Capnography - Physics, Waveform Analysis, and Clinical Applications

Quick Answer

Capnography is the continuous measurement and graphical display of carbon dioxide (CO₂) concentration in respiratory gases. It utilises infrared absorption spectroscopy at the characteristic CO₂ wavelength of 4.26 μm, following the Beer-Lambert law to determine concentration. Two main configurations exist: mainstream (sensor in breathing circuit, faster response ~20 ms, adds dead space) and sidestream (gas aspirated to remote sensor, ~150-200 mL/min sampling, transit delay 2-3 seconds). The capnogram displays four phases: Phase I (inspiratory baseline, zero CO₂), Phase II (expiratory upstroke), Phase III (alveolar plateau), and Phase 0 (inspiratory downstroke). End-tidal CO₂ (ETCO₂) normally underestimates PaCO₂ by 2-5 mmHg due to physiological dead space. Clinical applications include confirmation of endotracheal intubation (gold standard), detection of oesophageal intubation, monitoring ventilation adequacy, early warning of malignant hyperthermia (rising ETCO₂), assessment of CPR quality during cardiac arrest (ETCO₂ >10 mmHg indicates adequate compressions), detection of circuit disconnection, and identification of rebreathing (elevated baseline). Abnormal waveforms include the "shark fin" pattern (bronchospasm/COPD), cardiac oscillations, curare cleft (spontaneous breathing efforts), and rebreathing patterns. ANZCA PS18 mandates capnography for all general anaesthesia and confirms it as the gold standard for airway device placement confirmation.

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Physics Principles

Infrared Absorption Spectroscopy

Capnography relies on the principle that CO₂ molecules absorb infrared (IR) radiation at specific wavelengths due to their asymmetric molecular structure. The CO₂ molecule has three atoms arranged linearly (O=C=O) and exhibits asymmetric stretching vibrations that interact with electromagnetic radiation in the infrared spectrum. [1,2]

Key Physical Properties:

• CO₂ has a characteristic absorption peak at 4.26 μm (2350 cm⁻¹ wavenumber)
• This corresponds to the asymmetric stretch vibrational mode of the molecule
• The absorption is highly specific, minimising interference from other respiratory gases
• Only molecules with asymmetric dipole moments absorb infrared radiation (hence N₂ and O₂, being symmetric diatomic molecules, do not absorb IR and are "invisible" to the capnograph)

Beer-Lambert Law: The fundamental equation governing infrared absorption is:

I = I₀ × e^(-αCL)

Where:

• I = transmitted light intensity
• I₀ = incident light intensity
• α = absorption coefficient (wavelength-dependent constant)
• C = concentration of absorbing species (CO₂)
• L = path length through the sample

The absorbance (A) is proportional to concentration:

A = log(I₀/I) = αCL

This linear relationship between absorbance and concentration forms the basis of quantitative capnography, allowing direct conversion of light absorption to CO₂ partial pressure. [3]

Capnograph Components

A typical infrared capnograph consists of:

1. Infrared Source: Heated filament or LED emitting broadband IR radiation at 800-1000°C (some newer devices use specific IR diodes)

2. Sample Chamber (Cuvette): Transparent to IR radiation (sapphire or calcium fluoride windows), contains the gas sample, path length typically 5-15 mm

3. Optical Filters: Narrow bandpass filters (4.26 μm for CO₂) ensuring wavelength specificity; reference filter at 3.7 μm (non-absorbing wavelength) for drift compensation

4. Detector: Photoconductive or pyroelectric detectors sensitive to IR wavelengths; compares sample and reference beams to calculate absorption

5. Signal Processing: Analog-to-digital conversion, temperature compensation, linearisation algorithms, and waveform display

Interference and Compensation:

Several factors can cause measurement errors:

Nitrous oxide (N₂O) causes the most significant interference due to overlapping absorption spectra. At 70% N₂O, CO₂ readings may be overestimated by 10-15% without correction. Modern monitors automatically compensate when N₂O administration is detected. [4,5]

Mainstream vs Sidestream Capnography

Two fundamental configurations exist for respiratory gas sampling:

Mainstream (Non-Diverting) Capnography:

The sensor is located directly in the breathing circuit, typically between the endotracheal tube/supraglottic airway and the circuit Y-piece.

Technical Specifications (Mainstream):

• Response time (10-90%): 15-50 ms
• Dead space: 5-15 mL (adult sensors)
• Operating temperature: 41°C (prevents condensation)
• Warm-up time: 2-5 minutes
• Accuracy: ±2 mmHg or ±5% (whichever is greater)

Sidestream (Diverting) Capnography:

Gas is continuously aspirated from the breathing circuit through fine-bore tubing to a remote sensor unit.

Technical Specifications (Sidestream):

• Sampling rate: 50-250 mL/min (typically 150-200 mL/min)
• Sample tubing: 1.5-3 m length, 1.2-2.0 mm internal diameter
• Response time (10-90%): 150-400 ms (including transit time)
• Transit delay: 2-4 seconds depending on tubing length
• Rise time (inherent): 50-100 ms at sensor
• Water handling: Nafion tubing (permeable membrane) or water traps

Rise Time and Response Time:

Understanding timing specifications is critical for waveform interpretation:

• Rise time: Time for signal to change from 10% to 90% of final value (inherent sensor speed)
• Transit time: Time for gas to travel from sampling point to sensor (sidestream only)
• Total response time: Rise time + transit time

In paediatric patients with rapid respiratory rates (>40/min), inadequate response time causes waveform distortion and underestimation of ETCO₂. [6,7]

Calibration and Accuracy

Calibration Methods:

1. Factory calibration: Reference gases at known concentrations set zero point and span
2. User calibration: Zero calibration against room air (0% CO₂) should be performed regularly
3. Span calibration: Verification against reference gas (typically 5% CO₂) when accuracy is questioned

Accuracy Standards (ISO 21647):

• For CO₂ 0-40 mmHg: ±2 mmHg
• For CO₂ 40-76 mmHg: ±5% of reading
• For CO₂ >76 mmHg: ±10% of reading

Factors Affecting Accuracy:

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Capnogram Waveform Analysis

Normal Capnogram Phases

The normal capnogram is a real-time graphical representation of CO₂ concentration plotted against time or volume. Understanding each phase is essential for clinical interpretation:

Phase I (Inspiratory Baseline):

• CO₂ concentration: 0 mmHg (zero baseline)
• Duration: Variable (depends on inspiratory time)
• Represents: Fresh gas from circuit (dead space gas) containing no CO₂
• Clinical significance: Baseline should return to zero; elevation indicates rebreathing

Phase II (Expiratory Upstroke):

• CO₂ concentration: Rapid rise from 0 to plateau
• Duration: 0.1-0.2 seconds in healthy adults
• Represents: Transition from anatomical dead space to alveolar gas
• Slope determined by: V/Q matching, airway resistance, lung emptying patterns
• Normal angle: Approximately 70-80° from horizontal (steep)

Phase III (Alveolar Plateau):

• CO₂ concentration: Relatively constant, slight upward slope (0-3 mmHg)
• Duration: Majority of expiratory phase
• Represents: Alveolar gas from gas-exchanging lung units
• The peak value at end of Phase III = ETCO₂ (End-Tidal CO₂)
• Slight upslope normal due to progressive emptying of lung units with varying V/Q ratios

Phase 0 (Inspiratory Downstroke):

• CO₂ concentration: Rapid fall to zero
• Duration: 0.1-0.2 seconds
• Represents: Inspiration of fresh gas washing out CO₂
• Should be near-vertical in mainstream systems
• Slightly slower in sidestream due to mixing in sample line

Normal ETCO₂ Values:

• Adults: 35-45 mmHg (4.7-6.0 kPa)
• Neonates: 35-40 mmHg (4.7-5.3 kPa)
• These values are typically 2-5 mmHg below PaCO₂ [8,9]

ETCO₂ to PaCO₂ Gradient

The Pa-ETCO₂ gradient (also written PaCO₂-ETCO₂ or P(a-ET)CO₂) is a clinically important parameter reflecting the relationship between arterial and end-tidal CO₂.

Normal Gradient: 2-5 mmHg (PaCO₂ > ETCO₂)

This gradient exists because ETCO₂ represents the mean alveolar CO₂ concentration, which is diluted by:

1. Anatomical dead space (conducting airways with no gas exchange)
2. Alveolar dead space (ventilated but poorly perfused alveoli)

Physiological Basis:

• Well-perfused alveoli: PCO₂ ≈ PaCO₂ (40 mmHg)
• Poorly perfused alveoli: PCO₂ approaches inspired gas (0 mmHg)
• Mixed expired gas: Weighted average of all alveolar units
• ETCO₂ = end-expiratory alveolar PCO₂, but includes dilution from dead space

Widened Gradient (>5 mmHg):

Negative Gradient (ETCO₂ > PaCO₂):

Rarely, ETCO₂ may exceed PaCO₂ (negative gradient) in:

• Pregnancy (increased metabolic rate, respiratory alkalosis)
• Exercise recovery
• Low V/Q states with sampling from well-perfused lung regions
• Paediatric patients with rapid respiratory rates

Clinical Application: Serial Pa-ETCO₂ gradient monitoring can indicate:

• Response to volume resuscitation (gradient narrows with improved cardiac output)
• Progression of pulmonary embolism (gradient widens)
• Effectiveness of CPR (gradient correlates with coronary perfusion pressure) [10,11,12]

Abnormal Waveforms

Recognition of abnormal capnogram patterns is essential for prompt diagnosis and intervention:

1. Elevated Baseline (Rebreathing):

Diagnostic Test:

• Increase fresh gas flow to 10 L/min
• If baseline returns to zero → exhausted absorbent
• If baseline remains elevated → valve malfunction

2. "Shark Fin" Waveform (Bronchospasm/Obstruction):

• Prolonged, slanted Phase II (upstroke)
• Loss of distinct Phase II/III junction
• Steep, upward-sloping Phase III
• Seen in: Bronchospasm, COPD, asthma, kinked ETT, partial obstruction

The slope reflects uneven alveolar emptying—obstructed units empty more slowly, contributing higher CO₂ later in expiration.

3. Cardiac Oscillations:

• Small rhythmic undulations on Phase III plateau
• Frequency matches heart rate
• Caused by: Cardiac contractions "pumping" small volumes of gas
• Common in: Paediatric patients, low respiratory rates, small tidal volumes
• Generally benign, indicates close heart-lung relationship

4. Curare Cleft:

• Notch or dip in the alveolar plateau (Phase III)
• Caused by: Spontaneous breathing efforts against ventilator
• Indicates: Waning neuromuscular blockade or light anaesthesia
• Named after curare (historical neuromuscular blocker)
• Represents diaphragmatic contraction during expiratory phase

5. Sudden Drop to Zero:

• Immediate flat line at zero
• Causes: Circuit disconnection, ventilator failure, complete airway obstruction, oesophageal intubation, cardiac arrest
• ACTION: Immediately hand ventilate and check circuit connections

6. Exponential Decay:

• Gradual decline in ETCO₂ over several breaths
• Causes: Decreasing cardiac output, pulmonary embolism, hypovolaemia, air embolism
• Pattern reflects progressive reduction in CO₂ delivery to lungs

7. Sudden Increase in ETCO₃:

• Rapid rise above normal values
• Causes: Malignant hyperthermia (early sign), inadequate ventilation, CO₂ absorption from laparoscopy, release of tourniquet, NaHCO₃ administration, fever

8. Prolonged Expiratory Phase:

• Extended Phase III duration
• Normal waveform shape but prolonged cycle
• Causes: Slow respiratory rate, large tidal volumes, hypoventilation [13,14,15]

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Key Equations

Dead Space Calculations

Bohr Equation (Physiological Dead Space):

The Bohr equation calculates the dead space fraction using the dilution principle:

VD/VT = (PACO₂ - PĒCO₂) / PACO₂

Where:

• VD = Dead space volume
• VT = Tidal volume
• PACO₂ = Alveolar CO₂ partial pressure
• PĒCO₂ = Mixed expired CO₂ partial pressure

Enghoff Modification (Clinical Application):

Since alveolar CO₂ is difficult to measure directly, arterial CO₂ is substituted (assuming equilibration between arterial blood and well-perfused alveoli):

VD/VT = (PaCO₂ - PĒCO₂) / PaCO₂

Normal Values:

• Anatomical dead space: 2 mL/kg (approximately 150 mL in 70 kg adult)
• Physiological dead space: 30% of tidal volume (VD/VT = 0.30)
• During anaesthesia: VD/VT may increase to 0.35-0.45

Clinical Significance:

• VD/VT >0.5 associated with increased mortality in ARDS
• VD/VT >0.6 indicates severe V/Q mismatch
• Serial VD/VT monitoring tracks response to therapy [16,17]

Pa-ETCO₂ Gradient Estimation

Simplified Clinical Estimation:

Pa-ETCO₂ gradient ≈ (VD/VT × PaCO₂) × correction factor

For practical purposes in healthy patients:

• Normal gradient: 2-5 mmHg
• Each 10% increase in VD/VT adds approximately 2-3 mmHg to gradient

Alveolar Gas Equation Application:

PAO₂ = FiO₂ × (Pb - PH₂O) - (PaCO₂/R)

Where ETCO₂ can substitute for PaCO₂ in stable patients:

• PAO₂ ≈ FiO₂ × (Pb - PH₂O) - (ETCO₂/0.8) + gradient correction

This allows non-invasive estimation of alveolar oxygen tension. [18]

Volumetric Capnography Calculations

Volumetric capnography plots CO₂ concentration against expired volume rather than time, providing additional physiological information:

Volume of CO₂ per breath (VCO₂):

VCO₂ = ∫ FCO₂ × V̇ dt

Where FCO₂ is fractional CO₂ concentration and V̇ is flow rate.

Fowler Dead Space (Anatomical):

• Identified as the volume at which CO₂ concentration reaches 50% of alveolar value
• Represents conducting airways without gas exchange
• Normal: 2 mL/kg ideal body weight

Phase III Slope Analysis:

• Steep slope indicates V/Q heterogeneity
• Slope = ΔPCO₂/ΔVolume (units: mmHg/L or kPa/L)
• Normal slope <5 mmHg/L; increased in obstructive disease

VTCO₂,br (Volume of CO₂ eliminated per breath):

• Area under volumetric capnogram curve
• Useful for monitoring metabolic rate changes
• Approximately 15-20 mL CO₂/breath at rest (200 mL/min total) [19,20]

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Clinical Abnormalities

Oesophageal Intubation Detection

Capnography is the gold standard for confirming endotracheal tube placement and detecting oesophageal intubation.

Findings with Oesophageal Intubation:

• No sustained CO₂ waveform after 5-6 ventilations
• May see small, rapidly declining CO₂ spikes if carbonated beverages or gastric CO₂ present
• Waveform does not maintain consistent shape and height

Evidence:

• Silvestri et al. (2005): Zero unrecognised oesophageal intubations with continuous ETCO₂ monitoring
• False positive rate (absent CO₂ with correct placement): <1% (cardiac arrest, severe bronchospasm)
• False negative rate (CO₂ present with oesophageal placement): <2% (recent carbonated drinks)

ANZCA Position (PS18): Capnography is mandatory for confirmation of airway device placement. The presence of a normal capnogram waveform for at least 5 breaths confirms tracheal intubation. [21,22]

Bronchospasm and Airway Obstruction

Capnographic Features:

• "Shark fin" or "ramp" waveform
• Prolonged Phase II (slanted upstroke >0.3 seconds)
• Steep, continuously rising Phase III
• Loss of distinct Phase II/III junction
• ETCO₂ may be elevated (due to CO₂ retention) or decreased (due to air trapping)

Pathophysiology:

• Bronchoconstriction causes uneven alveolar emptying
• Obstructed lung units empty slowly, contributing CO₂ late in expiration
• Sequential emptying creates progressively rising CO₂ concentration

Severity Assessment:

Differential Diagnosis of Sloping Phase III:

• Bronchospasm (response to bronchodilators)
• ETT kink or obstruction
• Endobronchial intubation
• Mucous plug
• Foreign body aspiration [23,24]

Cardiac Arrest and CPR Monitoring

ETCO₂ during CPR reflects cardiac output generated by chest compressions and is an objective marker of CPR quality.

Physiological Basis:

• CO₂ is constantly produced by tissues (approximately 200 mL/min)
• CO₂ delivery to lungs depends entirely on cardiac output during arrest
• ETCO₂ ∝ Pulmonary blood flow ∝ Cardiac output from compressions

Clinical Targets (ARC/ANZCOR Guidelines):

Prognostic Value:

• ETCO₂ <10 mmHg after 20 minutes of high-quality CPR: Poor prognosis
• ETCO₂ persistently <20 mmHg: Low likelihood of survival
• ETCO₂ increase >10 mmHg during resuscitation: Positive prognostic indicator

ROSC Detection: A sudden, sustained increase in ETCO₂ (often to 35-50 mmHg) is frequently the earliest indicator of ROSC, preceding palpable pulse by several seconds. [25,26,27]

Circuit Disconnection and Leaks

Complete Disconnection:

• Immediate drop to zero baseline (flat line)
• Loss of all respiratory cycling
• No waveform visible

Partial Disconnection/Leak:

• Decreased ETCO₂ values
• Reduced waveform amplitude
• May see irregular waveform pattern
• Air entrainment dilutes exhaled CO₂

Differential Diagnosis of Sudden Zero:

1. Circuit disconnection (most common)
2. ETT dislodgement/extubation
3. Ventilator failure
4. Complete airway obstruction (no gas flow to sensor)
5. Cardiac arrest (no CO₂ delivery)
6. Massive pulmonary embolism

Management Protocol:

1. Immediately hand ventilate with 100% O₂
2. Check circuit connections systematically
3. Verify ETT position (direct laryngoscopy if needed)
4. Auscultate chest
5. Check ventilator function
6. Consider alternative diagnoses if circuit intact

Rebreathing Patterns

Definition: Rebreathing occurs when previously exhaled CO₂-containing gas is re-inhaled, manifesting as failure of the capnogram baseline to return to zero.

Circle System Causes:

Mapleson Circuit Causes:

• Inadequate fresh gas flow
• Baseline elevation progressive as FGF:MV ratio decreases

Clinical Consequences of Rebreathing:

• Increased inspired CO₂ → respiratory acidosis
• Increased work of breathing (compensatory hyperventilation)
• Elevated PaCO₂ and ETCO₂ values
• Sympathetic stimulation (tachycardia, hypertension)
• In extreme cases: narcosis, arrhythmias [28,29]

Malignant Hyperthermia

Capnographic Significance: An unexplained, rapidly rising ETCO₂ is often the earliest clinical sign of malignant hyperthermia, frequently appearing before temperature elevation.

Pathophysiology:

• Uncontrolled skeletal muscle hypermetabolism
• Massive CO₂ production (may exceed 3-4 times normal)
• Overwhelms ventilatory capacity despite constant minute ventilation

Typical Pattern:

• Baseline: Normal (zero)
• ETCO₂: Progressive rise despite adequate ventilation
• Rate of rise: 5-15 mmHg over 10-15 minutes
• Waveform shape: Normal initially, then may show increased slope

MH Clinical Grading (Larach et al.): ETCO₂ >55 mmHg (or >20% above baseline) with appropriate minute ventilation scores as a major criterion for MH diagnosis.

Management Implications:

• Immediately increase minute ventilation
• Discontinue volatile anaesthetics and succinylcholine
• Administer dantrolene 2.5 mg/kg IV
• Hyperventilate with 100% O₂ at 2-3 times normal minute ventilation
• Monitor for continued rise indicating treatment failure [30,31]

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Indigenous Health Considerations

Remote and Rural Anaesthesia Challenges

Aboriginal and Torres Strait Islander communities in remote Australia face unique challenges regarding monitoring equipment access and maintenance that directly impact capnography use.

Equipment Availability: Many remote health clinics and small hospitals operate with limited monitoring equipment. While pulse oximetry is universally available, advanced capnography (particularly volumetric or mainstream systems) may not be present. The Royal Flying Doctor Service (RFDS) and retrieval services carry portable capnography, but initial stabilisation may occur without this monitoring modality.

Maintenance and Calibration:

• Remote locations experience delays in equipment servicing
• Calibration gases may not be readily available
• Humidity and temperature extremes (5-50°C) affect sensor accuracy
• Sample line replacements may be delayed by supply chain issues

Aeromedical Considerations: During RFDS retrieval at altitude (typically 6000-8000 feet cabin pressure), capnography readings require interpretation considering:

• Reduced barometric pressure affects displayed values
• Pa-ETCO₂ gradient may widen with altitude-related V/Q changes
• Equipment calibrated at sea level may show systematic errors

Cultural Safety in Communication

When explaining capnography monitoring to Aboriginal and Torres Strait Islander patients and families:

• Involve Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs) in explanations
• Use plain language rather than technical terminology
• Allow time for family discussion and questions
• Recognise that some communities prefer collective decision-making
• Consider interpreter services for patients whose first language is not English

Health Literacy: Explaining that capnography measures "the air coming out of your lungs" may be more meaningful than technical descriptions. Visual displays of the waveform can aid understanding when culturally appropriate.

Māori Health Considerations (New Zealand):

• Involve whānau (extended family) in discussions about monitoring
• Consider involvement of kaumātua (elders) for procedural consent
• Respect tikanga (cultural protocols) in monitoring placement
• Māori Health Workers can facilitate communication about equipment and its purpose

Health Equity Implications

Populations with higher rates of respiratory disease (including higher smoking rates and chronic lung disease in some Indigenous communities) may demonstrate altered capnography patterns:

• Baseline COPD/asthma causing "shark fin" waveforms
• Widened Pa-ETCO₂ gradients from V/Q mismatch
• Altered normal ranges requiring individual baseline assessment

Understanding these variations ensures appropriate interpretation and avoids misdiagnosis during anaesthesia. [32,33]

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Assessment Content

SAQ Practice Question 1 (20 marks)

Question: A 68-year-old man with severe COPD (FEV₁ 35% predicted) is undergoing laparoscopic cholecystectomy under general anaesthesia. Fifteen minutes after induction, the anaesthetist notes that the ETCO₂ is 52 mmHg with a characteristic "shark fin" capnogram waveform. An arterial blood gas shows PaCO₂ of 58 mmHg.

(a) Explain the physics principles underlying capnography measurement and describe how CO₂ concentration is determined from infrared absorption. (6 marks)

(b) Describe the normal capnogram phases and explain why this patient's waveform shows a "shark fin" pattern. (8 marks)

(c) Calculate and interpret the Pa-ETCO₂ gradient in this patient. What are the clinical implications for ventilation management? (6 marks)

Model Answer:

(a) Physics Principles (6 marks)

Infrared Absorption (3 marks):

• CO₂ molecules have asymmetric molecular structure (O=C=O) that absorbs infrared radiation at specific wavelengths
• Peak absorption occurs at 4.26 μm wavelength corresponding to asymmetric stretch vibrational mode
• Only molecules with asymmetric dipole moments absorb IR; symmetric molecules (O₂, N₂) are "invisible" to capnograph

Beer-Lambert Law (3 marks):

• The intensity of transmitted light decreases exponentially with gas concentration
• I = I₀ × e^(-αCL), where α = absorption coefficient, C = concentration, L = path length
• Absorbance is directly proportional to CO₂ concentration: A = αCL
• This linear relationship allows quantitative measurement of partial pressure
• Modern capnographs use reference wavelengths (3.7 μm) to compensate for drift and interference

(b) Capnogram Phases and "Shark Fin" Pattern (8 marks)

Normal Phases (4 marks):

• Phase I (Inspiratory baseline): CO₂ = 0 mmHg, represents fresh gas (dead space gas)
• Phase II (Expiratory upstroke): Rapid rise as anatomical dead space cleared, transition to alveolar gas
• Phase III (Alveolar plateau): Relatively constant CO₂ from alveoli, slight upward slope (0-3 mmHg); peak = ETCO₂
• Phase 0 (Inspiratory downstroke): Rapid fall to zero as fresh gas inspired

"Shark Fin" Pattern Explanation (4 marks):

• In severe COPD, airway obstruction causes uneven alveolar emptying
• Well-ventilated alveoli (short time constants) empty first, contributing lower CO₂
• Obstructed alveoli (long time constants) empty slowly, contributing higher CO₂ late in expiration
• This sequential emptying causes:
  • Prolonged Phase II (slanted rather than steep upstroke)
  • Steep, continuously rising Phase III (no plateau)
  • Loss of distinct Phase II/III junction
• The waveform resembles a shark's dorsal fin rather than the normal rectangular shape

(c) Pa-ETCO₂ Gradient Interpretation (6 marks)

Calculation (2 marks):

• Pa-ETCO₂ gradient = PaCO₂ - ETCO₂
• Gradient = 58 - 52 = 6 mmHg
• Normal gradient: 2-5 mmHg
• This patient's gradient is mildly widened

Interpretation (2 marks):

• Widened gradient indicates increased physiological dead space
• In COPD: V/Q mismatch with areas of high V/Q (ventilated but poorly perfused)
• Additionally, the elevated ETCO₂ (52 mmHg) indicates inadequate alveolar ventilation despite the gradient
• The CO₂ retention (PaCO₂ 58 mmHg) reflects both V/Q mismatch and likely inadequate minute ventilation

Clinical Implications (2 marks):

• ETCO₂ underestimates PaCO₂ in this patient—cannot rely solely on capnography for ventilation adequacy
• Management should include:
  • Increase minute ventilation (↑ respiratory rate and/or tidal volume) to target PaCO₂ closer to patient's baseline
  • Permissive hypercapnia may be acceptable if baseline PaCO₂ elevated
  • Consider pressure-controlled ventilation with adequate I:E ratio for COPD
  • Serial ABGs recommended rather than relying on ETCO₂ alone
  • Bronchodilator therapy if bronchospasm contributing

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SAQ Practice Question 2 (20 marks)

Question: During a general anaesthetic using a circle breathing system, you notice the capnogram baseline has risen to 8 mmHg and is no longer returning to zero during inspiration. The ETCO₂ is 48 mmHg.

(a) List the possible causes of an elevated capnogram baseline in a circle system and describe how you would differentiate between them. (10 marks)

(b) Explain the chemistry of CO₂ absorption in soda lime and describe the indicators of absorbent exhaustion. (6 marks)

(c) What are the physiological consequences of significant rebreathing if unrecognised? (4 marks)

Model Answer:

(a) Causes and Differentiation of Elevated Baseline (10 marks)

Possible Causes (5 marks):

Diagnostic Approach (5 marks):

1. Increase fresh gas flow to 10 L/min:

   • If baseline returns to zero → exhausted absorbent (high FGF bypasses absorbent)
   • If baseline remains elevated → valve malfunction (high FGF cannot overcome physical backflow)

2. Check absorbent canister:

   • Colour change (pH indicator) suggests exhaustion
   • Absence of heat generation during use
   • Note: Colour may revert after rest (false reassurance)

3. Listen for valve clicks:

   • Unidirectional valves should click with each breath
   • Silence suggests stuck valve

4. Manual bag ventilation:

   • Check for resistance pattern consistent with valve failure
   • Observe valve disc movement if visible

5. Replace components systematically:

   • Change absorbent canister first (most common cause)
   • If unresolved, replace valve assemblies

(b) Soda Lime Chemistry and Exhaustion Indicators (6 marks)

Chemical Reactions (3 marks):

Soda lime contains:

• Calcium hydroxide Ca(OH)₂: 75-80%
• Sodium hydroxide NaOH: 3-4%
• Potassium hydroxide KOH: 1%
• Water: 14-19%
• pH indicator dye

Reactions:

1. CO₂ + H₂O → H₂CO₃ (carbonic acid formation)
2. H₂CO₃ + 2NaOH → Na₂CO₃ + 2H₂O (fast reaction, regenerates water)
3. Na₂CO₃ + Ca(OH)₂ → CaCO₃ + 2NaOH (regenerates sodium hydroxide)

Net reaction: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O + Heat

The reaction is exothermic, releasing approximately 14 kcal/mol CO₂.

Exhaustion Indicators (3 marks):

• Colour change: Ethyl violet (white → purple) or other pH indicators
• Note: Colour may revert ("regeneration") after rest period—unreliable if absorbent rested
• Loss of heat generation: Fresh absorbent warms during CO₂ absorption
• Increased inspired CO₂: Capnogram baseline elevation
• Increased hardness: Calcium carbonate harder than hydroxide
• Time/volume: Typically 8-12 hours continuous use or 100-150 L CO₂ absorbed

(c) Physiological Consequences of Rebreathing (4 marks)

Respiratory Effects (2 marks):

• Progressive increase in PaCO₂ (respiratory acidosis)
• Increased work of breathing (compensatory hyperventilation attempts)
• Increased minute ventilation requirement
• If anaesthetised/paralysed: uncontrolled hypercapnia

Systemic Effects (2 marks):

• Cardiovascular: Tachycardia, hypertension (CO₂ is sympathomimetic)
• CNS: Increased cerebral blood flow, raised intracranial pressure
• Metabolic: Respiratory acidosis with pH depression
• Severe: CO₂ narcosis, arrhythmias, cardiovascular collapse

Recognition in Anaesthetised Patient:

• Tachycardia and hypertension are often first signs
• Increased bleeding in surgical field (vasodilation)
• Difficulty maintaining anaesthetic depth
• Post-operative delayed emergence

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Primary Viva Scenario (15 marks)

Examiner: "I'd like to discuss capnography. Can you start by explaining the physical principles that allow us to measure carbon dioxide concentration using infrared spectroscopy?"

Candidate: "Capnography measures CO₂ concentration using the principle that carbon dioxide molecules absorb infrared radiation at specific wavelengths. CO₂ has an asymmetric molecular structure—three atoms arranged as O=C=O—which allows it to undergo asymmetric stretching vibrations that interact with infrared radiation.

The key wavelength for CO₂ absorption is 4.26 micrometres, which corresponds to the asymmetric stretch vibrational mode. Only molecules with asymmetric dipole moments can absorb infrared radiation, which is why oxygen and nitrogen, being symmetric diatomic molecules, don't interfere with the measurement.

The quantitative relationship follows the Beer-Lambert law, which states that absorbance is proportional to concentration. Mathematically, the transmitted light intensity I equals the incident intensity I-zero multiplied by e to the power of negative alpha times C times L, where alpha is the absorption coefficient, C is the CO₂ concentration, and L is the path length through the sample.

The capnograph has an infrared source, typically a heated filament, which emits broadband radiation. This passes through the gas sample in a cuvette, then through a narrow bandpass filter at 4.26 micrometres, and finally reaches a detector. A reference wavelength at 3.7 micrometres, where CO₂ doesn't absorb, is used to compensate for drift and interference."

Examiner: "Good. What are the differences between mainstream and sidestream capnography?"

Candidate: "The two configurations differ in where the CO₂ sensor is located.

In mainstream capnography, the sensor sits directly in the breathing circuit, typically between the endotracheal tube and the Y-piece. The advantages are faster response time—typically 15 to 50 milliseconds—and no gas sampling required, which eliminates sample line blockage and moisture problems. However, the sensor adds weight to the circuit, around 15 to 30 grams, and adds dead space of 5 to 15 millilitres. The sensor is heated to 41 degrees Celsius to prevent condensation, which creates a theoretical burn risk if dislodged.

In sidestream capnography, gas is continuously aspirated from the circuit through fine-bore tubing at a rate of 150 to 200 millilitres per minute to a remote sensor. The advantages include a lightweight airway interface, the ability to analyse multiple gases including volatile agents, and the ability to monitor non-intubated patients using nasal cannulae. The disadvantages are a transit delay of 2 to 4 seconds, sample line blockage from secretions, the need for water traps, and some waveform smoothing due to mixing in the sample line.

For paediatric patients with rapid respiratory rates, mainstream is often preferred due to the faster response time, which provides more accurate waveform representation."

Examiner: "Describe the normal capnogram waveform and what each phase represents."

Candidate: "The normal capnogram has four phases when plotted as CO₂ concentration against time.

Phase I is the inspiratory baseline, where CO₂ concentration is zero. This represents the fresh gas from the circuit—essentially dead space gas—which contains no CO₂. The baseline should return to zero; any elevation indicates rebreathing.

Phase II is the expiratory upstroke, showing a rapid rise in CO₂ concentration. This represents the transition as anatomical dead space gas is washed out and alveolar gas begins to reach the sensor. The slope of this phase reflects lung emptying patterns, and it should be steep, at around 70 to 80 degrees from horizontal.

Phase III is the alveolar plateau, where CO₂ concentration is relatively constant with perhaps a slight upward slope of 0 to 3 millimetres of mercury. This represents pure alveolar gas. The small upslope is normal and reflects the slight variation in V/Q ratios across the lung. The peak value at the end of Phase III is the end-tidal CO₂ or ETCO₂.

Phase 0 is the inspiratory downstroke, showing a rapid fall back to zero as fresh gas is inspired. This should be near-vertical in mainstream systems.

Normal ETCO₂ values are 35 to 45 millimetres of mercury in adults, and this is typically 2 to 5 millimetres of mercury lower than arterial CO₂."

Examiner: "How would you use capnography to monitor CPR quality during a cardiac arrest?"

Candidate: "During cardiac arrest, ETCO₂ directly reflects the cardiac output generated by chest compressions because CO₂ delivery to the lungs depends entirely on pulmonary blood flow.

The Australian Resuscitation Council guidelines suggest several applications:

First, for CPR quality assessment: ETCO₂ less than 10 millimetres of mercury suggests inadequate chest compressions—the team should focus on improving compression depth, rate, and allowing full chest recoil. ETCO₂ between 10 and 20 millimetres of mercury suggests marginal compressions, while values above 20 millimetres of mercury indicate adequate chest compressions.

Second, for ROSC detection: A sudden, sustained increase in ETCO₂—often rising from low values to 35 to 50 millimetres of mercury—is frequently the earliest indicator of return of spontaneous circulation, often appearing before a palpable pulse. This is because the heart begins pumping CO₂-rich venous blood back to the lungs.

Third, for prognostication: ETCO₂ persistently below 10 millimetres of mercury after 20 minutes of high-quality CPR is associated with poor prognosis. However, ETCO₂ should never be used as a sole criterion to terminate resuscitation.

Fourth, for adrenaline response monitoring: Repeated adrenaline may cause transient ETCO₂ increases due to vasoconstriction improving coronary perfusion.

The continuous waveform also confirms airway device position throughout resuscitation—a flat line should prompt immediate verification of tube position."

Examiner: "What capnographic findings would raise your concern for malignant hyperthermia?"

Candidate: "Malignant hyperthermia causes an unexplained, rapidly rising ETCO₂, which is often the earliest clinical sign, frequently appearing before temperature elevation.

The pathophysiology involves uncontrolled skeletal muscle hypermetabolism triggered by volatile anaesthetics or succinylcholine. This causes massive CO₂ production—potentially 3 to 4 times normal—which overwhelms ventilatory capacity.

The typical capnographic pattern shows a normal zero baseline, but ETCO₂ progressively rising despite constant minute ventilation. The rate of rise is typically 5 to 15 millimetres of mercury over 10 to 15 minutes. Initially, the waveform shape remains normal, though it may show an increased Phase III slope as CO₂ production increases.

The Malignant Hyperthermia Clinical Grading Scale by Larach and colleagues considers ETCO₂ greater than 55 millimetres of mercury, or more than 20% above baseline with appropriate minute ventilation, as a major criterion for diagnosis.

If I suspected MH based on rising ETCO₂, I would immediately increase minute ventilation to 2 to 3 times normal, discontinue volatile anaesthetics and switch to total intravenous anaesthesia, administer dantrolene 2.5 milligrams per kilogram intravenously, hyperventilate with 100% oxygen, and continue monitoring for response. Continued ETCO₂ rise despite increased ventilation and dantrolene would indicate treatment failure requiring additional dantrolene doses."

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