A 28-year-old woman from Delhi presents with a 3-day history of severe vomiting following food poisoning. On examination, she is alert but appears dehydrated. Vital signs: BP 100/62 mmHg, HR 110/min, RR 18/min. Laboratory findings: pH 7.52, PaCO₂ 48 mmHg, HCO₃⁻ 38 mEq/L, Na⁺ 138 mEq/L, K⁺ 2.8 mEq/L, Cl⁻ 92 mEq/L. What is the primary acid-base disturbance?

Explanation

## Analysis of Acid-Base Status ### Step 1: Identify the Primary Disorder - pH = 7.52 → **alkalemia** (normal 7.35–7.45) - HCO₃⁻ = 38 mEq/L → **elevated** (normal 22–26 mEq/L) - PaCO₂ = 48 mmHg → **elevated** (normal 35–45 mmHg) **Key Point:** When pH is elevated AND HCO₃⁻ is elevated, the primary disorder is **metabolic alkalosis**. The elevated PaCO₂ represents the expected respiratory compensation (hypoventilation to retain CO₂ and buffer the alkalemia). ### Step 2: Assess Respiratory Compensation Using the Standard Formula for Metabolic Alkalosis In metabolic alkalosis, the respiratory system hypoventilates (retains CO₂) to partially correct the elevated pH. **Expected PaCO₂** using the compensation formula for metabolic alkalosis: $$\text{Expected PaCO}_2 = 0.7 \times [\text{HCO}_3^-] + 21 \pm 2$$ $$= 0.7 \times 38 + 21 \pm 2 = 26.6 + 21 \pm 2 = 47.6 \pm 2 = 45.6–49.6 \text{ mmHg}$$ **Observed PaCO₂** = 48 mmHg, which falls **squarely within** the expected compensatory range (45.6–49.6 mmHg). This confirms **appropriate respiratory compensation** — there is NO concurrent respiratory acidosis. **High-Yield:** When the observed PaCO₂ falls within the expected compensatory range, there is NO second acid-base disorder. The diagnosis is simply **metabolic alkalosis with appropriate respiratory compensation** (Option D). ### Step 3: Clinical Context - Severe vomiting → loss of HCl → **metabolic alkalosis** ✓ - Dehydration → **contraction alkalosis** (volume depletion maintains alkalosis via aldosterone-driven HCO₃⁻ reabsorption) ✓ - Hypokalemia (K⁺ 2.8 mEq/L) → promotes H⁺ shift intracellularly, worsening alkalosis ✓ - Elevated PaCO₂ = expected hypoventilatory compensation ✓ **Clinical Pearl (Harrison's Principles of Internal Medicine, 21st ed.):** In metabolic alkalosis, respiratory compensation is achieved by hypoventilation, raising PaCO₂. The expected PaCO₂ = 0.7 × [HCO₃⁻] + 21 ± 2. A PaCO₂ within this range confirms appropriate compensation — NOT a concurrent respiratory acidosis. Option B (metabolic alkalosis with concurrent respiratory acidosis) would only be correct if PaCO₂ were clearly and significantly above the expected compensatory range (e.g., >50 mmHg). ### Diagnosis **Metabolic alkalosis with appropriate respiratory compensation** — Option D. ## Comparison of Options | Feature | Option D (Correct) | Option B (Incorrect) | |---------|-------------------|---------------------| | Primary disorder | Metabolic alkalosis | Metabolic alkalosis | | PaCO₂ interpretation | Within expected range → appropriate compensation | Claims above expected range → concurrent respiratory acidosis | | Expected PaCO₂ | 45.6–49.6 mmHg | Same | | Observed PaCO₂ | 48 mmHg (within range) | 48 mmHg (incorrectly claimed outside range) | | Conclusion | Single disorder | Incorrectly labels as mixed |