ICU Ventilators - Types and Modes

Quick Answer

ICU ventilators are sophisticated medical devices that provide mechanical respiratory support to critically ill patients. Modern ICU ventilators deliver positive pressure ventilation through various modes classified by control variable (volume or pressure), triggering mechanism (time, pressure, flow), and cycling mechanism (volume, time, flow, pressure). The fundamental modes include volume control (VC-CMV), pressure control (PC-CMV), SIMV, pressure support (PSV), and CPAP. Advanced modes such as PRVC, APRV/BiLevel, ASV, NAVA, and PAV+ provide adaptive support. The cornerstone of modern ventilation is lung-protective strategy: tidal volume 6 mL/kg ideal body weight, plateau pressure ≤30 cmH₂O, and driving pressure ≤15 cmH₂O, which reduces ARDS mortality by 22% (ARDSNet PMID: 10793162). Mode selection depends on underlying pathophysiology, patient effort, and clinical goals.

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CICM Exam Focus

What Examiners Expect

Second Part Written (SAQ):

Common SAQ stems:

• "Describe the classification of mechanical ventilation modes by control variable, triggering, and cycling mechanisms."
• "Compare and contrast volume control and pressure control ventilation. Discuss advantages, disadvantages, and clinical indications for each."
• "A patient with ARDS is receiving mechanical ventilation. The ventilator alarms show high peak pressure. Outline your approach to diagnosis and management."
• "Discuss the physics of gas flow as applied to mechanical ventilation."
• "Describe the features of NAVA and PAV+ that distinguish them from conventional ventilation modes."

SAQ scoring expectations:

• Systematic classification of ventilation modes
• Understanding of respiratory mechanics (compliance, resistance, work of breathing)
• Evidence-based mode selection for specific pathologies
• Waveform interpretation and troubleshooting
• Application of lung-protective ventilation principles

Second Part Hot Case:

Typical presentations:

• Mechanically ventilated patient with ARDS, requiring optimization of ventilator settings
• Patient with high airway pressures requiring troubleshooting
• Patient with patient-ventilator dyssynchrony
• Weaning failure requiring mode adjustment

Examiners assess:

• Systematic assessment of ventilator settings and patient-ventilator interaction
• Recognition of waveform abnormalities
• Safe adjustment of ventilator parameters
• Application of evidence-based protocols (ARDSNet)
• Recognition and management of complications

Second Part Viva:

Expected discussion areas:

• Physics of gas flow (laminar vs turbulent, Poiseuille's law, Reynolds number)
• Ventilator classification and mode taxonomy
• Triggering and cycling mechanisms
• Mode selection for specific clinical scenarios
• Waveform interpretation and troubleshooting
• Advanced modes: NAVA, PAV+, ASV
• ARDS ventilation strategy

Examiner expectations:

• Fluent discussion of respiratory mechanics
• Safe, consultant-level ventilator management
• Evidence-based practice (ARDSNet, PROSEVA, ACURASYS)
• Systematic troubleshooting approach
• Understanding of modern adaptive modes

Common Mistakes

• Confusing peak pressure with plateau pressure
• Not calculating ideal body weight for tidal volume calculation
• Misclassifying ventilation modes (e.g., confusing SIMV with PSV)
• Failing to consider auto-PEEP in obstructive lung disease
• Not recognizing patient-ventilator dyssynchrony patterns
• Inappropriate mode selection for clinical scenario
• Forgetting driving pressure as key mortality determinant

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Key Points

Must-Know Facts

1. Ventilator Classification: ICU ventilators are classified by pressure type (positive vs negative), power source (pneumatic, electric, combined), control variable (volume, pressure, dual), trigger (time, pressure, flow), and cycling (volume, time, flow, pressure).

2. Gas Flow Physics: Flow is laminar at low velocities (Poiseuille's law: R ∝ 1/r⁴) and turbulent at high velocities (Reynolds number >2000). Turbulent flow increases resistance and work of breathing.

3. Respiratory Mechanics Equation: Paw = (V̇ × Raw) + (VT / C) + PEEP, where Paw = airway pressure, V̇ = flow, Raw = airway resistance, VT = tidal volume, C = compliance.

4. Volume Control (VCV): Delivers a set tidal volume regardless of lung compliance. Peak pressure varies with resistance and compliance. Guarantees minute ventilation but may cause VILI if pressures uncontrolled.

5. Pressure Control (PCV): Delivers breaths to a set pressure limit. Tidal volume varies with lung compliance. Limits barotrauma but may result in inadequate ventilation if compliance worsens.

6. Lung-Protective Ventilation: ARDSNet protocol - VT 6 mL/kg IBW, plateau pressure ≤30 cmH₂O, driving pressure ≤15 cmH₂O reduces ARDS mortality by 22% (PMID: 10793162).

7. Driving Pressure: ΔP = Plateau pressure - PEEP. Most important predictor of ARDS mortality. Target ≤14-15 cmH₂O (PMID: 25693014).

8. Advanced Modes: PRVC combines volume targeting with pressure delivery; NAVA uses diaphragm electrical activity for synchronization; PAV+ delivers proportional assistance based on real-time mechanics.

9. Patient-Ventilator Dyssynchrony: Occurs in 25-80% of mechanically ventilated patients. Asynchrony Index >10% associated with increased mortality and prolonged ventilation (PMID: 23774337).

10. Auto-PEEP: Intrinsic PEEP from incomplete exhalation. Common in obstructive lung disease. Causes hemodynamic compromise and triggering failure. Detected by end-expiratory hold.

Memory Aids

SOAPME - Ventilator Setup Check:

• Suction (available and working)
• Oxygen (source, concentration, backup)
• Airway (ETT position, cuff pressure 20-30 cmH₂O)
• Pressure (circuit, alarms set appropriately)
• Monitors (waveforms, SpO₂, capnography)
• Electrics (power, backup battery)

DOPE - Acute Deterioration on Ventilator:

• Displacement (ETT malposition, right main bronchus)
• Obstruction (secretions, kinked ETT, biting)
• Pneumothorax (tension pneumothorax)
• Equipment failure (circuit disconnect, ventilator malfunction)

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Definition & Epidemiology

Definition

An ICU ventilator is a sophisticated medical device that provides controlled or assisted respiratory support by generating gas flow to deliver positive (or rarely negative) pressure to the airways, facilitating gas exchange in patients with respiratory failure.

Mechanical ventilation is defined as any method of respiratory support that employs a mechanical device to assist or replace spontaneous breathing.

Classification Systems:

Epidemiology

International Data:

• Approximately 40% of ICU patients require invasive mechanical ventilation (PMID: 27942923)
• Median ventilation duration: 3-5 days in general ICU populations
• Mean ventilation duration: 7-10 days (skewed by prolonged ventilation cases)
• Acute respiratory failure accounts for 20-30% of ICU admissions (PMID: 27071966)

Australian/NZ Data (ANZICS APD):

• Mechanical ventilation in 35-45% of Australian ICU admissions
• Median ventilation duration: 2.1 days (IQR 0.9-5.4 days)
• Prolonged mechanical ventilation (>7 days) in 15-20% of ventilated patients
• Tracheostomy rate: 8-12% of patients ventilated >48 hours

Risk Factors for Prolonged Ventilation:

• Non-modifiable: Age >65, pre-existing lung disease, neuromuscular disease
• Modifiable: Deep sedation, inadequate nutrition, delirium
• Iatrogenic: Excessive fluids, ventilator-induced diaphragm dysfunction (VIDD)

High-Risk Populations:

• Aboriginal and Torres Strait Islander peoples: 1.5-2× higher rates of respiratory failure requiring mechanical ventilation; associated with higher rates of chronic lung disease, pneumonia, and sepsis
• Māori: 1.3-1.5× higher rates of ICU admission for respiratory failure
• Remote/rural populations: Delayed access to ICU, higher acuity at presentation, retrieval challenges

Outcomes:

• ICU mortality: 15-25% for mechanically ventilated patients
• Hospital mortality: 25-35% for mechanically ventilated patients
• ARDS mortality: 35-45% (improved with lung-protective ventilation)
• 1-year mortality: 40-50% for ARDS survivors
• Functional recovery: 50-60% return to baseline at 6 months

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Applied Basic Sciences

This section bridges First Part basic sciences with Second Part clinical practice

Physics of Gas Flow

Laminar vs Turbulent Flow

Understanding gas flow physics is fundamental to ventilator function and airway management:

Laminar Flow:

• Smooth, parallel streamlines of gas molecules
• Occurs at low flow velocities in straight, smooth tubes
• Governed by Poiseuille's Law:
  • Resistance ∝ (8 × length × viscosity) / (π × radius⁴)
  • R ∝ 1/r⁴ - halving radius increases resistance 16-fold
• Resistance depends on gas viscosity (not density)
• Lower work of breathing

Turbulent Flow:

• Chaotic, disorganized gas movement with eddies
• Occurs at high flow velocities, branching points, airway obstruction
• Reynolds Number predicts transition:
  • Re = (velocity × diameter × density) / viscosity
  • "Re < 2000: Laminar flow"
  • "Re > 4000: Turbulent flow"
  • "Re 2000-4000: Transitional flow"
• Resistance depends on gas density (not viscosity) - basis for heliox therapy
• Higher work of breathing

Clinical Applications:

• Heliox (helium-oxygen mixture): Reduces density, promotes laminar flow, reduces work of breathing in upper airway obstruction (PMID: 9175038)
• ETT diameter: Critical determinant of resistance (R ∝ 1/r⁴); 7.0 mm ETT has 2× resistance of 8.0 mm ETT
• High flow rates: Increase turbulence, increase resistance, increase ventilator work

Equation of Motion

The fundamental equation governing mechanical ventilation:

Paw = (V̇ × Raw) + (VT / C) + PEEP

Where:

• Paw = Airway pressure
• V̇ = Inspiratory flow rate
• Raw = Airway resistance
• VT = Tidal volume
• C = Compliance
• PEEP = Positive end-expiratory pressure

This equation explains:

• Peak pressure = Flow-resistive component + Elastic component + PEEP
• Plateau pressure = Elastic component + PEEP (no flow during inspiratory pause)

Compliance and Resistance

Static Compliance (Cst):

• Measure of lung and chest wall distensibility
• Cst = VT / (Pplat - PEEP)
• Normal: 50-80 mL/cmH₂O in mechanically ventilated patients
• Decreased in: ARDS, pneumonia, pulmonary edema, pulmonary fibrosis, chest wall restriction, obesity
• Increased in: Emphysema (loss of elastic recoil)

Dynamic Compliance (Cdyn):

• Includes resistive component (flow present)
• Cdyn = VT / (Ppeak - PEEP)
• Always less than static compliance (includes airway resistance)

Airway Resistance (Raw):

• Raw = (Ppeak - Pplat) / V̇
• Normal: < 5 cmH₂O/L/sec in intubated patients
• Increased in: Bronchospasm, secretions, ETT obstruction, kinked circuit

Interpreting Pressure Differences:

Work of Breathing

Work of breathing (WOB) is the energy required to move gas into and out of the lungs:

WOB = Pressure × Volume

Components:

• Elastic work: Overcoming lung and chest wall recoil (60-70%)
• Resistive work: Overcoming airway resistance (30-40%)

Normal WOB: 0.3-0.6 J/L (2-3% of total body oxygen consumption)

Increased WOB in critical illness:

• Decreased compliance (↑ elastic work)
• Increased resistance (↑ resistive work)
• Tachypnea (↑ resistive work due to higher flows)
• Can reach 25-50% of total oxygen consumption in respiratory failure

Clinical Implications:

• Mechanical ventilation reduces WOB, redirects oxygen to other organs
• Excessive ventilator support causes ventilator-induced diaphragm dysfunction (VIDD) (PMID: 18725455)
• Inadequate support causes respiratory muscle fatigue
• Goal: Appropriate level of support to unload respiratory muscles without complete rest

Relevant Pharmacology

Neuromuscular Blocking Agents (NMBAs):

Rocuronium:

• Class: Non-depolarizing aminosteroid NMBA
• Mechanism: Competitive acetylcholine receptor antagonist at neuromuscular junction
• ICU Indication: ARDS (facilitates lung-protective ventilation), severe dyssynchrony
• Dosing: 0.6-1.2 mg/kg bolus, 0.3-0.6 mg/kg/hr infusion (titrate to TOF 1-2/4)
• Evidence: ACURASYS trial - 48-hour cisatracurium infusion reduced 90-day mortality in severe ARDS (PMID: 20843245)
• Duration: Intermittent boluses preferred over continuous infusion (reduce ICU-acquired weakness risk)

Cisatracurium:

• Class: Non-depolarizing benzylisoquinolinium NMBA
• Advantage: Organ-independent Hofmann elimination
• ICU Indication: ARDS, renal/hepatic dysfunction
• Dosing: 0.1-0.2 mg/kg bolus, 1-3 mcg/kg/min infusion
• Evidence: ROSE trial - no mortality benefit from early NMBA in moderate-severe ARDS with light sedation strategy (PMID: 30779531)

Sedation for Mechanical Ventilation:

Propofol:

• First-line sedative for short-term ventilation
• Dosing: 1-4 mg/kg/hr (avoid >4 mg/kg/hr or >48 hours - PRIS risk)
• Advantage: Rapid onset/offset, facilitates daily sedation interruption

Dexmedetomidine:

• Alpha-2 agonist with sedative and analgesic properties
• Advantage: Minimal respiratory depression, reduced delirium (PMID: 26903335)
• Dosing: 0.2-1.5 mcg/kg/hr (no loading in hemodynamically unstable)

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Ventilator Classification

Negative vs Positive Pressure Ventilation

Negative Pressure Ventilation (NPV):

• Historical: Iron lung (tank ventilator) developed for polio epidemics
• Mechanism: Creates sub-atmospheric pressure around chest wall, causing air to flow into lungs
• Mimics physiological inspiration
• Modern applications: Cuirass ventilators for chronic respiratory failure, neuromuscular disease
• Advantages: No intubation, preserves upper airway function
• Disadvantages: Bulky, limits patient access, upper airway obstruction risk

Positive Pressure Ventilation (PPV):

• Current standard of care for acute respiratory failure
• Mechanism: Delivers supra-atmospheric pressure to airways, inflating lungs
• Types: Invasive (ETT/tracheostomy) and Non-invasive (mask)
• Advantages: Precise control, overcomes airway obstruction, universal applicability
• Disadvantages: Hemodynamic effects (↓ venous return), requires airway device (invasive)

Transport vs ICU Ventilators

Australian Retrieval Considerations:

• Royal Flying Doctor Service (RFDS): Hamilton T1, Oxylog 3000+
• NETS/PIPER Paediatric: Babylog VN500, Servo-i
• Adult retrieval (CareFlight, MedSTAR): LTV 1200, Oxylog 3000+, Hamilton C1
• Altitude considerations: Hypobaria affects delivered FiO₂ and PEEP

Key Transport Ventilator Features:

• Robust construction for transport environment
• Clear alarm systems audible in noisy environments
• Long battery life (minimum 2 hours at high settings)
• Ability to function on limited gas supplies
• MRI-conditional for radiology transfers

ICU Ventilator Platforms

Major ICU Ventilator Manufacturers (Australia/NZ):

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Triggering Mechanisms

Time-Triggered (Mandatory Breaths)

• Ventilator initiates breath at preset time interval determined by set respiratory rate
• No patient effort required
• Used in: CMV (Controlled Mandatory Ventilation), deeply sedated/paralyzed patients
• Formula: Time interval = 60 seconds / Respiratory Rate
• Example: RR 12 = breath every 5 seconds

Pressure-Triggered (Assisted Breaths)

• Patient generates negative pressure by inspiratory effort
• Ventilator detects pressure drop below trigger sensitivity threshold
• Trigger sensitivity: Typically -0.5 to -2 cmH₂O below baseline
• Advantages: Simple, widely available
• Disadvantages: Higher work of breathing to trigger, auto-PEEP can prevent triggering

Auto-PEEP and Trigger Failure:

• If auto-PEEP = 8 cmH₂O and trigger = -2 cmH₂O
• Patient must generate -10 cmH₂O effort to trigger breath
• Results in ineffective triggering (wasted efforts)
• Solution: Apply external PEEP (80-85% of auto-PEEP) or increase trigger sensitivity

Flow-Triggered (Assisted Breaths)

• Ventilator delivers continuous bias flow through circuit (typically 2-5 L/min)
• Patient inspiratory effort creates flow differential detected by sensors
• Flow trigger sensitivity: Typically 1-3 L/min reduction in expiratory flow
• Advantages: Lower work of breathing than pressure triggering (PMID: 8989133)
• Most modern ICU ventilators use flow triggering as default

Flow Triggering Mechanism:

1. Continuous base flow circulates through circuit
2. Patient inspiratory effort diverts flow into lungs
3. Flow sensor detects difference between inspiratory and expiratory flow
4. When difference exceeds trigger threshold, breath is initiated

Neural Triggering (NAVA)

Neurally Adjusted Ventilatory Assist (NAVA) uses diaphragm electrical activity (Edi) detected via specialized nasogastric catheter with electrode array.

Mechanism:

• Electrodes positioned at diaphragm level (T8-L2)
• Edi signal reflects respiratory drive
• Trigger: Edi rise above threshold (typically 0.5 μV)
• Cycling: Edi decrease to 70% of peak
• Assist level: Proportional to Edi signal

Advantages:

• Near-zero trigger delay (<50 ms)
• Immune to auto-PEEP effects
• Improved synchrony (PMID: 21602655)
• Preserved respiratory variability
• Feedback from lung stretch receptors (Hering-Breuer reflex)

Disadvantages:

• Requires specialized catheter placement
• Cost ($200-300 per catheter)
• Signal artifacts (cardiac, esophageal contractions)
• Not available on all ventilator platforms

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Cycling Mechanisms

Volume-Cycled

• Inspiration terminates when set tidal volume delivered
• Airway pressure varies depending on compliance and resistance
• Used in: Volume Control (VC-CMV, VC-SIMV)
• Advantage: Guaranteed tidal volume delivery (minute ventilation control)
• Disadvantage: No pressure limit - risk of high airway pressures and VILI

Time-Cycled

• Inspiration terminates after set inspiratory time (Ti)
• Used in: Pressure Control (PC-CMV, PC-SIMV), APRV
• Ti typically 0.8-1.2 seconds (adults)
• I:E ratio determined by Ti and respiratory rate
• Advantage: Predictable inspiratory time, facilitates I:E ratio manipulation
• Disadvantage: Tidal volume varies with compliance changes

Flow-Cycled

• Inspiration terminates when inspiratory flow decays to set percentage of peak flow
• Typical cycling threshold: 25% of peak inspiratory flow (adjustable 10-50%)
• Used in: Pressure Support Ventilation (PSV)
• Allows patient to determine inspiratory time
• Improved patient-ventilator synchrony

Clinical Adjustment:

• Higher cycling threshold (e.g., 50%): Earlier termination, shorter Ti
• Lower cycling threshold (e.g., 10%): Later termination, longer Ti
• Obstructive disease (COPD): Higher threshold prevents breath stacking
• Restrictive disease (ARDS): Lower threshold allows adequate tidal volume

Pressure-Cycled

• Inspiration terminates when airway pressure reaches set limit
• Safety mechanism in volume-controlled modes (high-pressure limit)
• Used in: Early generation ventilators, pressure-limited infant ventilators
• Rarely used as primary cycling mechanism in modern ICU ventilators

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Conventional Ventilation Modes

Volume Control (VC-CMV, VC-SIMV)

Volume Control - Continuous Mandatory Ventilation (VC-CMV):

• Also called: Assist-Control (A/C), Volume Assist-Control (VAC)
• Control Variable: Volume
• Trigger: Time or patient effort (whichever comes first)
• Cycling: Volume (preset tidal volume delivered)
• Limit: Flow rate (constant during inspiration)

Set Parameters:

• Tidal volume (VT): 6-8 mL/kg ideal body weight
• Respiratory rate (RR): 12-20/min
• Inspiratory flow rate: 40-80 L/min (or I:E ratio)
• FiO₂: Titrate to SpO₂ target
• PEEP: 5-20 cmH₂O depending on pathology

Waveform Characteristics:

• Flow-time: Square wave (constant flow) or decelerating (if selected)
• Pressure-time: Progressive rise to peak, then plateau if inspiratory pause set
• Volume-time: Linear increase during inspiration

Advantages:

• Guaranteed tidal volume and minute ventilation
• Simple to set and monitor
• Predictable blood gas results

Disadvantages:

• Peak pressures vary with compliance and resistance
• No automatic pressure limiting (risk of VILI)
• Fixed flow may not match patient demand (flow starvation)

Volume Control - SIMV (VC-SIMV):

• Delivers set number of mandatory volume-controlled breaths
• Patient can take additional spontaneous breaths between mandatory breaths
• Spontaneous breaths can be unsupported (CPAP) or supported (PSV)
• Historically used for weaning (now less favored - PMID: 11445675)

Pressure Control (PC-CMV, PC-SIMV)

Pressure Control - Continuous Mandatory Ventilation (PC-CMV):

• Control Variable: Pressure
• Trigger: Time or patient effort
• Cycling: Time (preset inspiratory time)
• Limit: Pressure (preset pressure level)

Set Parameters:

• Inspiratory pressure (Pinsp): 10-30 cmH₂O above PEEP
• Inspiratory time (Ti): 0.8-1.2 seconds
• Respiratory rate (RR): 12-20/min
• Rise time: 0-0.4 seconds
• FiO₂ and PEEP

Waveform Characteristics:

• Pressure-time: Square wave (rapid rise to set pressure, maintained)
• Flow-time: Decelerating pattern (high initial flow, exponential decay)
• Volume-time: Exponential rise, plateau as flow decreases

Advantages:

• Direct pressure control limits barotrauma risk
• Decelerating flow improves gas distribution (V/Q matching)
• More comfortable for breathing patients
• May improve oxygenation in ARDS (PMID: 7876025)

Disadvantages:

• Tidal volume varies with compliance and resistance
• No guaranteed minute ventilation
• Requires close monitoring as condition changes

Pressure Control - SIMV (PC-SIMV):

• Mandatory breaths are pressure-controlled, time-cycled
• Spontaneous breaths supported by PSV or unsupported (CPAP)

Pressure Support Ventilation (PSV)

Classification:

• Control Variable: Pressure
• Trigger: Patient (flow or pressure triggered) - NO time-triggered breaths
• Cycling: Flow (typically 25% of peak inspiratory flow)
• Limit: Pressure

Set Parameters:

• Pressure support level: 5-20 cmH₂O above PEEP
• PEEP: 5-10 cmH₂O
• FiO₂: Titrate to target SpO₂
• Flow cycling threshold: 25% default (adjustable 10-50%)
• Rise time: 0-0.4 seconds

Critical Feature: PSV is a purely spontaneous mode - patient MUST trigger every breath. Backup apnea ventilation is essential.

Mechanism:

1. Patient initiates inspiratory effort
2. Ventilator pressurizes circuit to set pressure level
3. Flow delivered to maintain set pressure
4. As lung fills, flow decelerates
5. When flow reaches cycling threshold, inspiration terminates
6. Patient controls respiratory rate, inspiratory time, and tidal volume

Advantages:

• Improved patient-ventilator synchrony
• Patient controls breathing pattern
• Reduced work of breathing while maintaining respiratory muscle activity
• Preferred mode for weaning (PMID: 7898076)

Disadvantages:

• Requires intact respiratory drive
• No guaranteed minute ventilation
• Tidal volume varies with patient effort and lung mechanics
• Backup ventilation essential for apnea

Clinical Applications:

• Weaning: Gradual reduction in PS level (typically reduce by 2-4 cmH₂O steps)
• Spontaneous Breathing Trial (SBT): PSV 5-8 cmH₂O + PEEP 5 cmH₂O (or T-piece equivalent)
• NIV: Primary mode for non-invasive ventilation

Continuous Positive Airway Pressure (CPAP)

Classification:

• Pressure mode: Constant positive pressure throughout respiratory cycle
• Trigger: Patient-initiated only
• Support: No inspiratory pressure augmentation (unlike PSV)

Set Parameters:

• CPAP level: 5-15 cmH₂O
• FiO₂: Titrate to target SpO₂

Mechanism:

• Maintains positive airway pressure throughout inspiration and expiration
• Patient breathes spontaneously against constant pressure
• No pressure support during inspiration (differentiates from BiPAP/PSV)

Physiological Effects:

• Increases functional residual capacity (FRC)
• Recruits atelectatic lung units
• Improves oxygenation (reduces shunt fraction)
• Reduces work of breathing by offsetting auto-PEEP
• Supports cardiac function in heart failure (reduces afterload)

Indications:

• Mild hypoxemic respiratory failure
• Cardiogenic pulmonary edema (PMID: 16840402)
• Obstructive sleep apnea
• Spontaneous breathing trial component
• Post-extubation support

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Advanced Ventilation Modes

Pressure Regulated Volume Control (PRVC)

Classification: Dual-control mode (volume-targeted, pressure-controlled)

Mechanism:

1. First breath: Delivered at low pressure to measure compliance
2. Ventilator calculates required pressure to achieve target VT
3. Subsequent breaths: Pressure adjusted breath-by-breath (max 3 cmH₂O change)
4. Maintains target VT with minimum necessary pressure

Set Parameters:

• Target tidal volume: 6-8 mL/kg IBW
• Inspiratory time: 0.8-1.2 seconds
• Respiratory rate: 12-20/min
• PEEP and FiO₂
• Pressure limits: Usually set 5-10 cmH₂O above calculated requirement

Advantages:

• Guaranteed tidal volume with pressure-limited delivery
• Automatic adaptation to changing compliance
• Decelerating flow pattern (improved gas distribution)
• Limits barotrauma while ensuring minute ventilation

Disadvantages:

• May mask deterioration (pressure slowly creeps up)
• Patient effort affects tidal volume delivery
• Potential for undershoot or overshoot of target VT
• Complex algorithm may cause unexpected behavior

Clinical Applications:

• Conditions with fluctuating compliance (resolving bronchospasm, fluid shifts)
• Weaning phase with variable mechanics
• When both pressure control benefits and volume guarantee desired

Airway Pressure Release Ventilation (APRV/BiLevel)

Classification: Pressure-controlled, time-cycled, inverse-ratio mode

Concept: Prolonged high CPAP (Phigh) with brief releases (Tlow) for ventilation

Set Parameters:

• Phigh (High pressure): 20-35 cmH₂O (mean airway pressure target)
• Thigh (Time at Phigh): 4-6 seconds
• Plow (Low pressure): 0-5 cmH₂O
• Tlow (Release time): 0.3-0.8 seconds (terminates at 50-75% of peak expiratory flow)
• FiO₂

Key Principle: Tlow is set to terminate before complete exhalation, maintaining auto-PEEP and preventing derecruitment.

Mechanism:

1. Patient breathes spontaneously at Phigh level (majority of time)
2. Brief releases to Plow allow CO₂ elimination
3. Rapid return to Phigh maintains recruitment
4. Spontaneous breathing throughout cycle

Ventilation Equation:

• Minute ventilation = (Vrelease × Release frequency) + Spontaneous VT

Advantages:

• High mean airway pressure improves oxygenation
• Promotes spontaneous breathing (reduces sedation needs)
• Improved V/Q matching
• May reduce VILI (lung-protective effect debated)

Disadvantages:

• Complex to optimize
• Limited evidence for mortality benefit (PMID: 29493625)
• May worsen hemodynamic compromise
• Requires experienced users
• Potential for hyperinflation if Tlow too short

Contraindications:

• Obstructive lung disease (severe COPD, asthma)
• High minute ventilation requirements
• Hemodynamic instability
• Lack of institutional experience

Adaptive Support Ventilation (ASV)

Classification: Closed-loop, dual-control mode (Hamilton Medical proprietary)

Mechanism:

• Based on Otis equation (minimizing work of breathing)
• Targets optimal respiratory rate and VT combination
• Adjusts PS and mandatory RR to achieve target minute ventilation

Set Parameters:

• Patient height (for IBW calculation)
• Percent minute ventilation (%MV): 25-350%
• PEEP and FiO₂
• High pressure alarm

Algorithm:

1. Calculates target minute ventilation from IBW
2. Applies %MV setting (100% = normal predicted MV)
3. Determines optimal VT:RR ratio (minimizes work)
4. Adjusts support: Full support (mandatory breaths) → Partial support (PSV) → Minimal support (CPAP)

Advantages:

• Automatic adaptation to patient condition
• Facilitates weaning (reduces support as patient improves)
• Reduces clinician workload
• May reduce ventilation duration (PMID: 21336310)

Disadvantages:

• "Black box"
• clinician may not understand adjustments
• May not suit all clinical scenarios
• Limited to Hamilton ventilators
• Target minute ventilation may not be appropriate for all patients

IntelliVent-ASV (Advanced version):

• Adds automatic FiO₂ and PEEP adjustment based on SpO₂
• Adds automatic SBT initiation
• End-tidal CO₂ monitoring integration

Neurally Adjusted Ventilatory Assist (NAVA)

Classification: Neural-controlled pressure support mode (Maquet/Getinge proprietary)

Concept: Ventilator support directly proportional to patient's neural respiratory drive

Equipment:

• NAVA-enabled ventilator (Servo-i, Servo-u)
• Edi catheter: Specialized nasogastric tube with electrode array

Set Parameters:

• NAVA level: 0.5-4.0 cmH₂O/μV (proportionality constant)
• PEEP and FiO₂
• Trigger threshold (Edi trigger): 0.5 μV
• Backup ventilation settings (if Edi signal lost)

Mechanism:

1. Edi catheter detects diaphragm electrical activity
2. Edi signal amplitude reflects respiratory drive
3. Pressure support = NAVA level × Edi
4. Support increases and decreases with patient effort

Advantages:

• Near-perfect synchrony (neural triggering, neural cycling)
• Immune to auto-PEEP effects on triggering
• Preserved respiratory variability
• Protects against over-assistance (Hering-Breuer reflex feedback)
• May reduce duration of ventilation in neonates (PMID: 28076227)

Disadvantages:

• Requires specialized catheter (cost, placement skill)
• Signal artifacts (cardiac, esophageal)
• Not available on all platforms
• Limited evidence for improved outcomes in adults
• Requires intact phrenic nerve and diaphragm function

Edi Catheter Positioning:

• Insertion: Via nose or mouth (like NGT)
• Positioning: Electrodes at diaphragm level (T8-L2)
• Verification: ECG pattern and Edi waveforms on screen
• Depth: Approximately NEX distance or formula-based

Clinical Applications:

• Difficult weaning with dyssynchrony
• Diaphragm function monitoring
• Neonatal ventilation
• Research into respiratory drive

Proportional Assist Ventilation Plus (PAV+)

Classification: Proportional support mode (Medtronic/Covidien proprietary)

Concept: Ventilator amplifies patient effort proportionally, based on real-time mechanics measurement

Mechanism:

1. Ventilator continuously measures compliance and resistance (using mini-pauses)
2. Patient effort generates flow and pressure signals
3. Ventilator applies proportional pressure to unload elastic and resistive work
4. Support = % of total work × measured mechanics

Set Parameters:

• % Support: 5-95% (proportion of work assumed by ventilator)
• PEEP and FiO₂
• Tube compensation: On/off

Equation: Paw = % Support × [(V × E) + (V̇ × R)]

Where E = elastance (1/compliance), R = resistance

Advantages:

• Synchrony proportional to effort (natural breathing feel)
• Automatic adaptation to changing mechanics
• Preserved respiratory variability
• No need to set VT or PS level
• Favorable respiratory muscle activity patterns

Disadvantages:

• Requires intact respiratory drive
• May under-support if patient has inadequate drive
• Complex algorithm
• Limited to Medtronic ventilators
• "Runaway" risk if compliance suddenly improves (excessive VT)

Comparison: NAVA vs PAV+:

---

Mode Selection

ARDS Ventilation Strategy

Lung-Protective Ventilation Protocol (ARDSNet):

Mode Selection for ARDS:

• Mild ARDS: Volume control or pressure control acceptable
• Moderate ARDS: Pressure control preferred (pressure limiting), consider APRV in experienced centers
• Severe ARDS: Pressure control, early prone positioning, consider NMBA (first 48 hours), ECMO referral if refractory

Adjuncts:

• Prone positioning: 12-16 hours/day in moderate-severe ARDS (PROSEVA trial - PMID: 23688302)
• NMBA: Early use in severe ARDS (ACURASYS - PMID: 20843245, but ROSE negative - PMID: 30779531)
• ECMO: Consider for refractory hypoxemia (P/F <80, pH <7.20 despite optimization)

Obstructive Lung Disease (COPD/Asthma)

Ventilatory Goals:

• Minimize auto-PEEP
• Adequate expiratory time
• Permissive hypercapnia
• Avoid breath stacking

Settings:

• Low respiratory rate (8-12/min)
• Lower VT (6 mL/kg or less)
• High inspiratory flow (60-80 L/min in VC) for shorter Ti
• I:E ratio ≥1:3 to 1:4
• PEEP: Cautious external PEEP (80-85% of auto-PEEP)

Mode Selection:

• Volume control with constant flow (predictable Ti)
• Avoid APRV (risk of hyperinflation)
• PSV during weaning (higher cycle threshold, 40-50%)

Neuromuscular Disease

Ventilatory Goals:

• Maintain adequate ventilation (often primary hypercapnia)
• Prevent atelectasis
• Facilitate communication and secretion clearance

Settings:

• Higher VT acceptable (8-10 mL/kg) if no lung injury
• Moderate PEEP (5-8 cmH₂O)
• NIV as first-line if feasible

Mode Selection:

• Volume control (guaranteed VT despite variable effort)
• BiPAP/PSV for non-invasive support
• Consider NAVA if diaphragm partially functional

Weaning from Mechanical Ventilation

Evidence-Based Approach (Esteban et al. PMID: 7898076):

• PSV weaning or SBT approach superior to SIMV weaning
• SIMV prolongs weaning compared to PSV (PMID: 11445675)

Weaning Protocol:

1. Daily screening for weaning readiness
2. Spontaneous breathing trial (SBT) if criteria met
3. SBT options: T-piece, PSV 5-8 + PEEP 5, automatic tube compensation
4. 30-120 minute trial duration
5. Extubate if SBT passed

SBT Failure Criteria:

• RR >35/min
• SpO₂ <90%
• HR >140 or change >20%
• SBP >180 or <90 mmHg
• Anxiety, diaphoresis, altered consciousness

---

Waveform Interpretation

Pressure-Time Waveform

Normal VC Waveform:

• Progressive rise during inspiration (resistive + elastic components)
• Peak at end-inspiration
• Plateau if inspiratory hold applied
• Decay to baseline/PEEP during expiration

Normal PCV Waveform:

• Rapid rise to set pressure
• Square wave during inspiration (maintained pressure)
• Decay to baseline/PEEP during expiration

Abnormal Patterns:

Flow-Time Waveform

Normal VC (Square Flow):

• Constant inspiratory flow (horizontal line)
• Abrupt transition to expiratory flow
• Exponential decay of expiratory flow to zero

Normal PCV:

• Decelerating inspiratory flow (peak then decay)
• Exponential decay of expiratory flow to zero

Auto-PEEP Detection:

• Expiratory flow does NOT return to zero before next breath
• Indicates incomplete exhalation
• Measured by end-expiratory hold maneuver

Dyssynchrony Patterns:

Volume-Time Waveform

Normal Pattern:

• Progressive increase during inspiration
• Plateau at peak volume
• Progressive decrease during expiration to baseline

Abnormal Patterns:

• Incomplete return to baseline: Air trapping, auto-PEEP
• Reduced peak volume: Leak, low compliance, auto-cycling

Pressure-Volume Loops

Clinical Applications:

• PEEP optimization (lower inflection point)
• Overdistension detection (upper inflection point, "beaking")
• Compliance assessment (slope of loop)

Interpretation:

• Wide loop: High work of breathing, high resistance
• Narrow loop: Low resistance, good compliance
• Beaking (flattening at top): Overdistension
• Lower inflection point: Opening pressure for collapsed alveoli

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Alarm Management

High-Priority Alarms

Medium-Priority Alarms

Troubleshooting Algorithm

DOPE Mnemonic for Acute Deterioration:

1. Displacement

   • Right main bronchus intubation
   • Accidental extubation
   • Action: Check ETT depth, auscultate bilaterally, consider laryngoscopy

2. Obstruction

   • Secretions/mucus plug
   • Kinked ETT or circuit
   • Patient biting ETT
   • Action: Pass suction catheter, check circuit, insert bite block

3. Pneumothorax

   • Tension pneumothorax (high pressures, hypotension)
   • Action: Examine for tracheal deviation, absent breath sounds; needle decompression if tension

4. Equipment Failure

   • Circuit disconnect
   • Ventilator malfunction
   • Gas supply failure
   • Action: Disconnect and bag-valve-mask ventilate, troubleshoot or replace equipment

---

Australian/NZ Context

ANZICS Guidelines

ANZICS-CORE Recommendations:

• Lung-protective ventilation for ARDS (VT 6 mL/kg IBW, Pplat ≤30 cmH₂O)
• Daily sedation interruption and spontaneous breathing trials
• Protocolized weaning strategies
• Prone positioning for moderate-severe ARDS

IC-2 CICM Equipment Guidelines:

• Regular equipment checks and maintenance
• Staff training on all ventilator platforms
• Backup ventilation capacity (manual and mechanical)
• Documented alarm management protocols

Retrieval Medicine Considerations

Aeromedical Transport:

• Altitude effects: Barometric pressure decreases → FiO₂ effectively decreases, PEEP may need adjustment
• Pressurized cabin: Commercial aircraft ~8,000 ft equivalent; RFDS unpressurized or cabin altitude ~6,000-8,000 ft
• Gas consumption: Limited cylinder capacity; calculate requirements for journey + contingency
• Battery life: Ensure adequate power for transport duration + delays

Ventilator Selection for Retrieval:

• Robust, lightweight design
• Long battery life (minimum 2-4 hours)
• Cylinder-independent operation or efficient gas use
• MRI-conditional if CT/MRI transfers required
• Familiar to retrieval team

Indigenous Health Considerations

Disparities in Respiratory Failure:

• Aboriginal and Torres Strait Islander Australians have 1.5-2× higher rates of respiratory failure requiring mechanical ventilation
• Associated with higher prevalence of COPD, bronchiectasis, pneumonia
• Delayed presentation and later-stage disease
• Geographic barriers to ICU access in remote communities

Culturally Safe Care:

• Involve Aboriginal Health Workers/Aboriginal Liaison Officers (AHW/ALO)
• Include extended family in discussions about ventilation and goals of care
• Be aware of cultural protocols (sorry business, men's/women's business)
• Provide interpreter services as needed
• Recognize that traditional medicine may be important to patient and family

Māori Health Considerations:

• 1.3-1.5× higher rates of ICU admission for respiratory failure
• Include whānau in decision-making
• Respect tikanga (cultural protocols)
• Māori Health Workers involvement
• Consideration of karakia (prayer) if appropriate

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Prognosis & Outcome Measures

Mortality

Short-Term Outcomes:

• Overall ICU mortality for mechanically ventilated patients: 15-25%
• Hospital mortality: 25-35%
• ARDS mortality: 35-45% (reduced from 60%+ with lung-protective ventilation)

Long-Term Outcomes:

• 90-day mortality: 30-40% for ARDS survivors
• 1-year mortality: 40-50% for ARDS survivors
• 5-year mortality: 50-60% for ARDS survivors

Morbidity

Post-Intensive Care Syndrome (PICS):

• Physical impairment: 50-70% at hospital discharge
• Cognitive impairment: 30-50% at 1 year (PMID: 23688302)
• Psychological sequelae: 20-30% PTSD, depression, anxiety

ICU-Acquired Weakness (ICUAW):

• Prevalence: 25-50% of patients ventilated >7 days
• Risk factors: NMBA use, steroids, sepsis, prolonged ventilation
• Associated with prolonged ventilation and mortality (PMID: 24557903)

Ventilator-Induced Diaphragm Dysfunction (VIDD):

• Occurs within 18-72 hours of controlled ventilation
• Diaphragm atrophy: 50% reduction in muscle cross-sectional area within 5-6 days (PMID: 18725455)
• Contributes to weaning failure and prolonged ventilation

Prognostic Factors

Good Prognostic Factors:

• Rapid improvement in oxygenation within 24 hours
• Low driving pressure (≤14 cmH₂O)
• Resolution of underlying cause
• Preserved respiratory drive
• Younger age, fewer comorbidities

Poor Prognostic Factors:

• High driving pressure (>15 cmH₂O) - strongest predictor (PMID: 25693014)
• Persistent hypoxemia despite optimization
• Multi-organ failure (high SOFA score)
• Prolonged mechanical ventilation (>7 days)
• Development of ARDS
• ICU-acquired complications (VAP, weakness)

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SAQ Practice

SAQ 1: Mode Selection and Ventilator Settings

Time Allocation: 10 minutes Total Marks: 20

Stem: A 62-year-old male (height 175 cm, weight 90 kg) is admitted to ICU with severe community-acquired pneumonia and ARDS. He was intubated in the emergency department for hypoxemic respiratory failure.

Current ventilator settings:

• Mode: Volume Control Assist-Control
• Tidal Volume: 650 mL
• Respiratory Rate: 16/min
• PEEP: 8 cmH₂O
• FiO₂: 0.8

Observations:

• HR: 110 bpm
• BP: 95/60 mmHg
• SpO₂: 88%

Ventilator readings:

• Peak pressure: 42 cmH₂O
• Plateau pressure: 38 cmH₂O

ABG (FiO₂ 0.8):

• pH: 7.28
• PaCO₂: 52 mmHg
• PaO₂: 58 mmHg
• HCO₃: 24 mmol/L
• Lactate: 2.4 mmol/L

---

Question 1.1 (8 marks)

Identify the problems with the current ventilator settings and calculate the appropriate parameters for this patient.

Question 1.2 (6 marks)

Outline your approach to optimizing this patient's ventilation in the first hour.

Question 1.3 (6 marks)

What additional interventions would you consider for this patient with moderate-severe ARDS if oxygenation does not improve?

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Model Answer

Question 1.1 (8 marks)

Problem Identification (4 marks):

1. Excessive tidal volume (1 mark)

   • Current: 650 mL
   • IBW (male, 175 cm): 50 + 0.91 × (175 - 152.4) = 70.6 kg
   • Current VT: 650/70.6 = 9.2 mL/kg IBW (target: 6 mL/kg = 424 mL)

2. Plateau pressure too high (1 mark)

   • Current: 38 cmH₂O (target: ≤30 cmH₂O)
   • Indicates VILI risk

3. High driving pressure (1 mark)

   • ΔP = Pplat - PEEP = 38 - 8 = 30 cmH₂O (target: ≤15 cmH₂O)
   • Major mortality predictor

4. Inadequate PEEP for severity (1 mark)

   • P/F ratio: 58/0.8 = 72.5 (severe ARDS, P/F <100)
   • PEEP 8 cmH₂O likely inadequate

Correct Parameter Calculations (4 marks):

---

Question 1.2 (6 marks)

Immediate Ventilator Optimization (6 marks):

1. Reduce tidal volume (1 mark)

   • Decrease to 420-450 mL (6-8 mL/kg IBW)
   • Expect Pplat reduction

2. Increase PEEP (1 mark)

   • Increase to 12-16 cmH₂O (ARDSNet higher PEEP table)
   • Monitor for hemodynamic effects

3. Adjust respiratory rate (1 mark)

   • Increase RR to 20-28/min to maintain minute ventilation
   • Accept permissive hypercapnia (pH ≥7.20)

4. Consider mode change (1 mark)

   • Switch to pressure control if plateau pressure remains high
   • Set inspiratory pressure to achieve VT 6-8 mL/kg

5. Optimize sedation (1 mark)

   • Ensure adequate sedation for lung-protective ventilation
   • Target RASS -2 to -3 initially

6. Monitor response (1 mark)

   • Repeat ABG in 30-60 minutes
   • Assess plateau pressure after changes

---

Question 1.3 (6 marks)

Additional ARDS Interventions (1 mark each):

1. Prone positioning

   • 12-16 hours per day for moderate-severe ARDS (P/F <150)
   • PROSEVA trial: Mortality reduction from 32.8% to 16% (NNT 6)

2. Neuromuscular blockade

   • Consider cisatracurium or rocuronium for 24-48 hours
   • Facilitates lung-protective ventilation
   • ACURASYS showed benefit (ROSE trial negative with light sedation strategy)

3. Recruitment maneuver with decremental PEEP trial

   • Caution: ART trial showed potential harm with aggressive recruitment
   • Conservative approach if used

4. Inhaled pulmonary vasodilators

   • Inhaled nitric oxide (iNO) or inhaled prostacyclin (iloprost)
   • Improves oxygenation; no mortality benefit

5. ECMO referral consideration

   • If P/F <80 despite optimization
   • Early referral to ECMO center (CESAR, EOLIA criteria)

6. Corticosteroids

   • Dexamethasone 20 mg × 5 days, then 10 mg × 5 days
   • DEXA-ARDS trial: Reduced ventilator-free days

---

Common Mistakes:

• Using actual body weight instead of ideal body weight
• Not calculating driving pressure
• Inadequate PEEP escalation in severe ARDS
• Forgetting to increase RR when reducing VT
• Not considering prone positioning early

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SAQ 2: Waveform Interpretation and Troubleshooting

Time Allocation: 10 minutes Total Marks: 20

Stem: You are called to review a 55-year-old female with COPD who was intubated 6 hours ago for an acute exacerbation. The nurse reports worsening respiratory distress and increasing ventilator alarms.

Current ventilator settings (Volume Control A/C):

• Tidal Volume: 450 mL
• Respiratory Rate: 18/min
• PEEP: 5 cmH₂O
• FiO₂: 0.5
• Inspiratory flow: 40 L/min

Ventilator readings:

• Peak pressure: 48 cmH₂O (alarm threshold 50)
• Plateau pressure: Unable to obtain (patient triggering)
• Set RR: 18, Total RR: 30
• Minute ventilation: 13.5 L/min

Observations:

• HR: 120 bpm
• BP: 85/50 mmHg
• SpO₂: 92%

The flow-time waveform shows that expiratory flow does not return to zero before the next breath is triggered.

---

Question 2.1 (7 marks)

Interpret the clinical picture and waveform findings. What is the likely diagnosis?

Question 2.2 (7 marks)

Outline your immediate management of this patient.

Question 2.3 (6 marks)

Describe the physics underlying this condition and explain why COPD patients are particularly at risk.

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Model Answer

Question 2.1 (7 marks)

Clinical Interpretation (4 marks):

1. Auto-PEEP (intrinsic PEEP) with dynamic hyperinflation (2 marks)

   • Expiratory flow not returning to zero = incomplete exhalation
   • Air trapping with each breath

2. Hemodynamic compromise from auto-PEEP (1 mark)

   • Hypotension (85/50 mmHg)
   • Reduced venous return from increased intrathoracic pressure

3. High peak pressure (1 mark)

   • Peak 48 cmH₂O approaching alarm limit
   • Combination of auto-PEEP and airway resistance

Waveform Analysis (3 marks):

---

Question 2.2 (7 marks)

Immediate Management (7 marks, 1 mark each):

1. Disconnect ventilator briefly

   • Allow complete exhalation (10-15 seconds)
   • Monitor SpO₂ and hemodynamics
   • Immediate improvement in BP confirms diagnosis

2. Sedate and consider paralysis

   • Propofol bolus and infusion increase
   • Consider short-term NMBA (rocuronium 0.6 mg/kg) for severe cases
   • Eliminates patient triggering, allows controlled ventilation

3. Reduce respiratory rate

   • Decrease to 8-12/min
   • Prolongs expiratory time

4. Reduce tidal volume if possible

   • Consider 400 mL
   • Reduces volume requiring exhalation

5. Increase inspiratory flow rate

   • Increase to 60-80 L/min
   • Shortens inspiratory time, lengthens expiratory time

6. Measure auto-PEEP

   • Perform end-expiratory hold maneuver (once patient sedated/paralyzed)
   • Document level (may be 10-20+ cmH₂O)

7. Consider applied external PEEP

   • Set PEEP to 80-85% of measured auto-PEEP
   • Reduces work of triggering
   • Do NOT add to already high auto-PEEP

Additional considerations:

• Bronchodilators (salbutamol, ipratropium)
• IV corticosteroids if not already given
• Exclude pneumothorax (CXR)
• Fluid resuscitation if hypotensive persists

---

Question 2.3 (6 marks)

Physics of Auto-PEEP (3 marks):

1. Expiratory time constant (1 mark)

   • τ (tau) = Resistance × Compliance
   • Complete exhalation requires 3-5 time constants
   • In COPD: High resistance + high compliance = long time constant

2. Inadequate expiratory time (1 mark)

   • If respiratory rate too high or I:E ratio too short
   • Expiratory flow cannot reach zero
   • Air is trapped at end-exhalation

3. Progressive hyperinflation (1 mark)

   • Each breath adds to trapped volume
   • Creates positive alveolar pressure at end-expiration (auto-PEEP)
   • Increases mean intrathoracic pressure → reduces venous return

Why COPD Patients Are at Risk (3 marks):

1. Increased airway resistance (1 mark)

   • Bronchospasm, mucus hypersecretion, airway inflammation
   • Slows expiratory flow

2. Loss of elastic recoil (1 mark)

   • Emphysematous destruction of alveolar walls
   • Reduced driving pressure for exhalation
   • Increased compliance prolongs time constant

3. Dynamic airway collapse (1 mark)

   • Loss of airway support (emphysema)
   • Airways collapse during forced exhalation
   • Flow limitation regardless of effort

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Viva Scenarios

Viva Scenario 1: ARDS Ventilation Strategy

Stem: "A 45-year-old female with COVID-19 pneumonia and ARDS is admitted to your ICU. Initial P/F ratio is 95 on FiO₂ 1.0 and PEEP 14 cmH₂O. Discuss your approach to mechanical ventilation."

Duration: 12 minutes (2 min reading + 10 min discussion)

---

Opening Question:

"What are your immediate priorities for this patient's ventilation?"

Expected Answer (2-3 minutes):

Classification and Goals:

• P/F 95 = severe ARDS (Berlin definition: P/F <100 with PEEP ≥5)
• Goals: Lung-protective ventilation, adequate oxygenation, avoid VILI

Initial Settings:

• Volume control or pressure control (either appropriate)
• Tidal volume 6 mL/kg IBW (calculate from height)
• Plateau pressure ≤30 cmH₂O
• Driving pressure ≤14-15 cmH₂O
• PEEP as per ARDSNet table (likely 14-18 cmH₂O for this FiO₂)
• Respiratory rate 20-30/min (titrate to pH ≥7.20)

Monitoring:

• Plateau pressure with each setting change
• Driving pressure calculation
• ABG in 30-60 minutes

---

Follow-up Question 1 (2-3 minutes):

"What is driving pressure and why is it important?"

Expected Answer:

Definition:

• Driving pressure (ΔP) = Plateau pressure - PEEP
• Represents the pressure required to distend the respiratory system by tidal volume
• ΔP = VT / Compliance

Clinical Importance:

• Strongest predictor of ARDS mortality (Amato 2015, PMID: 25693014)
• Meta-analysis of 3,562 patients: Mortality increases 41% per 7 cmH₂O increase
• More predictive than VT or plateau pressure alone
• Reflects lung strain (tidal volume relative to aerated lung size)

Target:

• ≤14-15 cmH₂O
• If driving pressure high despite low VT, indicates "baby lung"
• very little aerated lung available

---

Follow-up Question 2 (2-3 minutes):

"After 24 hours, P/F ratio has worsened to 70. What are your next steps?"

Expected Answer:

Immediate Assessment:

• Ensure lung-protective settings optimized
• Check for complications (pneumothorax, VAP, progression of ARDS)
• Verify plateau and driving pressure acceptable

Escalation of Care:

1. Prone positioning (highest priority)

   • 12-16 hours/day
   • PROSEVA trial: Mortality 16% vs 32.8% (NNT 6)
   • Absolute indication for P/F <150 with PEEP ≥5

2. Neuromuscular blockade

   • Consider cisatracurium 15 mg/hr infusion for 24-48 hours
   • ACURASYS showed benefit; ROSE negative with light sedation
   • Facilitates proning and ventilator synchrony

3. Higher PEEP strategy

   • Consider PEEP 18-22 cmH₂O
   • Monitor driving pressure and hemodynamics
   • Decremental PEEP trial after recruitment

4. ECMO referral

   • P/F <70 for >6 hours despite optimization
   • Early contact with ECMO center
   • EOLIA criteria: P/F <80 for >6h or <50 for >3h

---

Follow-up Question 3 (2-3 minutes):

"The family asks about the prognosis. How would you approach this discussion?"

Expected Answer:

Communication Approach:

• Private, quiet setting with adequate time
• Include key family members identified by patient/family
• Use clear, non-medical language

Prognostic Information:

• Severe ARDS mortality approximately 40-50%
• Each organ failure adds to mortality risk
• Response to treatment in first 48-72 hours important prognostic indicator

Cultural Considerations:

• If patient is Aboriginal or Torres Strait Islander: Involve AHW/ALO
• Respect cultural protocols (family decision-making, sorry business)
• Offer interpreter services if needed
• Acknowledge importance of family support and cultural practices

Documentation:

• Document family discussion in notes
• Record expressed wishes
• Plan for follow-up meeting as condition evolves

---

Viva Scenario 2: Mode Troubleshooting

Stem: "You are asked to review a 68-year-old male post-cardiac surgery who is difficult to wean from mechanical ventilation. The nursing staff report frequent ventilator alarms and patient agitation. Current mode is SIMV with PSV."

Duration: 12 minutes (2 min reading + 10 min discussion)

---

Opening Question:

"How would you assess this patient?"

Expected Answer (2-3 minutes):

Systematic Assessment:

History Review:

• Reason for intubation, duration of ventilation
• Previous weaning attempts and outcomes
• Cardiac surgery details (valve, CABG, complications)
• Sedation and delirium history

Physical Examination:

• Airway: ETT position, cuff pressure, secretions
• Breathing: Respiratory pattern, accessory muscles, auscultation
• Circulation: Hemodynamic stability, cardiac function
• Disability: Level of sedation (RASS), delirium (CAM-ICU)

Ventilator Assessment:

• Current settings (SIMV rate, PSV level, PEEP, FiO₂)
• Waveform analysis: Look for dyssynchrony patterns
• Alarm log: Identify specific alarm types and frequency
• Recorded parameters: VT variability, respiratory rate pattern

---

Follow-up Question 1 (2-3 minutes):

"The waveforms show frequent ineffective triggering and the patient appears to be making inspiratory efforts that don't result in ventilator breaths. What is the cause and how would you manage it?"

Expected Answer:

Diagnosis: Ineffective Triggering:

• Patient inspiratory effort insufficient to trigger ventilator
• Common causes: Auto-PEEP, weak respiratory muscles, inappropriate trigger sensitivity

Assessment:

• Check trigger sensitivity setting (should be flow trigger 1-2 L/min or pressure trigger -1 to -2 cmH₂O)
• Perform end-expiratory hold to measure auto-PEEP
• Assess diaphragm function (ultrasound if available)

Management:

1. Optimize trigger sensitivity

   • Increase sensitivity (lower threshold)
   • Switch from pressure to flow trigger

2. Address auto-PEEP if present

   • Apply external PEEP (80-85% of auto-PEEP)
   • Reduce respiratory rate if high

3. Consider mode change

   • Switch from SIMV to PSV (improved synchrony)
   • Consider NAVA if available (neural triggering)
   • Consider PAV+ (proportional assist)

4. Optimize sedation

   • Target RASS 0 to -1 for weaning
   • Address pain (adequate analgesia)
   • Treat delirium if present

---

Follow-up Question 2 (2-3 minutes):

"What are the disadvantages of SIMV for weaning and what is the evidence?"

Expected Answer:

SIMV Limitations:

• Unsupported breaths between mandatory breaths increase work of breathing
• Variable support level confuses respiratory control
• Associated with prolonged weaning duration

Evidence:

• Esteban et al. 1995 (PMID: 7898076): RCT comparing weaning methods

  • SIMV associated with longest weaning duration
  • T-piece and PSV weaning both superior to SIMV
  • Daily T-piece trial fastest to extubation

• Brochard et al. 1994 (PMID: 8291580): RCT comparing weaning methods

  • PSV weaning faster than SIMV (5.7 vs 9.9 days)
  • Success rate similar

Current Recommendations:

• SIMV is not recommended as primary weaning mode
• Daily spontaneous breathing trials (PSV 5-8 or T-piece) preferred
• Protocolized weaning reduces ventilation duration

---

Follow-up Question 3 (2-3 minutes):

"How would you set up a spontaneous breathing trial for this patient?"

Expected Answer:

Readiness Criteria:

• Improvement in underlying condition
• Adequate oxygenation (SpO₂ ≥90% on FiO₂ ≤0.4, PEEP ≤8)
• Hemodynamically stable (minimal vasopressors)
• Awake, following commands
• Adequate cough reflex

SBT Setup Options:

1. T-piece trial

   • Patient breathes through T-piece connected to oxygen source
   • No ventilator support
   • 30-120 minutes duration

2. PSV trial (preferred in many units)

   • PSV 5-8 cmH₂O (to overcome ETT resistance)
   • PEEP 5 cmH₂O
   • 30-120 minutes duration

3. Automatic Tube Compensation (ATC)

   • Ventilator compensates exactly for ETT resistance
   • PEEP 5 cmH₂O

Failure Criteria (terminate trial if):

• RR >35/min or <8/min
• SpO₂ <90%
• HR >140 or change >20%
• SBP >180 or <90 mmHg
• Signs of distress (diaphoresis, anxiety, altered consciousness)

Post-SBT Assessment:

• RSBI (Rapid Shallow Breathing Index) = RR/VT in L
• RSBI <105 predicts successful extubation
• Assess cough strength, secretion management