Which finding on ECG supports a ventricular tachycardia diagnosis when the rhythm is unstable?

• A. AV dissociation
• B. Narrow QRS with normal axis
• C. Regular P waves before each QRS
• D. Shortened PR interval

Answer: (C)

When a tachycardia is unstable, the key idea is that ventricular tachycardia presents as an independent ventricular rhythm, often with the atria not dictating the ventricular rate. The best clue is AV dissociation: P waves and QRS complexes march to different rhythms, with no fixed relationship between them. This means the ventricles are driven by an ectopic focus rather than by impulses traveling from the atria through the AV node.

Seeing regular P waves that consistently precede every QRS would imply the atrial rhythm is driving the ventricles in a 1:1 AV relationship, which is typical of a supraventricular tachycardia with intact conduction, not VT. So that pattern does not best support VT. In contrast, AV dissociation, along with wide, irregularly related QRS morphology, would be the stronger VT indicator in an unstable patient. Narrow QRS with a normal axis would favor SVT, and a short PR interval points to pre-excitation or SVT rather than VT.