To revisit our problem –

On room air – serum bicarbonate is 44

Clearly the patient has a metabolic alkalosis with respiratory compensation. The pO2 appears low, but …

Go to an A-a gradient calculator – and I calculate an A-a gradient of 20. While this was slightly elevated, I do not believe that it was clinically significant.

This patient had prolonged vomiting and volume contraction. This was apparently a classic volume contraction metabolic alkalosis. I present this patient because the pO2 suggests a pulmonary problem, and yet when one applies the A-a gradient calculator, that seems unlikely.

The lesson that I stress often on rounds is that unless sometimes the A-a gradient provides very valuable information.

While ABGs are painful, in cases like this one, they provide important information that pulse oximetry alone does not provide.