A 72-year-old woman with severe diarrhea for 10 days is brought to the hospital. Laboratory findings show: pH 7.32, PaCO₂ 32 mmHg, HCO₃⁻ 16 mEq/L, anion gap 10 mEq/L. What is the most common cause of the primary acid-base disturbance in this patient?

Explanation

## Acid-Base Interpretation ### Primary Disturbance Identification The patient has **metabolic acidosis** (pH < 7.35, HCO₃⁻ < 22 mEq/L) with a **normal anion gap** (10 mEq/L; normal 8–16). The low PaCO₂ (32 mmHg) represents appropriate respiratory compensation. ### Anion Gap Calculation $$\text{Anion Gap} = [Na^+] - ([Cl^-] + [HCO_3^-])$$ Normal anion gap metabolic acidosis indicates **hyperchloremic acidosis**, which occurs when bicarbonate is lost without accumulation of unmeasured anions. ### Most Common Cause: Diarrhea **Key Point:** Diarrhea causes **loss of bicarbonate in stool**. The colon normally reabsorbs HCO₃⁻ and secretes Cl⁻ in exchange. When diarrhea occurs, this compensatory mechanism is overwhelmed, leading to net bicarbonate wasting and hyperchloremic metabolic acidosis. ### Mechanism of Bicarbonate Loss in Diarrhea ```mermaid flowchart TD A[Diarrhea]:::outcome --> B[Increased intestinal transit]:::outcome B --> C[Reduced HCO₃⁻ reabsorption in colon]:::outcome C --> D[Increased Cl⁻ reabsorption to maintain electroneutrality]:::outcome D --> E[Net loss of HCO₃⁻ in stool]:::outcome E --> F[Hyperchloremic metabolic acidosis]:::outcome F --> G[Normal anion gap]:::outcome ``` ### Why Normal Anion Gap? **Clinical Pearl:** When bicarbonate is lost, chloride is retained to maintain electroneutrality and osmolarity. This creates a 1:1 exchange of HCO₃⁻ for Cl⁻, keeping the anion gap normal. If unmeasured anions were accumulating, the anion gap would be elevated. ### Respiratory Compensation The low PaCO₂ (32 mmHg) is the appropriate respiratory response. Using Winter's formula: $$\text{Expected PaCO₂} = 1.5 \times [HCO_3^-] + (±2) = 1.5 \times 16 ± 2 = 22–26 \text{ mmHg}$$ The actual PaCO₂ of 32 mmHg is *higher* than expected, suggesting a concurrent **respiratory acidosis** (either primary or from fatigue/altered mental status in an elderly patient). However, the primary disturbance is metabolic acidosis from bicarbonate loss. **High-Yield:** Diarrhea → hyperchloremic metabolic acidosis (normal anion gap). Vomiting → metabolic alkalosis (loss of H⁺ and Cl⁻). This distinction is critical for NEET PG. **Mnemonic:** HARDUPS causes of normal anion gap metabolic acidosis: - **H**yperalimentation (TPN) - **A**cetazolamide (carbonic anhydrase inhibitor) - **R**TA (renal tubular acidosis) - **D**iarrhea ← **Most common** - **U**reteral diversions - **P**ancreatic fistula - **S**aline administration [cite:Harrison 21e Ch 48]