D. Pulmonary embolus – Explanation

This woman has become acutely breathless from a pulmonary emboli. She is
hypoxic and, as a reflex to this, is hyperventilating (as evidenced by the low
PaCO2). As a result, she has developed an alkalosis.

The other options are much less likely to be the answer:

Diabetic ketoacidosis and methanol overdose → both causes acidosis. The
scenario that was given is alkalosis

Panic attacks → This does cause acute alkalosis via hyperventilation (and
therefore low PaCO2 and a high pH), but tends to happen in the absence of
hypoxia rather than as a response to it (as in pulmonary embolism).

Severe vomiting → causes a metabolic alkalosis (i.e. a high pH with a high
HCO3–). PaO2 is not likely to decrease

To understand this question, we have to go back to basics.

First, is the patient hypoxic?
– the Pa02 >10 kPa is normal

Second, is the patient acidaemic (pH <7.35) or alkalaemic (pH >7.45)

Third, what is the respiratory component: What has happened to the PaCO2?
– PaCO2 > 6.0 kPa suggests a respiratory acidosis (or respiratory compensation
for a metabolic alkalosis)
– PaCO2 < 4.7 kPa suggests a respiratory alkalosis (or respiratory compensation
for a metabolic acidosis)

Fourth, what is the metabolic component: What is the bicarbonate level/base
excess?
– bicarbonate < 22 mmol/l suggests a metabolic acidosis (or renal compensation
for a respiratory alkalosis)
– bicarbonate > 26 mmol/l suggests a metabolic alkalosis (or renal compensation
for a respiratory acidosis)

In summary
She has developed respiratory alkalosis with very minimal renal compensation
taking into account that renal compensation usually takes around 48 hours to
manifest.

ARTERIAL BLOOD GASES, CAUSES