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]

Branch 2 — The anion gap and the differential (expected answer)

The gap of 32 is high, meaning an unmeasured acid is present. The differential of a high-gap acidosis is KULT — ketones, uraemia, lactate, toxins. Given the ketotic breath and the glucose of 24, this is diabetic ketoacidosis. The metabolic acidosis of DKA is driven by ketoacids (beta-hydroxybutyrate and acetoacetate) that consume bicarbonate and widen the gap. [6]

Probe. "The examiner offers a delta ratio of 2.4 — what does it mean?" — A delta ratio above 2 means a metabolic alkalosis is layered on top of the high-gap acidosis. Here it would point to vomiting (volume contraction) compounding the DKA, which is common at presentation. [11]

Branch 3 — Corrected sodium and the fluid plan (expected answer)

The measured sodium of 128 is falsely low because the high glucose draws water into the vascular space. The corrected sodium is 128 plus 1.6 times the glucose excess divided by 5.5: 1.6 times (24 minus 5.5) over 5.5, which is about 5.4, giving a corrected sodium of about 133. The fluid plan begins with 10 to 20 mL per kilogram of 0.9 per cent saline to restore perfusion, then isotonic fluids, and insulin at 0.05 to 0.1 unit per kilogram per hour after the bolus. [1] [7]

Probe. "Why does the corrected sodium matter during treatment?" — A falling corrected sodium as the glucose falls is an early warning of cerebral oedema; the fluid plan is built on the corrected value, and a rapid fall demands slowing the fluids. [7]

Branch 4 — Cerebral oedema and the limits of bicarbonate (expected answer)

Cerebral oedema is the feared complication of paediatric DKA, driven by rapid fluid shifts and the osmotic gradient between plasma and brain. Watch for headache, drowsiness, bradycardia and a falling conscious level; treat with mannitol or hypertonic saline, slowed fluids, head-up positioning and airway protection. Bicarbonate is not routine, because it lowers the cerebrospinal fluid pH, generates carbon dioxide, and does not address the ketoacids; it is reserved for a pH below 7 with haemodynamic collapse. [6]

Probe. "The candidate proposes an insulin bolus — why is that wrong?" — An insulin bolus drives fluid shifts and potassium into the cell too fast and is associated with cerebral oedema; the standard is an infusion at 0.05 to 0.1 unit per kilogram per hour after the fluid bolus. [7]

Examiner's wrap

The five-step method, applied the same way every time, is what catches the mixed disorder and holds the interpretation together. Hold Winter's formula, the KULT differential, the corrected sodium, and the structured DKA protocol, and treat the child rather than the number. A venous or capillary gas tracks the trend well; reserve the arterial stick for oxygenation. [4] [11]