A 38-year-old woman with recurrent vomiting from gastric outlet obstruction is brought to the emergency department. Arterial blood gas analysis reveals pH 7.52, PaCO₂ 48 mmHg, HCO₃⁻ 38 mEq/L, and serum chloride 88 mEq/L. What is the most common cause of metabolic alkalosis in this patient?

Explanation

## Most Common Cause of Metabolic Alkalosis in Vomiting **Key Point:** Contraction alkalosis from combined volume depletion and chloride loss is the most common cause of metabolic alkalosis in vomiting, particularly gastric vomiting. ### Mechanism of Contraction Alkalosis **Mnemonic: CHIME** — Chloride, Hydrogen, loss from vomiting → Increased HCO₃⁻ reabsorption → Metabolic alkalosis → ECF contraction. 1. **Loss of HCl** in gastric fluid → direct loss of H⁺ and Cl⁻ 2. **Volume depletion** → activation of RAAS and sympathetic nervous system 3. **Increased proximal tubule HCO₃⁻ reabsorption** due to: - Reduced glomerular filtration rate (low GFR) - Increased angiotensin II (promotes HCO₃⁻ reabsorption) - Hypokalemia (promotes H⁺ secretion in collecting duct) 4. **Chloride depletion** prevents renal correction of alkalosis (chloride-responsive alkalosis) ### Chloride-Responsive vs. Chloride-Resistant Alkalosis | Feature | Chloride-Responsive | Chloride-Resistant | |---------|-------------------|-------------------| | **Mechanism** | Volume depletion + Cl⁻ loss | Mineralocorticoid excess or hypokalemia | | **Urine Cl⁻** | <10 mEq/L | >20 mEq/L | | **Treatment** | Normal saline (0.9% NaCl) | Treat underlying cause (K⁺, spironolactone) | | **Common causes** | Vomiting, diuretics, nasogastric suction | Primary hyperaldosteronism, Cushing's, hypokalemia | **High-Yield:** In this vignette, the **low serum chloride (88 mEq/L)** and **volume depletion** (from vomiting) indicate **chloride-responsive alkalosis**. This is contraction alkalosis and is the most common type in clinical practice. ### Clinical Features Supporting Contraction Alkalosis - Recurrent vomiting (loss of HCl) - Low serum chloride (88 mEq/L) — hallmark finding - Elevated HCO₃⁻ (38 mEq/L) - Elevated PaCO₂ (respiratory compensation) - Volume depletion (implied by vomiting) **Clinical Pearl:** Contraction alkalosis is **maintained** by chloride depletion and volume depletion. It will not resolve with fluid alone — **normal saline (0.9% NaCl)** is required to replete both volume and chloride. ### Why Other Options Are Wrong - **Excessive alkali ingestion:** Rare; would require deliberate ingestion of antacids or baking soda. No history provided. - **Primary hyperaldosteronism:** Causes chloride-resistant (not responsive) alkalosis; urine chloride would be >20 mEq/L, not low. Associated with hypertension, not vomiting. - **Diuretic use:** Would cause alkalosis, but no history of diuretic use is mentioned. Also causes chloride loss, but the primary insult here is gastric HCl loss. [cite:Harrison 21e Ch 280; KD Tripathi 8e Ch 50]