## Rationale for Amiodarone in Post-MI VT **Key Point:** Amiodarone is the drug of choice for acute termination and long-term suppression of ventricular tachycardia in the setting of acute myocardial infarction with reduced ejection fraction, regardless of haemodynamic stability. ### Why Amiodarone is Superior 1. **Broad-spectrum antiarrhythmic** — combines properties of all four Vaughan-Williams classes 2. **Efficacy in VT/VF** — highest conversion rate among antiarrhythmics (~80–90%) 3. **Safe in reduced EF** — minimal negative inotropic effect despite Class II properties 4. **Post-MI cardioprotection** — reduces mortality in acute MI with VT (ARREST trial) 5. **IV and oral formulations** — rapid onset (IV in 2–3 minutes) for acute haemodynamic instability 6. **No proarrhythmic paradox** — unlike Class I agents, does not increase mortality in structural disease ### Clinical Scenario Context ```mermaid flowchart TD A[Sustained VT post-MI<br/>LVEF 35%<br/>Haemodynamically unstable]:::outcome --> B{Immediate intervention?}:::decision B -->|Yes| C[Synchronized DC cardioversion<br/>+ IV Amiodarone]:::action B -->|No| D[IV Amiodarone bolus<br/>300 mg over 10-20 min]:::action C --> E[Amiodarone infusion<br/>1 mg/min × 6 hrs]:::action D --> E E --> F[Oral Amiodarone<br/>long-term suppression]:::action F --> G[ICD consideration<br/>if LVEF remains ≤35%]:::outcome ``` ### Comparison with Alternatives in Post-MI VT | Drug | Class | Efficacy | EF Safety | Post-MI Mortality | IV Available | Preferred | |------|-------|----------|-----------|-------------------|--------------|----------| | **Amiodarone** | I+II+III+IV | ✓✓✓ | ✓✓ (safe) | ↓ (ARREST) | ✓ | **YES** | | Procainamide | Ia | ✓ | ✗ (negative inotrope) | ↑ (CAST) | ✓ | No | | Mexiletine | Ib | ✓ | ✗ (weak) | ↑ (CAST) | ✗ | No | | Quinidine | Ia | ✓ | ✗ (negative inotrope) | ↑ (CAST) | ✗ | No | **High-Yield:** Class Ia (procainamide, quinidine) and Ib (mexiletine) agents are **contraindicated** in post-MI VT with reduced EF because they increase mortality (CAST trial). Amiodarone is the only Class I agent safe in structural disease. **Clinical Pearl:** In haemodynamically unstable VT, **synchronized DC cardioversion is the immediate intervention**, followed by IV amiodarone for rhythm stabilization and long-term suppression. Do not delay cardioversion to administer drugs. **Warning:** Amiodarone has significant systemic toxicity (thyroid, lung, liver, skin) with chronic use, but in acute post-MI VT, the benefit far outweighs the risk. Regular monitoring (TFTs, LFTs, CXR, ECG) is mandatory. **Mnemonic: CAST Principle** - **C**lass I antiarrhythmics (except amiodarone) - **A**re **S**uppressive in arrhythmias but - **T**oxic in structural disease (↑ mortality) ### Mechanism of Action Amiodarone blocks sodium channels (Class I), beta-adrenergic receptors (Class II), potassium channels (Class III), and calcium channels (Class IV), resulting in slowed conduction, prolonged refractoriness, and reduced automaticity across all cardiac tissues.
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