## Drug of Choice for Symptomatic Mobitz I AV Block **Key Point:** Atropine is the first-line pharmacological agent for symptomatic bradycardia and AV block in the acute setting, particularly in inferior MI where vagal tone is elevated. ### Mechanism of Action Atropine is an **anticholinergic agent** that blocks vagal (parasympathetic) effects on the AV node. In inferior MI, increased vagal tone and adenosine release impair AV nodal conduction. Atropine restores AV nodal function by: 1. Increasing AV nodal conduction velocity 2. Decreasing AV nodal refractoriness 3. Increasing heart rate and cardiac output ### Why Atropine Works in Inferior MI - Inferior MI causes **right coronary artery occlusion** → ischemia of the AV node - The AV node is **highly vagal-dependent** (parasympathetic innervation) - Increased adenosine and acetylcholine levels slow conduction - Atropine reverses this by antagonizing muscarinic receptors ### Clinical Application | Feature | Mobitz I (Wenckebach) | Mobitz II | Third-Degree | |---------|----------------------|-----------|---------------| | **Site of block** | AV node | Below AV node | Complete | | **Response to atropine** | Excellent | Poor | Poor | | **Pacing needed** | Rarely | Often | Always | | **Prognosis in inferior MI** | Usually transient | Often progresses | High mortality | **High-Yield:** Mobitz I (Wenckebach) in inferior MI is typically **vagal** in origin and responds excellently to atropine. Mobitz II and third-degree blocks are usually **infranodal** (below the AV node) and do NOT respond to atropine — these require **pacing**. ### Dosing - **IV bolus:** 0.5–1 mg IV, repeat every 3–5 minutes - **Maximum dose:** 3 mg (to avoid paradoxical bradycardia at very high doses) - **Onset:** Within 1–2 minutes **Clinical Pearl:** If atropine fails or is contraindicated (glaucoma, urinary retention), proceed directly to **temporary transcutaneous or transvenous pacing** rather than relying on catecholamines. ### Why Other Agents Are Suboptimal **Isoproterenol:** Beta-agonist; increases heart rate but increases myocardial oxygen demand in acute MI — **contraindicated** in ischemic heart disease. **Dopamine:** Inotrope; reserved for hypotension with cardiogenic shock, not first-line for AV block. Increases oxygen demand. **Amiodarone:** Class III antiarrhythmic; **slows AV nodal conduction further** — worsens block, not indicated. [cite:Harrison 21e Ch 297]
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