Mixed Acid Base Disorder

A mixed acid-base disorder is the presence of two or more primary disturbances at the same time. These are common in seriously ill patients and are frequently missed, because the pH may look normal or only mildly deranged while two significant opposing processes are in play.

Recognising them matters because each component may need separate treatment, and because a "normal" pH in a sick patient can be falsely reassuring.

Classic combinations include: the vomiting patient with diabetic ketoacidosis (metabolic acidosis plus metabolic alkalosis), salicylate poisoning (respiratory alkalosis plus raised anion gap metabolic acidosis), the exhausted asthmatic (respiratory acidosis in a patient who should be hyperventilating), severe sepsis with hepatic dysfunction, and the COPD patient on diuretics (respiratory acidosis plus metabolic alkalosis).

A normal pH does not exclude significant acid-base derangement, and this is the single most useful principle to carry into gas interpretation.

A systematic approach is what makes mixed disorders visible.

1. Assess pH: acidaemic, alkalaemic, or normal 2. Identify the primary disorder from the direction of PaCO₂ and bicarbonate 3. Check whether compensation is appropriate, using the expected formulae. Compensation that is inadequate or excessive indicates a second primary disorder, this is the step at which mixed disorders are usually caught:

• Metabolic acidosis: expected PaCO₂ = (1.5 × HCO₃) + 8 ± 2 (Winter's formula)
• Metabolic alkalosis: PaCO₂ rises roughly 0.7 mmHg per 1 mmol/L rise in HCO₃
• Acute respiratory acidosis: HCO₃ rises roughly 1 mmol/L per 10 mmHg rise in PaCO₂
• Chronic respiratory acidosis: HCO₃ rises roughly 3.5–4 mmol/L per 10 mmHg
• Acute respiratory alkalosis: HCO₃ falls roughly 2 mmol/L per 10 mmHg fall in PaCO₂
• Chronic respiratory alkalosis: HCO₃ falls roughly 4–5 mmol/L per 10 mmHg

4. Calculate the anion gap, corrected for albumin 5. Calculate the delta ratio where the anion gap is raised: (measured AG − 12) ÷ (24 − measured HCO₃)

• Below 0.4 suggests a pure normal anion gap acidosis
• 0.4–0.8 suggests a mixed raised and normal anion gap acidosis
• 1–2 suggests a pure raised anion gap acidosis
• Above 2 suggests a coexisting metabolic alkalosis, or a pre-existing chronic respiratory acidosis with a compensatory high bicarbonate

Remember that compensation never fully corrects the pH and never overshoots; an apparently over-compensated picture is a second disorder, not vigorous compensation.

• Treat each component according to its own cause; there is no treatment for a mixed disorder as such
• Prioritise by clinical threat rather than by biochemical magnitude: the exhausted asthmatic with a rising PaCO₂ needs airway and ventilatory support regardless of what the metabolic component is doing, and a patient in shock needs perfusion restored before anything else
• Beware the normal pH masking severe derangement. A patient with a pH of 7.38, a PaCO₂ of 20 and a bicarbonate of 12 is profoundly unwell despite a normal-looking pH, and the number that should prompt action is the bicarbonate and the work of breathing, not the pH
• Watch specifically for the patient whose respiratory compensation is failing: a PaCO₂ that is rising toward normal in a severely acidotic patient signals exhaustion and impending arrest
• Repeat gases serially, since mixed disorders evolve as treatment takes effect and as the patient tires or improves
• Consider salicylate poisoning specifically in any unexplained mixed respiratory alkalosis with metabolic acidosis, since it is treatable and rapidly fatal if missed
• Correct electrolytes alongside, particularly potassium, which shifts as pH changes

Referral: critical care for any patient with a significant mixed disorder and clinical instability, failing compensation, or diagnostic uncertainty; toxicology where poisoning is suspected.