Which electrocardiographic feature best distinguishes ventricular tachycardia (VT) from supraventricular tachycardia (SVT) with aberrant conduction in a 52-year-old man presenting with palpitations and syncope?

Explanation

## Distinguishing VT from SVT with Aberrancy ### AV Dissociation: The Gold Standard Discriminator **Key Point:** AV dissociation is the most specific finding for VT and is virtually diagnostic when present. It occurs because the ventricular focus depolarizes independently of atrial activity, allowing the atria to continue their own rhythm. ### Why AV Dissociation is Pathognomonic for VT 1. **Mechanism**: In VT, the ectopic ventricular focus generates impulses that do not conduct retrogradely to the atria reliably, or the atria continue sinus rhythm independently. 2. **Visibility**: When present on ECG, you may see: - P waves marching through the QRS complex and T wave at a different rate - Fusion beats (when a sinus impulse and VT impulse depolarize the ventricle simultaneously) - Capture beats (when a sinus impulse transiently captures the ventricles) **Clinical Pearl:** Fusion and capture beats are virtually pathognomonic for VT because they prove that the ventricles can be depolarized by both the ectopic focus AND normal conduction, confirming the ectopic origin is ventricular. ### Comparison Table: VT vs SVT with Aberrancy | Feature | VT | SVT with Aberrancy | |---------|----|-----------| | **AV Dissociation** | Present (60–80% of cases) | Absent (1:1 AV conduction) | | **QRS Duration** | Often > 140 ms | Usually 120–140 ms (both can overlap) | | **Fusion/Capture Beats** | Present | Absent | | **Concordance** | All positive or all negative in precordium | Rare | | **Brugada Criteria** | Positive | Negative | | **Josephson Sign** | RS interval > 100 ms | RS interval < 100 ms | **High-Yield:** While QRS duration > 120 ms is common in both, it is NOT specific. AV dissociation, fusion beats, and capture beats are the ONLY findings that definitively prove ventricular origin. ### Why Other Options Fail - **QRS > 120 ms**: Both VT and SVT with aberrancy can present with wide QRS. This is sensitive but not specific. - **Heart rate > 150 bpm**: Both arrhythmias can achieve this rate; rate alone does not discriminate. - **Narrow QRS in sinus rhythm**: This is a baseline finding and does not help distinguish the arrhythmia mechanism during tachycardia. **Tip:** When you see wide-complex tachycardia, always look for AV dissociation first. If present, VT is diagnosed. If absent, apply Brugada or Josephson criteria to differentiate.