College Answer

a)
Acute respiratory acidosis

At 38 weeks pregnancy the normal PaCO2 is <30 mmHg with a compensatory reduction in bicarbonate. The blood gases therefore indicate acute CO2 retention probably due to pain and narcotics. 
In the non-pregnant patient these results would indicate an uncompensated normal anion gap metabolic acidosis.

b)
A-a gradient – this is raised at 154 mmHg, suggesting shunt and/or V/Q mismatch. Possible explanations are the loss of FRC after abdominal surgery, segmental collapse/consolidation or aspiration

c)
Hb and WCC – the mild anaemia is physiological in pregnancy and the neutrophil leukocytosis is a normal feature during labour and early post-partum.

Discussion

This question closely resembles Question 18 from the first paper of 2006.

Instead of trying to justify why one identical question ended up in the O&G section and the other among the acid-base questions, I will instead focus on the acid-base and gas exchange abnormalities, for a second forgetting that the patient is pregnant. The obstetric flavour is dealt with in the other version of this discussion section.

For b)

There is a mild respiratory acidosis. The normal CO₂ of late pregnancy is around 30mmHg, which is generally sustained with a bicarbonate of 20. In this scenario the bicarbonate has not changed, and the CO₂ is elevated by 12mmHg.

That makes one think: is the pH influenced by any other disorder? One of the standard equations comes to mind. Every 1mmHg of change in Pa CO₂ leads to a 0.008 change in pH. The use of the standard equation yields an expected pH of 7.304 for this 12mmHg change in CO₂- very close to the measured pH (7.31)

The anion gap is normal if you calculate it without the potassium. It is 15.3 with potassium included, trivially elevated (by 3.3).

Now, as for the A-a gradient:

A-a gradient = (FiO2 x (760-47) - (PCO2 x 1.25) - PaO2

The humidity is always 100% so the left side of the equation is always FiO2 x 713.

So: (0.5 x 713) - (42 x 1.25)

Thus, the alveolar oxygen concentration is 303 mmHg, and the A-a gradient is 153 (303 - 150)

Given that there is a history of abdominal surgery and pregnancy, the causes of this could be
- Post-operative atelectasis 
- Aspiration 
- pulmonary thromboembolism or amniotic fluid embolism

for c), I note that a 15% drop in Hb is normal in pregnancy.

Normal acid-base changes in pregnancy are discussed elsewhere.