## Differentiation of Wide Complex Tachycardia **Key Point:** Adenosine challenge during tachycardia is the most specific bedside investigation to differentiate VT from SVT with aberrancy in a haemodynamically stable patient. ### Mechanism of Adenosine Challenge Adenosine causes transient AV block by hyperpolarizing AV nodal cells: - **SVT with aberrancy:** Tachycardia terminates abruptly (AV node is part of the re-entry circuit) - **Ventricular tachycardia:** Tachycardia continues unaffected (VT does not depend on AV nodal conduction) ### Why Adenosine is Preferred | Feature | Adenosine Challenge | EPS | Echo | CXR | |---------|-------------------|-----|------|-----| | **Specificity** | Very high (>95%) | Gold standard but invasive | Structural info only | Not diagnostic | | **Sensitivity** | High for SVT termination | Gold standard | Low for arrhythmia type | Not applicable | | **Timing** | Immediate bedside result | Requires lab setup | Structural assessment | Not relevant | | **Risk in haemodynamic stability** | Safe, reversible (t½ ~10 sec) | Invasive, risk of induction | Safe but non-diagnostic | Safe but non-diagnostic | | **Cost & availability** | Low, widely available | High, specialized centre | Moderate | Low | **High-Yield:** Adenosine is the **first-line diagnostic agent** for stable wide complex tachycardia of uncertain origin. A persistent tachycardia after adenosine strongly suggests VT. ### Clinical Pearl In haemodynamically unstable patients, **direct cardioversion** is preferred over adenosine. EPS is reserved for cases where diagnosis remains unclear after adenosine or when ablation is planned. **Warning:** Do not confuse adenosine's diagnostic role with its therapeutic role — adenosine terminates SVT but does not terminate VT, making it both diagnostic and therapeutic for SVT. [cite:Harrison 21e Ch 231]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.