A 52-year-old woman with a history of hypertension and chronic kidney disease (Stage 3b, eGFR 35 mL/min/1.73m²) is started on an ACE inhibitor for renal protection. Two weeks later, her serum potassium rises from 4.2 to 5.8 mEq/L and serum creatinine increases from 1.6 to 1.9 mg/dL. Which of the following BEST explains the hyperkalemia observed in this patient?

Explanation

## Hyperkalemia with ACE Inhibitors: The RAAS-Aldosterone Link ### The Mechanism of ACE Inhibitor-Induced Hyperkalemia **Key Point:** ACE inhibitors block the conversion of angiotensin I to angiotensin II. Angiotensin II is a potent stimulator of aldosterone secretion. Without angiotensin II, aldosterone levels fall, reducing potassium excretion in the collecting duct. ### Step-by-Step Pathway 1. **ACE Inhibition** → ↓ Angiotensin II formation 2. **Loss of Angiotensin II Signal** → ↓ Aldosterone secretion from zona glomerulosa 3. **Reduced Aldosterone** → ↓ ENaC (epithelial sodium channel) activity in collecting duct 4. **Impaired K⁺ Secretion** → ↑ Serum potassium 5. **Clinical Result** → Hyperkalemia, especially in CKD with reduced GFR ### Why This Patient Is at High Risk | Risk Factor | Mechanism | |-------------|----------| | CKD Stage 3b (eGFR 35) | Reduced renal mass → less potassium excretion capacity | | ACE inhibitor use | Blocks aldosterone stimulation | | Reduced GFR | Fewer nephrons to handle potassium load | | Mild baseline K⁺ (4.2) | Room for elevation before symptoms | **High-Yield:** The combination of ACE-I + CKD is a **red flag** for hyperkalemia. Risk is highest when GFR < 30 mL/min, but can occur at GFR 30–60 if other risk factors present (diabetes, NSAIDs, high dietary potassium). ### The RAAS-Aldosterone Axis in Potassium Homeostasis ```mermaid flowchart TD A[Angiotensin II]:::outcome --> B[Stimulates Aldosterone<br/>from Zona Glomerulosa]:::action B --> C[ENaC Activation<br/>in Collecting Duct]:::action C --> D[Na+ Reabsorption]:::action D --> E[K+ Secretion into Tubular Fluid]:::action E --> F[Urinary K+ Excretion]:::action G[ACE Inhibitor]:::action --> H[Blocks Angiotensin II<br/>Formation]:::action H --> I[↓ Aldosterone Secretion]:::urgent I --> J[↓ ENaC Activity]:::urgent J --> K[↓ K+ Secretion]:::urgent K --> L[Serum K+ Rises]:::urgent ``` **Clinical Pearl:** Monitor serum potassium and creatinine 1–2 weeks after starting ACE-I or ARB, especially in CKD. If K⁺ > 5.5 mEq/L, consider dose reduction, discontinuation, or addition of a potassium-lowering agent (e.g., finerenone, patiromer). ### Aldosterone's Role in Potassium Excretion **Mnemonic:** **ALDOSTERONE** = **A**ctivates **L**umen-facing **D**eposition of **O**pen **S**odium channels → **T**ubular **E**lectrolyte **R**eabsorption & **O**pposite-charge **N**ion **E**xcretion In the collecting duct: - Aldosterone ↑ ENaC → Na⁺ enters principal cell → lumen becomes negative → K⁺ is attracted into lumen → K⁺ secretion ↑ - No aldosterone → ENaC closed → Na⁺ stays in lumen → lumen stays positive → K⁺ secretion ↓ [cite:KD Tripathi 8e Ch 12; Harrison 21e Ch 297]