2003B11 Briefly describe the potential causes of a difference between measured
end-tidal and arterial partial pressure of carbon dioxide.

 

Causes:

·         Alveolar dead space

·         Incomplete alveolar emptying

·         Measurement error

 

Alveolar dead space:

Definition

·   Volume of air in non-perfused alveoli

·   West zone 1 of lung: PA > Pa > Pv
(Small volume near apices in health)

Effect on etCO2

·   Ideal alveolus: PaCO2 = PACO2 (i.e. ventilation = perfusion)

·   Alveolar dead space: PACO2 very low

·   End-tidal air = ideal + dead space; hence etCO2 < PaCO2

·   Normal 2-5mmHg difference. Due to V/Q scatter by gravity -> high V/Q ratio at apex

Causes of ↑

·   ↑ PA:

o IPPV, PEEP

·   ↓Pa: (↓RV output, ↓mPAP)

o Pulmonary vasodilators (e.g. milrinone)

o Negative inotropes (e.g. propofol)

o Reduced venous return (e.g. hypovolaemia)

Measurement

Alveolar dead space = physiological - anatomical

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

o Enghoff: substitute PaCO2 for PACO2 assuming steady state and minimal shunt

·   Anatomical dead space: from Fowler’s method

 

Incomplete alveolar emptying:

Causes

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

·   ↓ expiratory time (↑RR, ↓IE ratio)
(especially if both)

↑Variation in time constants

·   Time constant (τ) = resistance x compliance

·   ↑R,↑C -> ↑T:

o Slow rate of change

o ↓Ventilation

o ↑pCO2

                Empties late in expiration

·   ↓R,↓C -> ↓T:

o Fast rate of change

o ↑Ventilation

o ↑pCO2

o Empties early in expiration

·   Hence variation -> ↑α angle

 

Measurement error:

Machine

·   Confusion with other gas

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

o H2O vapour absorbs widely (causes falsely high pCO2)

o Inhaler propellant and halothane

o FIX: reference chamber AND water trap

·   Collision broadening

o Widening of the absorption peak for a gas when in the presence of another

o Due to collision between molecules raising their energy level

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

o FIX: use reference chambers

·   Ram-gas effect

o Pressure-drop across the sampling line

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

·   Lack of calibration

o FIX: auto regular three point calibration

Sampling

·   Dilution by FGF

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

o e.g. neonates and young children

·   Sampling line disconnection

o Complete disconnection: ambient air

o Partial disconnection: entrainment of some room air

·   Blockage by water condensation

Patient

·   Cardiogenic oscillation (if apnoeic)

·   (Also: if tidal volume very small, etCO2 may be very low due to anatomical deadspace)

 

 

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