A 72-year-old woman with a history of hypertension and diabetes presents with syncope. ECG shows complete heart block (third-degree AV block) with a ventricular escape rate of 35 bpm. After initial stabilization with temporary pacing, what is the drug of choice for long-term management while awaiting permanent pacemaker insertion?

Explanation

## Drug of Choice for Third-Degree AV Block (Temporary Bridge) **Key Point:** In third-degree (complete) AV block, the site of block is **below the AV node** (infranodal). Atropine does NOT work because the problem is not vagal — it is structural/ischemic damage to the His bundle or bundle branches. Isoproterenol is the temporary pharmacological bridge to maintain heart rate and perfusion until permanent pacing is available. ### Why Atropine Fails in Third-Degree Block | Feature | Mobitz I | Third-Degree | |---------|----------|---------------| | **Site of block** | AV node (vagal) | His bundle / bundle branches (structural) | | **Response to atropine** | ✓ Excellent | ✗ No response | | **Escape rhythm** | Usually adequate | Often slow & unreliable | | **Pacing required** | Rarely | Always | **High-Yield:** **Atropine works ONLY for AV nodal blocks** (Mobitz I, some second-degree blocks in inferior MI). It does **NOT work for infranodal blocks** (Mobitz II, third-degree). ### Why Isoproterenol Is Used as a Temporary Bridge **Isoproterenol** is a **non-selective beta-agonist** that: 1. **Increases heart rate** via beta-1 stimulation of the escape pacemaker 2. **Improves AV nodal conduction** (if any nodal component exists) 3. **Increases cardiac output** and perfusion pressure 4. **Provides temporary rate support** until permanent pacing is available ### Dosing & Administration - **IV infusion:** 2–10 mcg/min, titrated to achieve heart rate 60–80 bpm - **Onset:** Immediate (minutes) - **Duration:** Short (minutes after discontinuation) - **Monitoring:** Continuous ECG, blood pressure, oxygen saturation **Clinical Pearl:** Isoproterenol is a **temporary measure only**. It increases myocardial oxygen demand and can precipitate arrhythmias or angina. Permanent pacemaker insertion is definitive and should not be delayed. ### Why Other Agents Are Inappropriate **Atropine:** Does not work in infranodal (third-degree) blocks because the block is structural, not vagal. Wastes time and delays definitive management. **Theophylline:** A nonspecific adenosine antagonist; not indicated for AV block. No role in acute bradycardia management. **Dobutamine:** Inotrope with some chronotropic effect; less reliable for rate control than isoproterenol. Not first-line for pure bradycardia. ### Definitive Management **Permanent pacemaker insertion** is the gold standard for symptomatic third-degree AV block. Indications include: - Symptomatic bradycardia (syncope, hypotension, heart failure) - Escape rate < 40 bpm - Prolonged asystolic pauses - Infranodal block (Mobitz II, third-degree) [cite:Harrison 21e Ch 297]