Diabetic ketoacidosis — the full blood gas interpretation

(a) Classify the acid-base disorder (3 marks).

• pH 7.08 = severe acidaemia (under 7.35).
• Apply the direction rule: pH is low (acidosis) and PaCO2 is low (2.8 kPa) — they move in the same direction, so the primary disorder is metabolic acidosis. The low PaCO2 is the compensatory hyperventilation (Kussmaul breathing).
• HCO3 8 and base excess minus 20 confirm a severe metabolic acidosis of metabolic origin.
• Diagnosis so far: severe metabolic acidosis, almost certainly a high-anion-gap metabolic acidosis given the ketones and the lactate.^[10]

(b) Anion gap and delta ratio (4 marks).

• Anion gap = Na minus (Cl plus HCO3) = 132 minus (96 plus 8) = 132 minus 114 = 28 mmol per litre. Normal is 8 to 12, so this is a raised anion gap (high-anion-gap metabolic acidosis, HAGMA), driven here by ketones (5.8 mmol per litre) and a moderate lactate (4.2 mmol per litre). The mnemonic GOLD MARK applies: ketoacidosis.
• Delta ratio = (AG minus 12) divided by (24 minus HCO3) = (28 minus 12) divided by (24 minus 8) = 16 divided by 16 = 1.0. A ratio between 1 and 2 is a pure HAGMA — the fall in bicarbonate is fully accounted for by the rise in the anion gap.
• Corrected bicarbonate = HCO3 plus (AG minus 12) = 8 plus 16 = 24, within the normal range, confirming no coexisting metabolic alkalosis (which vomiting might have produced) and no coexisting normal-anion-gap acidosis.
• Interpretation: a single primary disorder — high-anion-gap metabolic acidosis from diabetic ketoacidosis — with no mixed metabolic process.

(c) Winter's formula and the compensation (3 marks).

• Expected PaCO2 = 1.5 times HCO3 plus 8, plus or minus 2 mmHg = 1.5 times 8 plus 8 = 20 mmHg, range 18 to 22 mmHg (2.4 to 2.9 kPa).
• The measured PaCO2 is 2.8 kPa (about 21 mmHg), which falls within the predicted range — the respiratory compensation is appropriate, and there is no coexisting respiratory disorder.
• If the measured PaCO2 had been higher than predicted, a coexisting respiratory acidosis would be present (for example exhaustion or sedation); lower would indicate a coexisting respiratory alkalosis (for example salicylate coingestion or fever). Neither is present here.^[1]

(d) Gas-driven management in the first hour (5 marks).

• Airway, breathing, circulation first — he is drowsy (GCS to be recorded); protect the airway and give high-flow oxygen if hypoxic.^[11]
• Fluid resuscitation — normal saline 0.9 percent, 1 to 1.5 litres in the first hour (15 to 20 mL per kg per hour absent cardiac compromise), then rate-guided by hydration, electrolytes and urine output; he is volume-depleted.^[6]
• Insulin — fixed-rate intravenous insulin infusion at 0.1 units per kg per hour after initial fluid resuscitation; monitor glucose, ketones, bicarbonate and potassium hourly for the response.^[6]
• Potassium — the hyperkalaemia will fall with insulin and fluids; start replacement (20 to 30 mmol potassium per litre of infusion fluid) once the level falls below 5.0 to 5.2, and delay or hold insulin until potassium is restored above 3.3.^[6]
• Treat the cause — search for the precipitant (infection, infarct, non-compliance); blood cultures, urinalysis, chest radiograph, troponin, ECG.
• Avoid bicarbonate here — the pH is 7.08, but he has a treatable cause (DKA) and the consensus reserves bicarbonate for a pH under 6.9 (100 mmol sodium bicarbonate in 400 mL sterile water with 20 mmol KCl over 2 hours until venous pH exceeds 7.0); the BICAR-ICU trial showed no overall outcome benefit from bicarbonate in severe acidaemia, with benefit confined to the acute-kidney-injury stratum.^[6][7]
• Monitoring — hourly ketones, glucose, potassium, and venous gas; the venous gas is sufficient for monitoring DKA and avoids repeated arterial puncture.^[8]
• Disposition — HDU or ICU depending on severity; a GCS under 12, pH under 7.1, or a potassium derangement warrants senior review.