Describe the factors that affect the partial pressure of CO2 in mixed venous blood.
It was expected candidates would define key concepts, particularly 'mixed venous'. Many
candidates knew some of the elements that contributed to mixed venous PCO2 but few
described all of the main factors. There was little mention of tissue capillary flow as a factor
affecting mixed venous CO2
This question is essentially identical to Question 7 from the first paper of 2011. However, here the comments are cold and abrupt, whereas in 2011 the college gave a full detailed breakdown of their expectations, even though they only wanted us to "briefly describe" this thing. On both occasions, the candidates were caught unprepared, and it was a blood bath. The pass rate for this SAQ in 2011 was 8%.
VCO2 = CO × k × (PvCO2 - PaCO2)
where
- VCO2 is the rate of CO2 production,
- CO is the cardiac output,
- PvCO2 - PaCO2 is the arteriovenous CO2 difference, and
- k is a coefficient used to describe the near-linear relationship between CO2 content and partial pressure in the blood.
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Kandel, Gabor, and Arnold Aberman. "Mixed venous oxygen saturation: its role in the assessment of the critically ill patient." Archives of internal medicine 143.7 (1983): 1400-1402.
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