A 58-year-old man with hypertension and type 2 diabetes mellitus is scheduled for elective open cholecystectomy. Preoperative labs: Na⁺ 138 mEq/L, K⁺ 4.2 mEq/L, Cl⁻ 102 mEq/L, HCO₃⁻ 24 mEq/L, Cr 1.1 mg/dL. He has been fasting for 8 hours. Intraoperatively, 2 L of normal saline (0.9% NaCl) is infused over 2 hours. At the end of surgery (3 hours total), his urine output is 180 mL, and postoperative labs show Na⁺ 135 mEq/L, Cl⁻ 108 mEq/L, HCO₃⁻ 20 mEq/L. Which of the following best explains the electrolyte abnormality observed?

Explanation

## Analysis of Perioperative Fluid Abnormality ### Clinical Presentation The patient received 2 L of normal saline (0.9% NaCl) intraoperatively. Postoperative labs reveal: - Sodium: 138 → 135 mEq/L (mild decrease) - Chloride: 102 → 108 mEq/L (significant increase) - HCO₃⁻: 24 → 20 mEq/L (decreased) This pattern is pathognomonic for **hyperchloremic metabolic acidosis**. ### Mechanism of Hyperchloremic Acidosis **Key Point:** Normal saline (0.9% NaCl) contains 154 mEq/L each of Na⁺ and Cl⁻. The chloride concentration exceeds physiologic plasma levels (96–106 mEq/L), making it a hyperchloremic solution. 1. Large-volume normal saline infusion → excess Cl⁻ load 2. Kidneys cannot excrete chloride as rapidly as sodium (chloride reabsorption is coupled to sodium in proximal tubule) 3. To maintain electroneutrality, HCO₃⁻ is excreted in exchange for Cl⁻ reabsorption 4. Net result: Cl⁻ accumulation + HCO₃⁻ loss → hyperchloremic metabolic acidosis ### Comparison of Intravenous Fluids | Fluid | Na⁺ (mEq/L) | Cl⁻ (mEq/L) | K⁺ (mEq/L) | pH Effect | Use | |-------|-------------|------------|-----------|-----------|-----| | **Normal saline (0.9%)** | 154 | 154 | 0 | Acidosis | General, but risk of hyperchloremic acidosis with large volumes | | **Lactated Ringer's** | 130 | 109 | 4 | Neutral/alkaline | Preferred for large-volume perioperative replacement | | **Plasmalyte** | 140 | 98 | 5 | Neutral | Balanced electrolyte solution | | **5% Dextrose in water** | 0 | 0 | 0 | Hypotonic | Maintenance only; risk of hyponatremia | **High-Yield:** In this case, 2 L of normal saline delivered 308 mEq of chloride—far exceeding the daily requirement and causing iatrogenic hyperchloremic acidosis. ### Why Sodium Decreased Slightly Despite hypertonic saline infusion, the mild hyponatremia (138 → 135) occurs because: - Chloride retention forces sodium excretion to maintain electroneutrality - Urine output was low (180 mL in 3 hours), limiting sodium loss but not preventing the osmotic effect **Clinical Pearl:** Hyperchloremic metabolic acidosis is a common but preventable complication of large-volume normal saline use. Balanced crystalloids (Lactated Ringer's, Plasmalyte) are now preferred for perioperative fluid therapy in most patients. **Mnemonic: CHLORIDE LOAD** — Cl⁻ excess → Loss of HCO₃⁻ → Orthostatic hypotension (rarely) → Renal compensation → Increased Cl⁻ reabsorption → Decreased HCO₃⁻ → Electrolyte imbalance.