Which antianginal drug is contraindicated in patients taking phosphodiesterase-5 inhibitors due to risk of severe, potentially fatal hypotension?

Explanation

## Nitrates and Phosphodiesterase-5 Inhibitors: Absolute Contraindication **Key Point:** Organic nitrates (nitroglycerin, isosorbide dinitrate, isosorbide mononitrate) are **absolutely contraindicated** in patients taking phosphodiesterase-5 (PDE-5) inhibitors (sildenafil, tadalafil, vardenafil) due to severe, potentially life-threatening hypotension. ### Mechanism of Interaction ```mermaid flowchart LR A["PDE-5 Inhibitor<br/>(sildenafil, tadalafil)"]:::action --> B["↓ PDE-5 activity<br/>in vascular smooth muscle"]:::outcome C["Nitrate<br/>(nitroglycerin)"]:::action --> D["↑ cGMP via NO pathway"]:::outcome B --> E["Accumulation of cGMP"]:::urgent D --> E E --> F["Massive vasodilation<br/>Severe hypotension"]:::urgent ``` 1. **PDE-5 inhibitors** block phosphodiesterase-5, which normally degrades cyclic GMP (cGMP) 2. **Nitrates** increase cGMP via the NO–guanylate cyclase pathway 3. **Combined effect:** Synergistic cGMP accumulation → profound vasodilation → **refractory hypotension** (SBP may drop >30 mmHg) 4. **Clinical consequence:** Myocardial infarction, stroke, or death in susceptible patients ### High-Yield Facts **Washout periods required:** - **Sildenafil (Viagra):** 24 hours (half-life ~4 hours, but effects persist) - **Tadalafil (Cialis):** 48 hours or longer (half-life ~17 hours; daily formulation contraindicated indefinitely) - **Vardenafil (Levitra):** 12 hours **Clinical Pearl:** Always ask patients about erectile dysfunction medications before prescribing nitrates. Tadalafil's long half-life makes it particularly problematic; some guidelines recommend avoiding nitrates entirely in patients on daily tadalafil. **Warning:** This is a **high-yield NEET PG trap question.** Students often confuse nitrate interactions with other antianginals (calcium blockers, beta-blockers), which do NOT have this contraindication. ## Why Other Antianginals Are Safe | Drug | Mechanism | PDE-5 Interaction? | |---|---|---| | **Diltiazem** (Ca²⁺ blocker) | Direct vascular smooth muscle relaxation | No — different pathway | | **Propranolol** (β-blocker) | Sympathetic inhibition | No — no cGMP involvement | | **Ranolazine** (metabolic modifier) | Inhibits late Na⁺ current | No — no vasodilatory pathway | [cite:Harrison 21e Ch 297]