A 62-year-old woman with hypertension and depression is on lisinopril 10 mg daily and sertraline 100 mg daily. She develops a urinary tract infection and is prescribed trimethoprim-sulfamethoxazole (TMP-SMX) double-strength twice daily. After 5 days, she reports dizziness and fatigue. Serum potassium is 6.2 mEq/L (normal 3.5–5.0). What is the primary mechanism of hyperkalemia in this case?

Explanation

## Multi-Drug Hyperkalemia Interaction **Key Point:** This is a classic **triple-drug hyperkalemia** scenario involving two potassium-sparing agents (lisinopril and TMP-SMX) with additive effects on renal potassium handling. ## Mechanism of Each Drug | Drug | Mechanism | Effect on K^+^ | |------|-----------|----------------| | **Lisinopril (ACE inhibitor)** | Blocks angiotensin II → reduces aldosterone secretion | ↓ Aldosterone → ↓ K^+^ excretion in collecting duct | | **TMP-SMX** | Trimethoprim blocks epithelial Na^+^ channels in collecting duct (similar to amiloride) | ↓ Na^+^ reabsorption → ↓ K^+^ secretion | | **Sertraline** | Minimal direct effect on potassium (not a primary culprit here) | Negligible | ## Pathophysiology of Hyperkalemia ```mermaid flowchart TD A[ACE Inhibitor + TMP-SMX started]:::action --> B[Reduced aldosterone secretion]:::outcome B --> C[Decreased ENaC activity in collecting duct]:::outcome C --> D[Reduced Na+ reabsorption]:::outcome D --> E[Reduced electrical driving force for K+ secretion]:::outcome E --> F[Hyperkalemia K+ = 6.2 mEq/L]:::urgent F --> G[Symptoms: dizziness, fatigue, arrhythmia risk]:::urgent ``` **High-Yield:** The combination of ACE inhibitors/ARBs + potassium-sparing diuretics or TMP-SMX is a high-risk triad for hyperkalemia, especially in: - Renal impairment (eGFR < 60) - Diabetes mellitus - Elderly patients - Acute illness or dehydration ## Clinical Management 1. **Immediate:** Check ECG for peaked T waves, widened QRS (signs of cardiac toxicity) 2. **Discontinue:** TMP-SMX (switch to alternative antibiotic, e.g., nitrofurantoin or cephalexin) 3. **Consider:** Reduce lisinopril dose or switch to alternative antihypertensive 4. **Acute treatment** (if K^+^ > 6.5 or ECG changes): - Calcium gluconate (cardiac membrane stabilization) - Insulin + dextrose (shifts K^+^ intracellularly) - Beta-2 agonist (albuterol) - Diuretics or potassium binders (patiromer, sodium zirconium cyclosilicate) **Clinical Pearl:** TMP-SMX is often overlooked as a hyperkalemia culprit because it is not a classic potassium-sparing agent, but its epithelial sodium channel blockade mimics amiloride's effect.