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?

Question

Accepted Answer

D. Additive potassium-sparing effects of lisinopril and TMP-SMX, both reducing renal potassium excretion. ## 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.

Answer

A. Sertraline-induced syndrome of inappropriate antidiuretic hormone (SIADH) leading to volume depletion and hyperkalemia

Answer

B. Direct inhibition of Na-K-ATPase pump by TMP-SMX in the distal tubule

Answer

C. Lisinopril-induced acute kidney injury reducing glomerular filtration rate and potassium clearance

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

Explanation

Multi-Drug Hyperkalemia Interaction

Mechanism of Each Drug

Pathophysiology of Hyperkalemia

Clinical Management

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