A 62-year-old woman with chronic atrial fibrillation and a rapid ventricular response (heart rate 135 bpm) presents to the emergency department. Her ECG shows irregular narrow-complex tachycardia with an absent P wave and a ventricular rate of 130–140 bpm. She is hemodynamically stable. What is the drug of choice for rate control in this acute presentation?

Explanation

## Acute Rate Control in Atrial Fibrillation **Key Point:** In hemodynamically stable patients with rapid atrial fibrillation, intravenous calcium channel blockers (verapamil or diltiazem) are first-line agents for acute rate control. ## Mechanism of Action Diltiazem (and verapamil) are non-dihydropyridine calcium channel blockers that: - Slow AV nodal conduction by blocking L-type calcium channels - Increase AV nodal refractoriness - Reduce ventricular response rate in atrial fibrillation - Onset of action: 2–5 minutes (IV); peak effect: 5–10 minutes ## Comparison of Rate-Control Agents in Acute AF | Agent | Route | Onset | Peak | Hemodynamic Effect | Use in Stable AF | |-------|-------|-------|------|-------------------|------------------| | **Diltiazem** | IV | 2–5 min | 5–10 min | Mild ↓ BP, ↓ HR | **First-line** | | **Verapamil** | IV | 3–5 min | 5–10 min | Moderate ↓ BP, ↓ HR | Alternative | | **Amiodarone** | IV | 5–30 min | 30–60 min | Variable; ↓ BP risk | Unstable/resistant | | **Digoxin** | Oral/IV | 30–60 min | 4–6 hrs | Vagomimetic | Chronic use; slow onset | **High-Yield:** Diltiazem is preferred over verapamil in acute AF because it has a faster onset, better hemodynamic tolerance, and fewer negative inotropic effects. Verapamil is an acceptable alternative but is less commonly chosen in acute settings. ## Why Diltiazem Is Optimal in This Case 1. **Rapid onset:** Achieves rate control within 5–10 minutes 2. **Hemodynamic stability:** Minimal myocardial depression; safe in stable patients 3. **Efficacy:** Reduces ventricular rate by 20–30% in most patients 4. **No need for loading:** Single IV bolus often sufficient **Clinical Pearl:** Diltiazem IV bolus is typically 0.25 mg/kg over 2 minutes, followed by a second dose if needed. A continuous infusion can be started for sustained rate control. ## Why Other Agents Are Not First-Line Here - **Amiodarone:** Reserved for hemodynamically unstable patients or when rate control fails; slower onset and greater hemodynamic risk - **Digoxin:** Slow onset (30–60 minutes); better for chronic rate control; less effective in high adrenergic states - **Verapamil:** Similar efficacy to diltiazem but slightly slower onset and greater negative inotropic effect [cite:Harrison 21e Ch 226]