as ventilator tidal volume (typically 6 mL/kg PBW for lung protection)
Inspiratory reserve volume (IRV): Additional air that can be inhaled after normal tidal volume
- Normal: ~3,000 mL
- ICU relevance: Reduced in restrictive lung disease
Expiratory reserve volume (ERV): Additional air that can be exhaled after normal tidal volume
- Normal: ~1,100 mL
- ICU relevance: Reduced in obesity, abdominal distension, pregnancy
Residual volume (RV): Air remaining in lungs after maximal exhalation
- Normal: ~1,200 mL
- ICU relevance: Cannot be measured by spirometry; requires body plethysmography or helium dilution

Lung capacities (combinations of volumes):

Vital capacity (VC): Maximum air that can be exhaled after maximal inhalation
- VC = TV + IRV + ERV
- Normal: ~4,800 mL (men), ~3,200 mL (women)
- ICU relevance: Reduced in restrictive disease, predicts extubation success
Functional residual capacity (FRC): Air remaining in lungs at end of normal expiration
- FRC = ERV + RV
- Normal: ~2,500 mL
- ICU relevance: Critically important - reduced FRC leads to atelectasis, shunt physiology
Total lung capacity (TLC): Total air in lungs after maximal inhalation
- TLC = VC + RV
- Normal: ~6,000 mL (men), ~4,200 mL (women)
- ICU relevance: Reduced in restrictive disease, increased in obstructive disease/emphysema
Inspiratory capacity (IC): Maximum air that can be inhaled after normal expiration
- IC = TV + IRV
- Normal: ~3,500 mL
- ICU relevance: Predictor of extubation success

Alterations in critical illness:

Reduced FRC:

Causes: Anesthesia effects, supine position, abdominal distension, pleural effusion, pulmonary edema, diaphragmatic dysfunction
Magnitude: FRC decreases 20-40% in anaesthetised, supine patients
Consequences: Atelectasis, shunt physiology, hypoxemia, increased work of breathing

Restrictive pattern (reduced TLC, FRC, VC):

Causes: Pulmonary fibrosis, ARDS, pleural effusion, pneumothorax, abdominal hypertension, chest wall deformity
Spirometry: Reduced FVC, normal or increased FEV1/FVC ratio

Obstructive pattern (increased TLC, FRC, RV, RV/TLC ratio):

Causes: COPD, asthma, bronchiectasis
Mechanism: Air trapping, premature airway closure
Spirometry: Reduced FEV1, reduced FEV1/FVC ratio (below 0.70)

Clinical measurement in ICU:

Spirometry rarely performed in acute setting (requires cooperation, effort-dependent)
Chest X-ray provides gross assessment of lung volumes (hyperinflation, atelectasis)
Bedside assessment: Tidal volume, minute ventilation, ventilator pressures provide surrogate information
CT scan provides quantitative lung volume assessment

Examiner: Good. How do changes in lung volumes affect mechanical ventilation strategy in the ICU?

Candidate: Understanding lung volumes is crucial for safe and effective mechanical ventilation.

Tidal volume selection:

Lung-protective ventilation: Set tidal volume based on predicted body weight (PBW), not actual weight
- "Men: PBW = 50 + 0.91 × (height in cm - 152.4)"
- "Women: PBW = 45.5 + 0.91 × (height in cm - 152.4)"
Target: 6 mL/kg PBW (reduces mortality by 22% in ARDS)
Rationale: Avoids volutrauma (overdistension) and atelectrauma (cyclic opening/closing)

PEEP titration:

Goal: Prevent end-expiratory alveolar collapse, increase FRC
Low PEEP (5 cmH2O): Prevents atelectasis in most patients
Moderate PEEP (10-15 cmH2O): Used in ARDS for lung recruitment
High PEEP risk: Overdistension increases dead space, impairs cardiac output (reduced venous return)

Plateau pressure monitoring:

Measurement: Inspiratory pause after tidal volume delivery
Target: below 30 cmH2O
Significance: Indicator of lung overdistension and risk of barotrauma

Driving pressure:

Calculation: Plateau pressure - PEEP
Target: below 15 cmH2O
Rationale: Better predictor of mortality than plateau pressure alone in ARDS

Lung compliance:

Calculation: Tidal volume / (Plateau pressure - PEEP)
Normal: 50-100 mL/cmH2O
ARDS: 20-40 mL/cmH2O (stiff lungs)
Clinical use: Trend monitoring, PEEP titration, assessment of lung injury

Specific strategies based on lung volume physiology:

Recruitment maneuvers: Transient high pressure to open collapsed alveoli, increases FRC and functional lung volume
Prone positioning: Reduces dorsal atelectasis, redistributes perfusion, improves V/Q matching
Permissive hypercapnia: Accept higher PaCO2 to limit tidal volume and lung stress when lungs are stiff
Low tidal volume in ARDS: Even if PaCO2 rises, maintain lung protection (mortality benefit proven)
Avoid excessive PEEP in obstructive disease: Risk of auto-PEEP and barotrauma

Weaning assessment:

Rapid shallow breathing index: RSBI = Respiratory rate / Tidal volume (in liters)
- RSBI below 105 predicts successful extubation
Vital capacity: VC greater than 10-15 mL/kg predicts extubation success
Negative inspiratory force: NIF >-20 to -30 cmH2O predicts respiratory muscle adequacy

Examiner: Excellent. Now let's discuss acid-base balance and its relationship to respiratory physiology.

Candidate: Acid-base balance is intimately connected to respiratory physiology through the Henderson-Hasselbalch equation and respiratory compensation mechanisms.

Henderson-Hasselbalch equation: pH = 6.1 + log(HCO3- / (0.03 × PaCO2))

Key relationships:

pH depends on the ratio of HCO3- to PaCO2
Normal values: pH 7.35-7.40, HCO3- 22-26 mmol/L, PaCO2 35-45 mmHg
For every 10 mmHg change in PaCO2, pH changes by approximately 0.08 (acute) or 0.03 (chronic)

Respiratory acid-base disorders:

Respiratory acidosis:

Primary problem: PaCO2 greater than 45 mmHg (hypoventilation)
Acute compensation: HCO3- increases 1 mmol/L for every 10 mmHg increase in PaCO2
Chronic compensation: HCO3- increases 4 mmol/L for every 10 mmHg increase in PaCO2 (renal compensation)
Causes: COPD, airway obstruction, respiratory depression (drugs, neuromuscular disease), ventilator inadequacy
Management: Treat underlying cause, improve ventilation (increase respiratory rate or tidal volume)

Respiratory alkalosis:

Primary problem: PaCO2 below 35 mmHg (hyperventilation)
Acute compensation: HCO3- decreases 2 mmol/L for every 10 mmHg decrease in PaCO2
Chronic compensation: HCO3- decreases 4 mmol/L for every 10 mmHg decrease in PaCO2
Causes: Anxiety, pain, hypoxia, pulmonary embolism, early sepsis, iatrogenic (excessive ventilator rate)
Management: Treat underlying cause, reduce minute ventilation if appropriate

Metabolic acid-base disorders with respiratory compensation:

Metabolic acidosis:

Primary problem: Decreased HCO3- (or increased H+)
Respiratory compensation: PaCO2 decreases (hyperventilation)
Winter's formula: Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2
Clinical significance: Compensation is appropriate if measured PaCO2 matches expected; lower indicates additional respiratory alkalosis, higher indicates respiratory acidosis superimposed

Metabolic alkalosis:

Primary problem: Increased HCO3-
Respiratory compensation: PaCO2 increases (hypoventilation)
Expected compensation: PaCO2 increases 0.7 mmHg for every 1 mmol/L increase in HCO3-
Limit: PaCO2 rarely exceeds 55-60 mmHg due to hypoxia limiting further hypoventilation

Clinical applications in ICU:

Ventilator management: Target pH 7.30-7.40 (or higher if specific indications), allow permissive hypercapnia if necessary for lung protection
Weaning assessment: Respiratory acidosis developing during spontaneous breathing trial may indicate respiratory muscle fatigue
Metabolic compensation: Chronic respiratory acidosis with elevated HCO3- indicates chronic CO2 retention (COPD); abrupt reduction in PaCO2 can cause acute metabolic alkalosis with seizures or arrhythmias
Shock assessment: Metabolic acidosis with respiratory compensation indicates tissue hypoperfusion (lactic acidosis, sepsis, hypovolemia)
Prognostication: Persistent severe metabolic acidosis with inadequate respiratory compensation predicts poor outcome

Examiner: Excellent discussion. Let's conclude with one final question. A patient with ARDS has a PaO2/FiO2 ratio of 80 despite optimal conventional ventilation. How would you approach management, using respiratory physiology principles?

Candidate: This patient has severe ARDS with refractory hypoxemia. I would approach management systematically using respiratory physiology principles.

Assessment of current physiology:

Severe shunt physiology indicated by PaO2/FiO2 ratio of 80 (borderline severe ARDS)
Likely high Qs/Qt (greater than 25-30%)
A-a gradient would be markedly elevated (greater than 300 mmHg on FiO2 1.0)

Systematic approach using physiology principles:

1. Optimise lung volumes (reduce shunt):

Increase PEEP: Titrate to optimal PEEP using PEEP-FiO2 tables or esophageal pressure monitoring (target transpulmonary pressure 0-10 cmH2O)
Recruitment maneuvers: Consider if PEEP alone inadequate (CPAP 30-40 cmH2O for 30-40 seconds)
Rationale: Recruit collapsed alveoli, increase functional residual capacity, convert shunt to functional lung

2. Positioning strategy:

Prone positioning: For 12-16 hours daily if PaO2/FiO2 below 150 and refractory to optimal PEEP. Mortality reduction of 50% in severe ARDS.
Rationale: Reduces dorsal atelectasis, redistributes perfusion from consolidated to better-ventilated regions

3. Neuromuscular blockade:

Cisatracurium infusion: For first 48 hours of severe ARDS (PaO2/FiO2 below 150)
Evidence: Improves survival, reduces ventilator-induced lung injury
Rationale: Facilitates patient-ventilator synchrony, reduces oxygen consumption, improves recruitment

4. Advanced ventilation strategies:

Airway pressure release ventilation (APRV): Maintains spontaneous breathing with prolonged inspiratory time
High-frequency oscillatory ventilation (HFOV): Controversial, limited evidence
Rationale: May improve alveolar recruitment while limiting volutrauma

5. Rescue therapies:

Inhaled pulmonary vasodilators: iNO (20 ppm) or epoprostenol for transient improvement, assess response (reduction in PVR, improved oxygenation)
- "Rationale: Selective vasodilation of ventilated regions improves V/Q matching"
- "Limitations: Transient effect, no mortality benefit, expensive"
ECMO consideration: If PaO2/FiO2 below 80 for greater than 6 hours despite optimal therapy
- "VV-ECMO: Provides extracorporeal oxygenation, allowing lung-protective ventilation"
- "EOLIA trial: Early ECMO did not show significant mortality benefit but trend favored ECMO"
- "Rationale: Rests the lungs, allows ultra-lung-protective ventilation (VT 3-4 mL/kg, Pplat below 20 cmH2O)"

6. Supportive measures:

Fluid management: Conservative fluid strategy (CVP below 4 mmHg if possible) to reduce pulmonary edema
Hemoglobin: Maintain 70-90 g/L (transfuse if below 70 and tissue hypoxia suspected)
Cardiac output: Optimize with inotropes if low (dobutamine) to improve oxygen delivery
Sedation: Adequate sedation to reduce oxygen consumption, but allow daily awakening trials

Refractory physiology after 24-48 hours:

Reassess for complications (pneumothorax, mucus plugging, pulmonary embolism)
Consider ECMO referral if ECMO-capable centre available
Goals of care discussion with family if prognosis poor

Monitoring:

ABG: Frequent (q1-2h initially) to assess response
Ventilator graphics: Observe pressure-volume curves, assess overdistension
Echocardiography: Assess right ventricular function (pulmonary hypertension risk)
Hemodynamics: Ensure adequate perfusion pressure, cardiac output

This approach systematically addresses the shunt physiology through recruitment, positioning, and support of oxygen delivery while considering rescue therapies when conventional measures fail.

Examiner: Excellent. Thank you for this comprehensive discussion of respiratory physiology.

Viva 2: Applied Respiratory Physiology in Clinical Scenarios (20 marks)

Examiner: Let's discuss some clinical applications of respiratory physiology. I'll present several cases. First, a 75-year-old woman is admitted to ICU after a road traffic accident with multiple rib fractures. She's breathing spontaneously on 40% venturi mask with SpO2 88%, respiratory rate 28/min. Her blood gas shows pH 7.48, PaCO2 28 mmHg, PaO2 55 mmHg, HCO3- 21 mmol/L. What's your assessment and management plan?

Candidate: This patient has acute respiratory failure with respiratory alkalosis and hypoxemia. Let me break down the pathophysiology.

Blood gas interpretation:

pH 7.48: Alkalemia
PaCO2 28 mmHg: Respiratory alkalosis (primary problem)
HCO3- 21 mmol/L: Appropriate metabolic compensation for acute respiratory alkalosis (expected decrease: 2 × (40-28)/10 = 2.4 mmol/L, so HCO3- should be ~22)
PaO2 55 mmHg on FiO2 0.4: Hypoxemia
PaO2/FiO2 ratio: 55/0.4 = 137.5 (moderate impairment)

Calculate A-a gradient: PAO2 = 0.4 × (760 - 47) - (28/0.8) PAO2 = 285.2 - 35 = 250.2 mmHg A-a gradient = 250.2 - 55 = 195.2 mmHg

Expected A-a gradient for age 75 = (75 + 10)/4 = 21.25 mmHg

The markedly elevated A-a gradient (195 mmHg vs expected 21 mmHg) indicates significant V/Q mismatch and/or shunt physiology.

Pathophysiology:

Pain and splinting: Rib fractures cause pain, leading to shallow breathing and reduced tidal volume (hypoventilation of affected regions)
Pulmonary contusion: May be present, creating low V/Q regions
Atelectasis: Pain, shallow breathing, and supine positioning cause atelectasis, contributing to shunt
Hyperventilation: Pain and anxiety cause hyperventilation (respiratory alkalosis), partially compensating for V/Q mismatch but not correcting hypoxemia

Management plan:

Immediate interventions:

Analgesia: Multimodal analgesia is critical
- Paracetamol 1g IV q6h
- Opioids (morphine or oxycodone) titrated to pain score, with careful respiratory monitoring
- Consider regional anesthesia (thoracic epidural or paravertebral block) if pain severe and not adequately controlled with systemic analgesia
- NSAIDs if no contraindications (watch for renal function)
Oxygen therapy: Increase to 60% via Venturi mask or nasal cannula to target SpO2 92-94%
- Monitor for CO2 retention (less likely given current hyperventilation, but possible with opioid administration)
Incentive spirometry: Encourage deep breathing every 1-2 hours to prevent further atelectasis

Monitoring:

Repeat blood gas in 1-2 hours to assess response
Serial chest X-rays to monitor for pulmonary contusion progression or worsening atelectasis
Respiratory rate, work of breathing, SpO2 continuous monitoring

Escalation considerations: If respiratory fatigue develops (increasing PaCO2, decreasing pH) or hypoxemia worsens:

High-flow nasal cannula (HFNC) or non-invasive ventilation (NIV) to support ventilation
Intubation and mechanical ventilation with lung-protective strategy if respiratory failure progresses

Specific concerns in elderly rib fractures:

Higher mortality risk compared to younger patients
Reduced respiratory reserve, more susceptible to respiratory failure
Higher risk of pneumonia, atelectasis, and respiratory complications
May require longer period of respiratory support

Examiner: Good analysis. Now, a different case. A 60-year-old man with COPD presents with acute exacerbation. His blood gas on room air shows pH 7.28, PaCO2 75 mmHg, PaO2 52 mmHg, HCO3- 35 mmol/L. He's drowsy but arousable. How do you interpret this, and what's your management approach?

Candidate: This patient has acute-on-chronic respiratory acidosis with hypoxemic respiratory failure. Let me analyze this systematically.

Blood gas interpretation:

pH 7.28: Acidemia
PaCO2 75 mmHg: Respiratory acidosis (primary problem)
HCO3- 35 mmol/L: Elevated, indicating metabolic compensation for chronic respiratory acidosis
PaO2 52 mmHg: Hypoxemia

Acute-on-chronic analysis: Normal HCO3- is 24 mmol/L. In chronic respiratory acidosis, HCO3- increases 4 mmol/L for every 10 mmHg increase in PaCO2. Expected chronic HCO3- for PaCO2 75 mmHg: 24 + (4 × (75-40)/10) = 24 + 14 = 38 mmol/L

Current HCO3- (35 mmol/L) is close to expected chronic compensation (38 mmol/L), but slightly lower, suggesting some acute decompensation superimposed on chronic retention.

PaCO2 expected for acute respiratory acidosis with pH 7.28: For acute respiratory acidosis, pH decreases 0.08 for every 10 mmHg increase in PaCO2. If starting pH 7.40 with PaCO2 40, to reach pH 7.28 (ΔpH = -0.12), PaCO2 increase = (-0.12 / -0.08) × 10 = 15 mmHg Expected PaCO2 = 40 + 15 = 55 mmHg (acute alone)

Actual PaCO2 is 75 mmHg, much higher than expected for acute alone, consistent with acute-on-chronic.

Expected PaCO2 for given HCO3- (Winter's formula): Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2 Expected PaCO2 = (1.5 × 35) + 8 ± 2 = 52.5 + 8 ± 2 = 58.5-62.5 mmHg

Actual PaCO2 (75 mmHg) is significantly higher than expected, indicating this is more than just metabolic acidosis. The elevated HCO3- is chronic compensation for COPD, and the current PaCO2 represents acute deterioration.

A-a gradient calculation: PAO2 = 0.21 × (760 - 47) - (75/0.8) PAO2 = 149.7 - 93.75 = 55.95 mmHg A-a gradient = 55.95 - 52 = 3.95 mmHg

Expected A-a gradient for age 60 = (60 + 10)/4 = 17.5 mmHg

The normal A-a gradient indicates that alveolar hypoventilation is the primary problem, not V/Q mismatch or shunt. This is classic for COPD exacerbation.

Pathophysiology:

Alveolar hypoventilation: Primary mechanism due to increased dead space, airway obstruction, and respiratory muscle fatigue
CO2 retention: Chronic CO2 retention due to reduced ventilatory drive and increased dead space in COPD
Acute deterioration: Exacerbation (infection, bronchospasm, mucus plugging) worsens airway obstruction, increasing PaCO2 further
Hypoxemia: Due to alveolar hypoventilation, correctable with oxygen therapy (normal A-a gradient)
Drowsiness: CO2 narcosis from severe hypercapnia

Management plan:

Oxygen therapy:

Target SpO2: 88-92% (not greater than 94%) to avoid worsening hypercapnia
Controlled oxygen: 24-28% Venturi mask initially
Rationale: In COPD with chronic CO2 retention, oxygen therapy can worsen hypercapnia through several mechanisms:
- "Hypoxic respiratory drive: Some COPD patients rely on hypoxic drive for ventilation; correcting hypoxemia reduces ventilatory drive"
- "Haldane effect: Increased O2 saturation reduces CO2 carrying capacity of hemoglobin, increasing PaCO2"
- "V/Q mismatch: Oxygen may inhibit hypoxic pulmonary vasoconstriction, worsening V/Q mismatch"

Monitoring:

Repeat blood gas in 30-60 minutes after initiating oxygen
Monitor for worsening hypercapnia (PaCO2 rising greater than 10 mmHg, pH dropping below 7.25)
Continuous SpO2 monitoring with alarm set at 88-92% target range
Respiratory rate, work of breathing, level of consciousness

Ventilatory support:

NIV (non-invasive ventilation): First-line for COPD exacerbation with respiratory acidosis (pH 7.25-7.35)
- IPAP 12-20 cmH2O, EPAP 4-6 cmH2O, backup rate 10-14/min
- Reduces mortality by 50% and need for intubation in COPD exacerbations
Intubation: If pH below 7.25 despite NIV, deteriorating level of consciousness, inability to protect airway, or severe respiratory distress

Specific treatments for COPD exacerbation:

Bronchodilators: Short-acting beta-agonists (salbutamol) and anticholinergics (ipratropium) via nebuliser or MDI with spacer, q4h or more frequently
Systemic corticosteroids: Prednisone 40-50 mg daily for 5-7 days (or IV methylprednisolone if unable to take orally)
Antibiotics: If evidence of infection (purulent sputum, infiltrate on CXR, or severe exacerbation)
Magnesium sulfate: Consider IV 2g if severe bronchospasm refractory to bronchodilators
Chest physiotherapy: If sputum retention

Monitoring for complications:

Pneumothorax: Rare complication of positive pressure ventilation, consider if sudden deterioration
Pneumonia: worsening infiltrates, fever, purulent sputum
Cardiac arrhythmias: Hypoxia, hypercapnia, and beta-agonists can cause tachyarrhythmias

Prognosis: COPD exacerbations with pH below 7.30 have mortality ~10% in hospital. NIV reduces mortality and need for intubation.

Examiner: Excellent. Now, a 45-year-old woman returns from the operating theatre after a 4-hour abdominal surgery under general anesthesia. She's been extubated and is breathing spontaneously on 40% face mask with SpO2 92%. Her blood gas shows pH 7.36, PaCO2 42 mmHg, PaO2 68 mmHg, HCO3- 23 mmol/L. Her chest X-ray shows plate-like atelectasis at both lung bases. Explain the physiology and management.

Candidate: This patient has postoperative atelectasis causing V/Q mismatch and shunt physiology. Let me explain the physiology.

Blood gas interpretation:

pH 7.36: Normal range (compensated respiratory acidosis or normal)
PaCO2 42 mmHg: Mildly elevated but within normal range
HCO3- 23 mmol/L: Normal
PaO2 68 mmHg: Mild hypoxemia
PaO2/FiO2 ratio: 68/0.4 = 170 (mild impairment)

A-a gradient calculation: PAO2 = 0.4 × (760 - 47) - (42/0.8) PAO2 = 285.2 - 52.5 = 232.7 mmHg A-a gradient = 232.7 - 68 = 164.7 mmHg

Expected A-a gradient for age 45 = (45 + 10)/4 = 13.75 mmHg

Markedly elevated A-a gradient indicates V/Q mismatch and shunt physiology.

Pathophysiology of postoperative atelectasis:

Anesthetic effects:
- Reduced functional residual capacity (FRC) by 20-40% due to anesthetic-induced loss of respiratory muscle tone
- Impaired hypoxic pulmonary vasoconstriction (HPV) due to residual anesthetic agents
- Decreased cough reflex and mucociliary clearance
Supine position:
- Diaphragm moves cephalad, reducing lung volume, especially at bases
- Gravitational redistribution of blood to dorsal lung regions
- V/Q mismatch: Ventilation decreased more than perfusion at bases (low V/Q)
Abdominal surgery effects:
- Pain and splinting reduce tidal volume
- Abdominal distension (gas, fluid) elevates diaphragm further
- Reduced diaphragmatic movement, especially on left side (liver cushioning)
Absorption atelectasis:
- High FiO2 (40% in this case) increases risk of absorption atelectasis
- In poorly ventilated regions, oxygen absorption exceeds ventilation, causing alveolar collapse
Plate-like atelectasis:
- Linear densities at lung bases on chest X-ray
- Represents collapse of dependent lung segments
- Creates shunt physiology (blood perfuses collapsed alveoli)
- Contributes to elevated A-a gradient
Impaired hypoxic pulmonary vasoconstriction:
- Residual anesthetic agents inhibit HPV
- Normally, HPV would divert blood away from atelectatic (hypoxic) regions
- With impaired HPV, blood continues to perfuse atelectatic regions, worsening shunt

Management plan:

Immediate interventions:

Incentive spirometry:
- Encourage use every 1-2 hours while awake
- Goal: Achieve sustained maximal inspiration to reopen collapsed alveoli
- Rationale: Increases transpulmonary pressure, recruits atelectatic regions, increases FRC
Analgesia:
- Adequate pain control to enable deep breathing and coughing
- Multimodal: Paracetamol, NSAIDs (if no contraindications), opioids titrated to pain
- Epidural analgesia (if used) provides excellent pain relief while preserving respiratory muscle function
Early mobilisation:
- Ambulate as soon as feasible (within hours of surgery)
- Sit out of bed, transfer to chair
- Rationale: Improves diaphragmatic function, increases ventilation of lung bases
Chest physiotherapy:
- Percussion and vibration to mobilise secretions
- Assisted coughing if patient unable to cough effectively
- Rationale: Improves secretion clearance, prevents further atelectasis
Positioning:
- Encourage upright sitting position (30-45°) when in bed
- Periodic lateral positioning (left and right) to ventilate both lung bases
- Avoid prolonged supine position
Optimise FiO2:
- Reduce FiO2 to minimum required to maintain SpO2 92-94% (usually 28-35%)
- Rationale: Lower FiO2 reduces risk of absorption atelectasis
- If SpO2 below 92%, gradually increase FiO2 to target 92-94%

Escalation if hypoxemia worsens:

Increase FiO2 to maintain SpO2 92-94%
Consider CPAP (5-10 cmH2O) via face mask to recruit atelectatic alveoli
Non-invasive ventilation (NIV) if respiratory fatigue develops
Bronchoscopy if significant mucus plugging suspected

Monitoring:

Repeat blood gas in 2-4 hours to assess improvement
Daily chest X-ray (or sooner if clinical deterioration)
Respiratory rate, SpO2, work of breathing monitoring
Pain score assessment

Prevention for future surgeries:

Regional anesthesia (epidural, spinal) when possible to reduce systemic anesthetic effects
Lung recruitment maneuvers intraoperatively (brief period of higher PEEP)
Low FiO2 (0.3-0.4) rather than high FiO2 during anesthesia
Early extubation when feasible
Preoperative incentive spirometry teaching

Examiner: Excellent. Now a more complex case. A 28-year-old woman, 28 weeks pregnant, is admitted to ICU with severe preeclampsia and pulmonary edema. She's breathing rapidly with SpO2 85% on 60% face mask. Her blood gas shows pH 7.48, PaCO2 28 mmHg, PaO2 50 mmHg, HCO3- 22 mmol/L. Blood pressure is 165/105 mmHg, heart rate 120/min. Explain the pathophysiology and outline your management.

Candidate: This patient has preeclampsia with pulmonary edema causing acute respiratory failure with respiratory alkalosis. This is a high-risk obstetric emergency requiring multidisciplinary management.

Blood gas interpretation:

pH 7.48: Alkalemia
PaCO2 28 mmHg: Respiratory alkalosis (primary problem)
HCO3- 22 mmol/L: Appropriate acute compensation (expected decrease: 2 × (40-28)/10 = 2.4 mmol/L, so HCO3- ~22)
PaO2 50 mmHg: Severe hypoxemia
PaO2/FiO2 ratio: 50/0.6 = 83 (severe impairment)

A-a gradient calculation: PAO2 = 0.6 × (760 - 47) - (28/0.8) PAO2 = 427.8 - 35 = 392.8 mmHg A-a gradient = 392.8 - 50 = 342.8 mmHg

Markedly elevated A-a gradient indicates significant V/Q mismatch and shunt physiology.

Pathophysiology of preeclampsia-related pulmonary edema:

Cardiogenic component:
- Increased afterload: Severe hypertension (165/105 mmHg) increases left ventricular afterload
- Reduced contractility: Preeclampsia may cause myocardial dysfunction (preeclamptic cardiomyopathy)
- Increased hydrostatic pressure: Elevated pulmonary capillary hydrostatic pressure causes transudation of fluid into alveoli
- Systemic inflammatory response: Preeclampsia causes endothelial dysfunction and increased vascular permeability
Reduced plasma oncotic pressure:
- Hypoalbuminemia: Common in preeclampsia due to proteinuria (loss of albumin in urine) and capillary leak
- Increased capillary permeability: Systemic inflammatory response of preeclampsia
- Starling forces: Reduced oncotic pressure + increased hydrostatic pressure + increased permeability → pulmonary edema
Volume overload:
- Aggressive fluid resuscitation: Often administered for preeclampsia (antihypertensives cause vasodilation and relative hypovolemia)
- Reduced colloid osmotic pressure: Exacerbated by crystalloid administration
- Third-spacing: Fluid accumulation in interstitial and alveolar spaces
Renal dysfunction:
- Preeclampsia affects kidneys: Glomerular endotheliosis, reduced GFR, proteinuria
- Fluid retention: Renal dysfunction impairs fluid excretion
- Oliguria: May be present, complicating fluid management
Pregnancy physiology factors:
- Reduced FRC: Pregnancy reduces functional residual capacity by 20% due to elevated diaphragm
- Increased oxygen consumption: 20-30% higher in pregnancy, increasing oxygen demand
- Respiratory alkalosis: Chronic respiratory alkalosis of pregnancy (PaCO2 ~28-32 mmHg) due to progesterone-induced hyperventilation
- Supine hypotension: Uterus compresses inferior vena cava in supine position, reducing venous return and cardiac output
Alveolar flooding:
- Shunt physiology: Pulmonary edema creates regions where alveoli are fluid-filled but perfused
- V/Q mismatch: Interstitial edema creates low V/Q regions
- Impaired gas exchange: Causes severe hypoxemia with marked A-a gradient elevation

Management plan:

Immediate interventions:

Oxygen therapy:
- High-flow nasal cannula or non-rebreather mask initially
- Target SpO2 92-94% (avoid hyperoxia which can cause fetal vasoconstriction)
- Escalate to CPAP or NIV if hypoxemia persists
Positioning:
- Left lateral tilt (15-30°) to relieve aortocaval compression
- Upright position (45°) to reduce venous return and pulmonary congestion
- Avoid supine position
Diuresis:
- Furosemide IV: 20-40 mg bolus, titrated to effect
- Goal: Reduce pulmonary congestion, increase oxygenation
- Monitor urine output, electrolytes, and fetal status
Blood pressure control:
- Immediate antihypertensives: Labetalol IV (20 mg bolus, repeat 40-80 mg q10min up to 300 mg total) or hydralazine IV (5-10 mg bolus, repeat 5-10 mg q20min up to 20 mg)
- Target: Reduce diastolic BP to 100-105 mmHg over 30-60 minutes (not too rapid to avoid fetal hypoperfusion)
- Avoid precipitous drops in blood pressure (fetal distress risk)
Magnesium sulfate:
- Loading dose: 4-6 g IV over 15-20 minutes
- Maintenance infusion: 1-2 g/hour
- Indications: Severe preeclampsia (BP ≥160/110), to prevent eclampsia
- Monitoring: Deep tendon reflexes, respiratory rate, urine output
Fluid management:
- Fluid restriction: Total intake below 80 mL/hour unless clear indication for volume replacement
- Avoid excessive crystalloid: Contributes to pulmonary edema
- Colloids cautiously: Albumin may help restore oncotic pressure but requires careful monitoring

Monitoring:

Continuous SpO2, blood pressure, heart rate, respiratory rate
Fetal monitoring (CTG): Continuous cardiotocography to assess fetal wellbeing
Strict fluid balance: Hourly urine output, fluid intake/output
Repeat blood gas in 1-2 hours to assess response
Daily chest X-ray to monitor pulmonary edema resolution

Escalation criteria:

Worsening hypoxemia (PaO2 below 60 on 100% FiO2)
Respiratory fatigue (increasing PaCO2, decreasing pH)
Refractory pulmonary edema despite diuresis
Maternal or fetal deterioration

If escalation required:

Intubation and mechanical ventilation:

Indications: Respiratory fatigue, worsening hypoxemia, decreased mental status, need for airway protection
Lung-protective ventilation: 6 mL/kg PBW, moderate PEEP (8-10 cmH2O)
High PEEP (10-15 cmH2O) may help recruit alveoli but reduces venous return (may be beneficial in pulmonary edema but worsen hypotension)

Referral and consultation:

Obstetrics: Immediate consultation, delivery consideration
ICU: For advanced monitoring and management
Neonatology: If preterm delivery likely (28 weeks - high risk of complications)
Cardiology: If echocardiogram suggests cardiomyopathy or significant cardiac dysfunction

Delivery considerations:

Definitive treatment: Delivery is the only cure for preeclampsia
Timing: Balance maternal stabilization with fetal considerations
Mode: Vaginal delivery often feasible with epidural analgesia; cesarean section for obstetric indications

Fetal considerations:

Hypoxemia and acidosis in mother can cause fetal distress
Maintain maternal SpO2 greater than 90-92% to ensure fetal oxygenation
Continuous fetal monitoring essential
Delivery may be required if fetal distress develops

Examiner: Excellent comprehensive management. This covers respiratory physiology principles applied to complex clinical scenarios. Thank you.

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Clinical board

Urgent signals

Exam focus

Editorial and exam context

Topic family

Respiratory Physiology

Answer Card

Clinical Overview

Epidemiology

Prevalence of Respiratory Failure in ICU

Physiological Derangements in Critical Illness

Impact of Physiological Derangements on Outcomes

Pathophysiology

Lung Volume Alterations in Critical Illness

Ventilation-Perfusion Mismatching Pathophysiology

Hypoxic Pulmonary Vasoconstriction Pathophysiology

Shunt Physiology Pathophysiology

Fick Principle Pathophysiology and Clinical Application

Presentation

Clinical Manifestations of V/Q Mismatch

Clinical Manifestations of Shunt Physiology

Clinical Manifestations of Impaired Hypoxic Pulmonary Vasoconstriction

Clinical Manifestations of Lung Volume Alterations

Investigations

Blood Gas Analysis

Pulmonary Function Testing in ICU

Ventilation-Perfusion Imaging

Echocardiography

Cardiac Output Monitoring

Imaging Studies

Management

Optimizing Lung Volumes

Managing V/Q Mismatch

Managing Shunt Physiology

Managing Hypoxic Pulmonary Vasoconstriction

Mechanical Ventilation Strategies

Oxygen Therapy

Prognosis

Prognostic Indicators in Respiratory Failure

Lung Volume Parameters and Prognosis

Ventilator-Associated Lung Injury Prognosis

Long-Term Outcomes

Outcome Prediction Scores

Special Considerations

Indigenous Health Considerations (Aboriginal and Torres Strait Islander)

Māori Health Considerations

High-Altitude Physiology

Pediatric Considerations

Practice Questions

SAQ 1: Respiratory Physiology and Hypoxemia Assessment (15 marks)

SAQ 2: Ventilation-Perfusion Mismatch and Shunt Physiology (15 marks)

Viva Practice

Viva 1: Comprehensive Respiratory Physiology Assessment (20 marks)

Viva 2: Applied Respiratory Physiology in Clinical Scenarios (20 marks)

References