2021B10 Discuss possible causes for the PaCO2 differing from the ETCO2 in an anaesthetised, intubated patient on IPPV.




·     Gap in health 2-5mmHg (PaCO2 > etCO2)

·     Due to alveolar, anatomical and apparatus dead space


1.   Dead space

2.   Incomplete alveolar emptying

3.   Measurement error

4.   Sampling error


1.Dead space:


·     That portion of the tidal volume that does not undergo gas exchange


·     Apparatus dead space (distal to Y piece)

·     Physiological dead space

o  Anatomical dead space (conducting airways)

o  Alveolar dead space (non-ventilated alveoli; most important)


·     Ideal alveolus: PACO2 = PaCO2 40mmHg

·     Dead space: PACO2 ≈ 0.4mmHg

·     End-tidal air is a mix of both, hence etCO2 < PaCO2



·     Bohr equation VD/VT = (PACO2 – PECO2) / PACO2

·     Enghoff mod: substitute PaCO2 for PACO2 (assumes steady state, minimal shunt)



·     Estimable, or measurable using product information



·     Estimated by Fowler’s method

·     Normal 2mL/kg


Description automatically generated



·     = total – anatomical – apparatus

Alveolar DS

a.k.a. West Zone 1:

·     PA > Pa > Pv

·     hence vascular collapse

·     Small volume near apices in health when upright

↑PA if:

·     IPPV ( airway pressure)

↓Pa if:

·     Pulmonary vasodilators (e.g. milrinone)

·     Negative inotropes (e.g. propofol)

·     Reduced venous return (e.g. hypovolaemia)

Effect of GA:

·     Non-ventilated alveoli

·     ↑Hypoventilated alveoli (due to maldistribution of V and Q)

Anatomical DS

Cause of absolute increase:

·     ↑Tidal volume (plateau at VT ~350mL)

·     Bronchodilation (sevoflurane, pregnancy)

Cause of absolute decrease:

·     ↓↓Tidal volume

o  Axial streaming

o  Cardiac impulse -> mixing

(Note ↓VT increases relative dead space)

·     LMA or ETT (bypasses upper airway)

Apparatus DS

·     Device and tubing distal to Y junction

·     Important in small children

·     ETT/LMA: VD is 1/2 of VT

·     Face mask: VD is 2/3 of VT


2.Incomplete alveolar emptying:


·     Variation in time constants (e.g. asthma, COPD)

·     ↓Expiratory time (↑RR, ↓IE ratio)

·     (especially if both are present)

↑Variation in time constants

·     Time constant (τ) = resistance x compliance

·     Slow lung units: ↑R,↑C

o Slow rate of change

o ↓Ventilation

o ↑pCO2

               Empties late in expiration

·     Fast lungs units: ↓R,↓C

o Fast rate of change

o ↑Ventilation

o ↓pCO2

Empties early in expiration

·     Hence variation -> ↑α angle


Description automatically generated


3.Measurement error:


·     N2O 4.5nm, CO 4.7nM (causes falsely high pCO2)

·     H2O vapour absorbs widely (causes falsely high pCO2)

·     Inhaler propellant and halothane

·     FIX: reference chamber AND water trap

Collision broadening

·     Widening of the absorption peak for one gas in the presence of another

·     Due to collision between molecules raising their energy level

·     50% N2O widens CO2’s absorption peak by 10%

·     FIX: use reference chambers

Ram-gas effect

·     Pressure-drop across the sampling line

·     ↓ total pressure inside sample chamber -> ↓ CO2 partial pressure

Calibration failure

·     FIX: auto regular three point calibration


4.Sampling error:

Dilution by FGF

·     Sampling of FGF may occur in expiration if high RR and low VT

·     e.g. neonates and young children


·     Complete disconnection: ambient air

·     Partial disconnection: entrainment of some room air


·     By water condensation



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