2020A03 Discuss possible causes of for the PaCO2 differing from etCO2 in an anaesthetized, intubated patient on IPPV.



·         Dead space

·         Incomplete alveolar emptying

·         Measurement error

·         Sampling error


Dead space:


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


·   Apparatus dead space (circuit distal to Y piece)

·   Physiological DSAnatomical dead space (conducting airways)

·   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 each, hence etCO2 < PaCO2

·   Accounts for difference in health of 2-5mmHg



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

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



·   Estimable, or measurable using product information



·   Fowler’s method



= 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


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


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 a gas when 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


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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