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

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College Answer

The main points for a pass were


Patient factors:
• Normal value for the difference between end tidal and arterial C02
• Physiological factors e.g. alveolar dead space, failure to plateau of the capnograph
trace
• Pathological factors e.g. cardiac arrest, air embolism, asthma.

Equipment factors:
• Leaks
• Occlusion
• Sampling site errors etc

A common mistake was to state that dead space caused the difference between end tidal and
arterial carbon dioxide but not which type of dead space. The Bohr equation was not
required.