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

 

Intro:

Normal

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

·     Due to alveolar, anatomical and apparatus dead space

Causes

1.    Dead space

2.    Incomplete alveolar emptying

3.    Measurement error

4.    Sampling error

 

1.Dead space:

Definition

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

Components

·     Apparatus dead space (distal to Y piece)

·     Physiological dead space

o  Anatomical dead space (conducting airways)

o  Alveolar dead space (non-perfused alveoli)

Pathophysiology

·     Ideal alveolus: PACO2 = PaCO2 ≈40mmHg

·     Dead space: PACO2 ≈ 0.4mmHg

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

Measurement

Physiological:

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

o  If PA = ideal alveolar gas -> measure of anatomical dead space

o  If PA = end-expiratory gas -> measure of physiological dead space

·     Enghoff modification: substitute PaCO2 for PACO2

o  Measure of physiological dead space

o  Assumes stable cardiac output

o  Assumes PaCO2 = (ideal) PACO2
Hence inaccurate if large shunt

 

Anatomical:

·     Estimated by Fowler’s method

·     Volume of gas exhaled before pCO2 reaches a plateau

·     Normal 2mL/kg

Diagram Description automatically generated

 

Alveolar:

·     VD = physiological - anatomical

·     Minimal if healthy and conscious

 

Apparatus:

·     Estimable using product information

 

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:

Causes

·     ↑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

Chart Description automatically generated

 

3.Measurement error:

Confusion

·     N2O 4.5μm, CO 4.7μM (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

Disconnection

·     Complete disconnection: ambient air

·     Partial disconnection: entrainment of some room air

Blockage

·     By water condensation

 

 

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