IM23.8-12 | Acid Base Disorders and ABG Interpretation — Summary & Reflection

KEY TAKEAWAYS

The four primary acid-base disorders and their key features:

• Metabolic acidosis: low HCO₃⁻, pH <7.35. High AG (HAGMA — MUDPILES: DKA, lactic acidosis, uraemia, toxins) vs normal AG (diarrhoea, RTA, saline infusion). Apply Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. Calculate delta-delta: DDR <1 = concurrent NAGMA; DDR >2 = concurrent metabolic alkalosis.

• Metabolic alkalosis: high HCO₃⁻, pH >7.45. Causes: vomiting, diuretics, hyperaldosteronism. Maintained by Cl/volume depletion. Treat with 0.9% saline + KCl (chloride-responsive, urine Cl <20) or treat cause (chloride-resistant, urine Cl >20). Expected PaCO₂ = 24 + 0.7 × (HCO₃⁻ − 24).

• Respiratory acidosis: high PaCO₂, pH <7.35. Causes: COPD, opioids, NMJ disease, obesity. Acute compensation: HCO₃⁻ +1 per 10 mmHg CO₂. Chronic: +3.5 per 10 mmHg. Treat the cause; NIV for COPD; careful O₂ titration in chronic CO₂ retainers.

• Respiratory alkalosis: low PaCO₂, pH >7.45. Causes: anxiety, hypoxaemia (PE, pneumonia), sepsis, salicylates, pregnancy. Acute compensation: HCO₃⁻ −2 per 10 mmHg. Chronic: −5 per 10 mmHg.

Six-step approach: (1) pH direction; (2) identify primary disorder; (3) apply compensation formula — Winter's for metabolic acidosis; (4) anion gap (albumin-corrected); (5) delta-delta in HAGMA; (6) clinical correlation. A normal pH does not mean no disorder — opposite abnormalities in PaCO₂ and HCO₃⁻ = mixed disorder.

REFLECT

Return to the opening ABG. You now have the complete framework to work through it fully. pH 7.18 (severe acidaemia) → HCO₃⁻ 7 primary metabolic acidosis → Winter's formula: PaCO₂ expected 18.5 ± 2, measured 18 — appropriate compensation → AG = 138 − (98+7) = 33, markedly elevated → DDR = (33−12)/(24−7) = 21/17 = 1.24, pure HAGMA → clinical context: glucose 42 + ECG peaked T waves from hyperkalaemia = DKA. The entire diagnostic synthesis required six steps and approximately 90 seconds. Now reflect: which of these six steps would a physician skip if relying on intuition alone? Most commonly, they would skip Winter's formula and the delta-delta, and treat the DKA without noticing whether the respiratory compensation is adequate. Automaticity of this framework is built only by repeated practice — on paper, in simulation, and at the bedside. How many of the five cases in this module can you now work through correctly without looking at the analysis?