Question 4

Describe the underlying principles involved in the measurement of end  tidal CO2 (by infrared analysis), including sources of error and interference

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College Answer

Candidates were expected to at least mention and describe the following points –
absorption at the infrared spectrum; Beer-Lambert Law and it’s relevance to 
measurement of ETCO2; sources of error (effect of other gases, atmospheric 
pressure), sources of interference (gas sampling methods, heating), calibration, 
features of the sampling chamber that may cause error (glass construction, size), 
sampling rates, etc.
Answers provided by candidates lacked breadth and depth, indicated that there was 
generally a poor understanding of this topic. The usual mistake was to discuss the 
clinical reasons behind the reading not reflecting the PaCO2 rather than sources of 
error of the end tidal CO2. Candidates should review this topic from the references 
from which the answer was sought and texts included as recommended reading.