A 58-year-old man with hypertension is prescribed enalapril. Within 2 weeks, he develops a persistent dry cough and mild angioedema of the lips. His blood pressure is well-controlled at 128/82 mmHg. Serum creatinine and potassium are normal. Which classification of adverse drug reaction does this represent, and what is the underlying mechanism?

Explanation

## ACE Inhibitor Adverse Effects: Type A Reaction This is a **Type A (Augmented) reaction** caused by the **pharmacological consequence of ACE inhibition** — specifically, accumulation of **bradykinin and substance P**. ### Understanding Type A Reactions **Key Point:** Type A reactions are: - **Dose-dependent** (more drug = more effect) - **Predictable** — exaggeration of the drug's known pharmacology - **Common** (10–20% of patients) - **Reversible** upon dose reduction or discontinuation - Occur in any patient given sufficient drug exposure ### Mechanism of ACE Inhibitor–Induced Cough and Angioedema **High-Yield:** ACE inhibitors block the enzyme that degrades **bradykinin and substance P**. ```mermaid flowchart TD A[ACE Inhibitor administered]:::action --> B[ACE enzyme inhibited]:::outcome B --> C[Bradykinin accumulation in lungs & tissues]:::outcome C --> D[Stimulation of vagal C-fibers in airways]:::outcome D --> E[Persistent dry cough]:::outcome B --> F[Increased bradykinin in deep skin layers]:::outcome F --> G[Increased vascular permeability]:::outcome G --> H[Angioedema of lips/tongue/face]:::outcome ``` **Clinical Pearl:** The cough is **not** due to fluid overload or heart failure — it is a direct pharmacological effect of bradykinin on airway sensory nerves. ### Why This Is Type A, Not Type B | Feature | Type A | Type B | |---|---|---| | **Predictability** | Predictable; occurs in most patients | Unpredictable; rare, idiosyncratic | | **Dose-Dependency** | Yes — more drug = more effect | No — occurs at any dose in susceptible individuals | | **Mechanism** | Exaggeration of normal pharmacology | Abnormal immune or genetic response | | **Incidence** | Common (10–20%) | Rare (1–10 per 10,000) | | **ACE inhibitor cough** | Type A (bradykinin accumulation) | — | | **ACE inhibitor angioedema** | Type A (bradykinin-mediated vascular permeability) | Type B (if severe/idiosyncratic with genetic predisposition) | **Key Point:** The **mild angioedema with normal renal function and potassium** in this case is a Type A manifestation of bradykinin accumulation, not a Type B idiosyncratic reaction. If angioedema were severe or life-threatening, it might represent a Type B component, but the presentation here is classic Type A. ### Clinical Management **High-Yield:** Management of ACE inhibitor–induced cough: 1. **Dose reduction** — may relieve symptoms (Type A is dose-dependent) 2. **Switch to ARB** — angiotensin II receptor blockers do NOT inhibit bradykinin degradation; cough resolves 3. **Add NSAIDs** — can reduce bradykinin-mediated effects (rarely used) 4. **Discontinue** — if cough is intolerable **Warning:** Do NOT confuse this with: - ~~Type B reaction~~ — Type B would be unpredictable and occur in only rare individuals - ~~Type C reaction~~ — Type C is cumulative organ toxicity (e.g., renal insufficiency from chronic ACE inhibition in bilateral renal artery stenosis) **Mnemonic: "ACE-B" for bradykinin** - **A**CE inhibitor blocks enzyme - **C**auses accumulation of **B**radykinin - **B**radykinin → cough + angioedema (Type **A** reaction) ### Why ARBs Don't Cause Cough **Clinical Pearl:** ARBs block the **angiotensin II receptor**, not the ACE enzyme. Bradykinin is still degraded normally, so cough does NOT occur. This is why ARBs are preferred in patients intolerant of ACE inhibitor cough.