**Question:** Electrically shockable rhythm is

A. Ventricular tachycardia (VT) with a cycle length of ≤300 ms
B. Ventricular tachycardia (VT) with a cycle length of ≥300 ms and QRS duration of ≥120 ms
C. Atrial fibrillation (AF) with a rapid ventricular rate (>150 beats/min)
D. Ventricular fibrillation (VF)

**Correct Answer:** .

**Core Concept:** Electrical instability refers to rhythms that have a high likelihood of converting to ventricular fibrillation (VF), which can cause rapid and often fatal cardiac arrest. Electrical shock therapy, like cardioversion, aims to restore normal sinus rhythm by interrupting and resetting the re-entrant circuits causing the shockable rhythm.

**Why the Correct Answer is Right:**

Electrically shockable rhythms are characterized by abnormal electrical activity in the heart that can lead to life-threatening arrhythmias. The definition for an electrically shockable rhythm depends on the rhythm and its ability to convert to VF with electrical cardioversion.

In the case of ventricular tachycardia (VT), the definition considers cycle length and QRS duration. Electrically shockable VT is defined as:

A. Ventricular tachycardia (VT) with a cycle length of ≤300 ms (short cycle length increases the likelihood of re-entry and the risk of VF).

B. Ventricular tachycardia (VT) with a cycle length of ≥300 ms and QRS duration of ≥120 ms (long cycle length and prolonged QRS duration increase the risk of VF).

**Why Each Wrong Option is Incorrect:**

C. Atrial fibrillation (AF) with a rapid ventricular rate (>150 beats/min) is not considered shockable because it is a supraventricular rhythm. While AF can lead to hemodynamically unstable conditions, electrical shock therapy is not typically used for this rhythm.

D. Ventricular fibrillation (VF) is a result of the electrical instability of the ventricles, not a rhythm in itself. VF is a clinical sign of the electrical instability, not a rhythm to be treated by electrical cardioversion.

**Why the Correct Answer is Right:**

The correct answer (Option A) specifically targets the electrical instability associated with short cycle length and increased risk of VF. Ventricular tachycardia with a cycle length of ≤300 ms and QRS duration of <120 ms is considered shockable because it has a higher likelihood of re-entry and the risk of VF. Electrical cardioversion can restore sinus rhythm in these patients, potentially saving their lives. **Clinical Scenario:** Imagine a patient with chest pain and a shortened QRS complex (indicating short cycle length) who presents with a wide complex tachycardia (indicating prolonged QRS duration). This clinical scenario is most consistent with the correct answer (Option A). Electrical cardioversion would be a reasonable treatment