mixed acid-base disorder

**Definition:** Two or more simultaneous primary acid-base processes. Note: compensation is NOT a second disorder. A mixed disorder is present only when the observed compensatory response differs from the expected one. A single patient can have at most one respiratory disorder (acidosis or alkalosis, since the two cannot coexist) but can have up to two or three metabolic disorders, e.g., triple disorder: AG metabolic acidosis + non-gap metabolic acidosis/metabolic alkalosis + respiratory disorder.

**Stepwise approach (high-yield):** 1. Check pH: acidemia (<7.35) or alkalemia (>7.45). 2. Identify the primary process: low HCO3- with low pH = metabolic acidosis; high PaCO2 with low pH = respiratory acidosis; high HCO3- with high pH = metabolic alkalosis; low PaCO2 with high pH = respiratory alkalosis. 3. Check compensation: - Metabolic acidosis: Winter's formula, expected PaCO2 = 1.5 x [HCO3-] + 8 (±2). Measured PaCO2 higher than expected = concomitant respiratory acidosis. Lower than expected = concomitant respiratory alkalosis. (Rough check: PaCO2 should approximate the last two digits of pH.) - Metabolic alkalosis: PaCO2 rises ~0.7 mmHg per 1 mEq/L rise in HCO3-. - Respiratory acidosis: acute, HCO3- rises 1 per 10 mmHg PaCO2 increase; chronic, rises 3.5-4 per 10 mmHg. - Respiratory alkalosis: acute, HCO3- falls 2 per 10 mmHg PaCO2 decrease; chronic, falls 4-5 per 10 mmHg. 4. Calculate the anion gap (AG = Na+ - Cl- - HCO3-; normal ~8-12, with albumin correction: add 2.5 per 1 g/dL albumin below 4). 5. If AG is elevated, calculate the delta-delta (delta gap): ΔAG/ΔHCO3- = (AG - 12)/(24 - HCO3-). - Ratio <1 (about <0.4 to 0.8): concurrent non-gap (hyperchloremic) metabolic acidosis. - Ratio 1-2: pure AG metabolic acidosis. - Ratio >2: concurrent metabolic alkalosis (or pre-existing chronic respiratory acidosis with elevated baseline HCO3-).

**Classic examples:** - **Salicylate toxicity:** respiratory alkalosis (direct medullary stimulation) + AG metabolic acidosis (uncoupled oxidative phosphorylation, lactate/ketones). Adults classically present with this mix; children more often have predominant AG acidosis. - **Vomiting + diarrhea/laxative or diuretic use:** metabolic alkalosis + non-gap or gap acidosis. - **DKA with vomiting:** AG acidosis + metabolic alkalosis (delta ratio >2). - **COPD exacerbation with sepsis/lactic acidosis:** respiratory acidosis + AG metabolic acidosis (PaCO2 higher than Winter's predicts). - **Sepsis:** lactic acidosis + respiratory alkalosis. - **Cardiac arrest:** respiratory acidosis (hypoventilation) + lactic metabolic acidosis, causing profound acidemia. - **Cirrhosis:** chronic respiratory alkalosis + metabolic alkalosis (diuretics) or lactic acidosis. - **Renal failure + vomiting:** AG acidosis + metabolic alkalosis. - **Diuretics + hypokalemia:** metabolic alkalosis.

**Clinical significance:** - Opposing disorders (e.g., acidosis + alkalosis) may result in a near-normal pH despite markedly abnormal HCO3- and PaCO2. This is a clue to a mixed disorder. A normal pH with abnormal HCO3-/PaCO2 essentially always indicates a mixed disorder. - Additive disorders (e.g., respiratory acidosis + metabolic acidosis) lead to severe acidemia and are life-threatening. - Management: treat each underlying cause (e.g., fluids and insulin for DKA, ventilation for hypercapnia, antiemetics for vomiting). Avoid empirically giving bicarbonate without identifying the cause.

**Board pearls:** - Always compute Winter's formula and the delta ratio when the AG is elevated, as these are the most common tested calculations. - Metabolic acidosis AG mnemonic: MUDPILES or GOLDMARK. - Compensation never fully corrects the pH (exception: chronic respiratory alkalosis may nearly normalize it). - Respiratory compensation for metabolic disorders is rapid (minutes to hours). Renal compensation for respiratory disorders takes 3-5 days. - Check urine chloride in metabolic alkalosis: <20 mEq/L = saline-responsive (vomiting, diuretics already stopped); >20 = saline-resistant (hyperaldosteronism, Bartter/Gitelman, active diuretic use).