A 58-year-old man with a history of hypertension and coronary artery disease presents to the emergency department with palpitations and chest discomfort. ECG shows a regular wide-complex tachycardia at 180 bpm with a rate of 1:1 AV conduction. Blood pressure is 110/70 mmHg and he is haemodynamically stable. Intravenous adenosine 6 mg is administered but produces no effect on the arrhythmia. Which of the following is the most appropriate next step in pharmacological management?

Explanation

## Clinical Scenario Analysis This patient presents with a **haemodynamically stable wide-complex tachycardia** that is **unresponsive to adenosine**. The key diagnostic clue is the failure of adenosine to terminate the arrhythmia, which suggests this is **not an AV-nodal-dependent arrhythmia** (AVNRT or AVRT). ### Differential Diagnosis of Wide-Complex Tachycardia | Feature | Ventricular Tachycardia | AVNRT/AVRT | Atrial Flutter with aberrancy | |---------|------------------------|-----------|------------------------------| | **Adenosine response** | Usually no effect | Terminates or slows | May slow atrial rate | | **AV dissociation** | Present (pathognomonic) | Absent | Absent | | **1:1 AV conduction** | Rare | Common | Common | | **Haemodynamic stability** | Variable | Usually stable | Usually stable | **Key Point:** Adenosine resistance in a wide-complex tachycardia with 1:1 AV conduction strongly suggests **ventricular tachycardia (VT)** or an antidromic AVRT using an accessory pathway with rapid conduction. ### Why Amiodarone is the Best Choice **High-Yield:** Amiodarone is the **only antiarrhythmic drug with efficacy across all four Vaughan-Williams classes** (I, II, III, IV properties). It is the drug of choice for: - Haemodynamically stable VT (adenosine-resistant) - Polymorphic VT - Atrial fibrillation with rapid ventricular response - Antidromic AVRT **Clinical Pearl:** In a haemodynamically stable patient with adenosine-resistant wide-complex tachycardia, amiodarone 150 mg IV over 10 minutes is the standard first-line pharmacological intervention before considering electrical therapy. ### Mechanism of Amiodarone in VT ```mermaid flowchart TD A[Wide-complex tachycardia]:::outcome --> B{Adenosine responsive?}:::decision B -->|Yes| C[AVNRT/AVRT]:::outcome B -->|No| D{Haemodynamically stable?}:::decision D -->|No| E[Synchronized cardioversion]:::urgent D -->|Yes| F[IV Amiodarone 150 mg/10 min]:::action F --> G{Rhythm terminated?}:::decision G -->|Yes| H[Continue infusion, monitor]:::action G -->|No| I[Consider synchronized cardioversion]:::urgent ``` **Mnemonic: AMIO** — **A**ll classes, **M**ultiple mechanisms, **I**ncredible efficacy, **O**ften first-line for VT. **Dosing regimen:** - Loading: 150 mg IV over 10 minutes - Second dose: 150 mg IV over 10 minutes (if needed after 10 min) - Maintenance: 0.5–1 mg/min infusion [cite:Harrison 21e Ch 235]