Explain the causes of the difference between measured end tidal and arterial partial pressures of carbon dioxide

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College Answer

Points required to pass this question included the normal difference between end-tidal and
arterial partial pressure of C02 and the reasons for this. The patient factors that increase the
difference include increases in alveolar dead space and a slow rise of expired C02. Mention of
pathology e.g. pulmonary embolism and cardiac arrest gained extra marks. Equipment
factors needed to be included e.g. leaks, occlusion of sampling line. Candidates who failed
did not discuss alveolar dead space and very few adequately explained how it increased the
end tidal to arterial partial pressure difference.
There is an excellent graph of expired C02 in “Physiology for the Anaesthetist” by Power
and Kam which helps understand alveolar dead space.
Syllabus S2g B1g2
Reference: Power and Kam 1st edition p 84-88