18B03: Exam Report

Explain the causes of the differences between measured end tidal and arterial partial pressures of carbon dioxide (CO2).

29% of candidates passed this question.

The answer required an explanation of the causes of the difference between the PaCO2 and ETCO2. This required recognising how the end point of phase 3 of the capnograph trace corresponds with end tidal CO2. The difference is caused by the alveolar dead space. The

difference is normally very small in healthy adults with the ETCO2 being lower than the PaCO2. It is increased with increasing alveolar dead space. Many incorrectly attributed anatomical dead space as a contributor to the PaCO2-ETCO2 gradient. Discussion of the various types of dead space did not score marks. Marks were awarded for the processes that cause an increased gradient e.g. low cardiac output and pulmonary embolism. Recognising physiological factors such as increasing gradient with increasing age scored marks. Marks were not awarded for descriptions on how dead space is measured.

17A21: Exam Report

Explain the potential causes of a difference between the measured end tidal CO2 and the arterial partial pressure of CO2.

30% of candidates passed this question.

Many candidates didn’t distinguish between the different types of dead space. In general this topic was not well understood.

F12ii / 18B03 / 17A21: Explain the causes of the differences between measured end tidal and arterial partial pressures of carbon dioxide (CO2)


  • ETCO2 (32 – 42mmHg) = CO2 measured at the end of expiration by IR analysis → plotted against time
  • PaCO2 (35 – 45mmHg) = Partial pressure of CO2 in arterial blood → measures dissolved CO2 by Severinghaus electrode in ABG machine
    • Theoretically: ETO2 = PACO2 = PaCO2
    • But this is never the case → normal difference 5% in health

Causes of ↑ Difference

Patient Factors ↓ETCO2

  • ↑Anatomical DS
    • Volume of conducting airways
    • No role in gas exchange
    • Normally 2mL/kg
  • ↑Alveolar DS
    • Volume of gas reaching alveoli that doesn’t take part in gas exchange
      • WZ1: PA > Pa > PV
      • ↑PA: IPPV, PEEP
      • ↓Pa: cardiac arrest, PE

Patient Factors ↑PaCO2 / ↑ETCO2

  • ↑VT + ↓RR
    • Allows more time for “slow alveoli” to empty
  • ↑Production e. MH
  • ↑Absorptione. Pneumoperitoneum

Measurement Factors ETCO2

  • Sample line too long → gas trapped in line & not sampled
  • Interference from other gases (N2O) = ↑ETCO2 due to interference with IR absorber KA COLLISION BROADENING
  • Excess H2O in H2O trap
  • Loss of calibration
  • Air leakage → entraining room air/loss of expired air

Measurement Errors PaCO2

  • Collection issue → non-arterial sample
  • Temp correction
  • Loss of calibration of ABG machine