Acid-Base Disorders: A Step-by-Step Approach for Boards and Wards
Acid-base questions are a guaranteed component of every major exam — and they reward students who have a systematic approach. Memorize the five steps, learn the compensation formulas, and these questions become free points.
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The 60-second version
pH <7.35 = acidosis; pH >7.45 = alkalosis
Anion gap = Na − (Cl + HCO3); normal 8–12 mEq/L
MUDPILES for elevated anion gap metabolic acidosis
Step 1: Is the pH acidic (<7.35) or alkalotic (>7.45)? Step 2: Look at PaCO2 — is it elevated (respiratory acidosis) or low (respiratory alkalosis)? Step 3: Look at HCO3 — is it elevated (metabolic alkalosis) or low (metabolic acidosis)? Step 4: Is there appropriate compensation? Step 5: If metabolic acidosis, calculate the anion gap (Na − [Cl + HCO3], normal 8–12 mEq/L).
Metabolic acidosis (Winter's formula): expected PaCO2 = 1.5(HCO3) + 8 ± 2. Metabolic alkalosis: expected PaCO2 = 0.7(HCO3) + 21 ± 2. Respiratory acidosis (acute): HCO3 increases 1 per 10 CO2 increase. Respiratory acidosis (chronic): HCO3 increases 3.5 per 10 CO2 increase. If measured value differs significantly from expected, a second disorder is present.
Classic Vignette Patterns
Salicylate toxicity: mixed respiratory alkalosis + anion gap metabolic acidosis (direct stimulation of respiratory center + organic acid production). Vomiting: metabolic alkalosis (loss of HCl). Diarrhea: normal anion gap metabolic acidosis (loss of HCO3). COPD exacerbation: respiratory acidosis with chronic compensation (elevated HCO3). Anxiety/hyperventilation: respiratory alkalosis.
Test yourself
Quick check — 3 questions
1. ABG shows pH 7.28, PaCO2 20, HCO3 9. Anion gap = 22. Winter's formula gives expected PaCO2 = 1.5(9)+8 = 21.5. What is the diagnosis?
Explanation. The measured PaCO2 (20) matches Winter's formula prediction (21.5 ± 2). This is a pure elevated anion gap metabolic acidosis with appropriate respiratory compensation — no second disorder.
2. A patient presents with confusion after ingesting antifreeze. ABG: pH 7.18, PaCO2 28, HCO3 10. Anion gap = 26. Which category does this fall into?
Explanation. Ethylene glycol (antifreeze) is in MUDPILES — it causes an elevated anion gap metabolic acidosis via glycolate and oxalate production. The elevated AG (26 vs normal 8–12) confirms this.
3. A patient with bulimia presents with pH 7.52, PaCO2 48, HCO3 38. What is the primary disorder?
Explanation. The pH is alkalotic. HCO3 is elevated (primary driver) and PaCO2 is elevated (appropriate respiratory compensation — hypoventilation to retain CO2). This is metabolic alkalosis, classically from repeated vomiting with loss of HCl.
Frequently asked questions
What are the most common causes of metabolic alkalosis?
The two dominant causes are vomiting/NG suction (loss of HCl, leaving bicarbonate behind) and diuretic use (furosemide, thiazides cause urinary loss of Cl and volume contraction). Less common: primary hyperaldosteronism (Conn's syndrome), Cushing's syndrome, Bartter/Gitelman syndromes.
How do you detect a mixed acid-base disorder?
Calculate the expected compensation and compare it to the actual measured value. If PaCO2 is higher than predicted by Winter's formula in metabolic acidosis, there's also a respiratory acidosis. If HCO3 is lower than expected in respiratory alkalosis, there's also a metabolic acidosis (classic in salicylate toxicity).
What is the delta-delta ratio and when do you use it?
The delta-delta (ΔΔ) ratio = (measured AG − 12) / (24 − measured HCO3). Used when you have an elevated AG metabolic acidosis to detect a hidden metabolic alkalosis or normal AG acidosis. Ratio <1 suggests concurrent normal AG acidosis; ratio >2 suggests concurrent metabolic alkalosis.
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