Describe a set of arterial blood gases in a pregnant woman at term and the reasons for these values.

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College Answer

For a good answer candidates were expected to describe the respiratory alkalosis 
and metabolic compensation associated with pregnancy. Candidates were expected 
to write a set of arterial blood gases showing a compensated respiratory alkalosis 
with a normal to slightly high P02. Values within +/- 5% of those expected, and found, 
in listed references would have scored candidates marks. 

Candidates were then expected to mention that the PaO2 is high despite a 20% 
increase in oxygen consumption (relate that to increase in alveolar ventilation, 
decrease in PaCO2, and alveolar gas equation; PaCO2 is low due to tidal volume 
increase by 35%, Although anatomical dead space increases VD/VT in unchanged, 
despite CO2 production increased by increased basal metabolic rate; HCO3
decreases because of increased renal excretion of HCO3 due to inhibition of renal 
secretion of hydrogen ions and ammonium; base deficit reflects the renal loss of 
HCO3. 
Some candidates described a low maternal P02, none commented on how the 
maternal P02 and PC02 enhance gas exchange to the foetus. While electrolyte 
values are often measured on arterial blood gas analysis they do not form part of an 
arterial blood gas and so comments on electrolytes gained no marks. The double 
Bohr and double Haldane effects were not required to answer this question. Once 
again, this question highlighted the importance of a structured approach to the 
answer, thus enhancing a candidates opportunity to cover all key areas and put their 
knowledge across.
Syllabus: O1, 2a
References: Nunn’s Applied Respiratory Physiology, Lumb, 6th edition, Chp 14