Blood gas, electrolyte and acid-base interpretation — branching viva

Branching viva — blood gas, electrolyte and acid-base interpretation

The examiner releases the stem and then branches into four probes. A strong candidate names the five steps first, applies Winter's formula without prompting, and defends the choice of sample and the cerebral-oedema risk in DKA. ^{[4] [11]}

Opening (examiner)

"A three-year-old arrives breathing deeply and rapidly with a ketotic breath. The capillary gas shows pH 7.07, PaCO2 14, bicarbonate 4, sodium 132, chloride 96 and glucose 24. Walk me through your interpretation." ^[6]

Branch 1 — The five-step method (expected answer)

Step one, the pH of 7.07 names a severe acidaemia. Step two, the PaCO2 of 14 is low, which is a compensatory respiratory alkalosis and not the driver. Step three, the bicarbonate of 4 is very low with the low pH, so the primary disturbance is a metabolic acidosis. Step four, Winter's formula gives an expected PaCO2 of 1.5 times 4 plus 8, equals 14, within the band of 12 to 16, so the compensation is appropriate and there is no second respiratory disorder. Step five, the anion gap is 132 minus 96 plus 4, equals 32, which is high. ^{[4] [11]}

Probe. "Is a capillary gas reliable here?" — For pH and bicarbonate, yes: a warmed capillary sample mirrors arterial pH and bicarbonate closely. For oxygenation, no: only an arterial sample measures PaO2. Here the question is the acid-base trend, so a capillary or venous gas is appropriate. ^[4]