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
This question is virtually identical to Question 3 from the second paper of 2018 and Question 21 from the first paper of 2017. Interestingly, though the examiners here mention that "candidates who failed
did not discuss alveolar dead space", one must walk some sort of fine line here, because in 2018 "discussion of the various types of dead space did not score marks".
Satoh, Kenichi, et al. "Evaluation of differences between PaCO2 and ETCO2 by Age as measured during general anesthesia with patients in a supine position." Journal of Anesthesiology 2015 (2015).
Nunn, J. F., and D. W. Hill. "Respiratory dead space and arterial to end-tidal CO2 tension difference in anesthetized man." Journal of Applied Physiology 15.3 (1960): 383-389.
Shankar, Kodali Bhavani, et al. "Arterial to end tidal carbon dioxide tension difference during caesarean section anaesthesia." Anaesthesia 41.7 (1986): 698-702.
Fletcher, R., and B. Jonson. "Deadspace and the single breath test for carbon dioxide during anaesthesia and artificial ventilation: effects of tidal volume and frequency of respiration." British journal of anaesthesia 56.2 (1984): 109-119.
Donnellan, Michael E. "Capnography: Gradient PACO2 and PETCO2." Applied Technologies in Pulmonary Medicine. Karger Publishers, 2011. 126-131.
Kodali, Bhavani Shankar. "Capnography outside the operating rooms." Anesthesiology: The Journal of the American Society of Anesthesiologists 118.1 (2013): 192-201.