Five steps to the diagnosis — blood gas interpretation in a sick child

Structured clinical encounter — interpretation and management leadership

This station tests whether the candidate interprets a blood gas methodically, defends each step, and leads a cerebral-oedema-safe plan for diabetic ketoacidosis. Marks reward the five-step order, Winter's formula, the anion gap, the corrected sodium, and the structured fluid and insulin protocol. [4] [11]

Stem

A three-year-old is brought in breathing deeply and rapidly, drowsy but rousable, with the smell of ketones on the breath. The capillary gas shows pH 7.07, PaCO2 14 mmHg, bicarbonate 4 mmol per litre, base excess negative twenty-four, sodium 132, chloride 96, potassium 5.4 and glucose 24 mmol per litre. The team looks to you for the interpretation and the plan. [6]

Candidate tasks

1. Interpret the gas using the five-step method aloud (3 minutes). Name the primary disturbance (acidaemia, metabolic acidosis), judge the respiratory compensation with Winter's formula, and calculate the anion gap. [11]
2. Name the diagnosis and defend the choice of sample (2 minutes). The high gap with ketones and hyperglycaemia is DKA; defend why a capillary gas is adequate for the trend but not for oxygenation. [4] [6]
3. State the corrected sodium and why it matters (2 minutes). Correct the sodium for hyperglycaemia (measured plus 1.6 mmol per litre per 5.5 mmol per litre of glucose above normal) and explain that a falling corrected sodium warns of cerebral oedema. [1]
4. Lead the structured fluid and insulin plan (3 minutes). Give 10 to 20 mL per kilogram of 0.9 per cent saline to restore perfusion, start insulin at 0.05 to 0.1 unit per kilogram per hour after the bolus, replace potassium as it falls, and add dextense when the glucose reaches 14 to 17. [7]
5. Anticipate cerebral oedema and the limits of bicarbonate (2 minutes). Name the warning signs (headache, drowsiness, bradycardia), the treatment (mannitol or hypertonic saline, slowed fluids, head-up positioning), and explain why bicarbonate is reserved for a pH below 7 with haemodynamic collapse. [6] [7]

Examiners' discussion points

• Why five steps in order? Because a glance at the pH misses a mixed disorder; the compensation step and the anion gap catch what the pH hides. [11]
• Defend Winter's formula. Expected PaCO2 equals 1.5 times bicarbonate plus 8, within plus or minus 2. A measured PaCO2 of 14 with a bicarbonate of 4 gives an expected 14, so the compensation is appropriate. [11]
• The anion gap. 132 minus 96 plus 4 equals 32, which is high. KULT — ketones, uraemia, lactate, toxins — and the ketotic breath with glucose of 24 confirms DKA. [6]
• The corrected sodium. Measured 128 is false; the corrected value of about 133 to 134 drives the fluid plan, and a fall during treatment warns of cerebral oedema. [1]

Marking grid (out of 20)