A 52-year-old man from Delhi presents with a 6-month history of epigastric pain, heartburn, and regurgitation. He has been self-medicating with sodium bicarbonate antacid 4–5 times daily. On examination, he appears well, BP 128/82 mmHg, HR 88/min. Serum sodium is 148 mEq/L (normal 135–145), serum chloride is 92 mEq/L (normal 96–106), and serum bicarbonate is 32 mEq/L (normal 22–26). Arterial pH is 7.48. What is the most likely acid–base disturbance and the primary mechanism underlying his electrolyte abnormality?

Explanation

## Clinical Presentation & Pathophysiology This patient has developed **metabolic alkalosis with contraction alkalosis** from chronic sodium bicarbonate antacid abuse. ### Mechanism of Alkalosis **Key Point:** Sodium bicarbonate antacids neutralize gastric acid by the reaction: $$\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2$$ This removes H⁺ ions from the body and generates alkalemia. Chronic use leads to: 1. **Direct H⁺ loss** — bicarbonate neutralizes gastric acid, removing H⁺ from the body 2. **Chloride depletion** — HCl neutralization removes Cl⁻; the body excretes excess Na⁺ and HCO₃⁻ in urine 3. **Volume contraction** — loss of NaCl leads to extracellular fluid (ECF) contraction 4. **Contraction alkalosis** — the kidneys reabsorb HCO₃⁻ avidly in response to volume depletion, perpetuating alkalosis ### Laboratory Findings Explained | Parameter | Finding | Mechanism | |-----------|---------|----------| | **pH** | 7.48 (↑) | Metabolic alkalosis from HCO₃⁻ excess | | **HCO₃⁻** | 32 mEq/L (↑) | Excess bicarbonate from antacid | | **Na⁺** | 148 mEq/L (↑) | Hypernatremia from water loss and Na⁺ retention (contraction) | | **Cl⁻** | 92 mEq/L (↓) | Chloride depletion from HCl neutralization | **High-Yield:** The **low chloride level is the key diagnostic clue** for antacid-induced metabolic alkalosis. Chloride-responsive alkalosis (low urinary Cl⁻) is treated with saline replacement, not just stopping the antacid. ### Clinical Pearl **Milk-Alkali Syndrome** is the chronic form of this condition, characterized by: - Metabolic alkalosis - Hypercalcemia (from milk calcium absorption) - Renal dysfunction (from hypercalcemia) - Occurs with prolonged intake of absorbable antacids (calcium carbonate, sodium bicarbonate) + milk/dairy This patient is at risk of progressing to milk-alkali syndrome if he continues sodium bicarbonate and increases dairy intake. ### Why Sodium Bicarbonate Is Problematic **Warning:** Sodium bicarbonate is an **absorbable antacid** — it is absorbed into the bloodstream and causes systemic alkalosis, unlike non-absorbable antacids (aluminum hydroxide, magnesium hydroxide) which do not cause alkalosis. **Mnemonic: SCAMP** — Systemic alkalosis from absorbable antacids: - **S**odium bicarbonate - **C**alcium carbonate - **A**bsorbable - **M**ilk-alkali syndrome risk - **P**roton pump inhibitors (PPIs) are preferred for chronic use ## Management 1. **Stop sodium bicarbonate antacid** — switch to non-absorbable antacids (magnesium hydroxide, aluminum hydroxide) 2. **IV normal saline** — restores chloride and corrects contraction alkalosis 3. **PPI therapy** — omeprazole or pantoprazole for long-term GERD management (does not cause alkalosis) 4. **Monitor electrolytes** — recheck Na⁺, Cl⁻, HCO₃⁻, and pH after treatment [cite:KD Tripathi 8e Ch 49]