• Differential diagnosis of wide complex tachycardia (WCT) where the underlying rhythm is unclear.
• As a fast, single-lead (aVR) alternative to the 4-step Brugada criteria.
• Particularly useful when precordial leads (V1-V6) are unavailable or difficult to interpret.

During SVT with bundle branch block, the initial activation of the ventricles occurs via the normal specialized conduction system, which typically moves AWAY from lead aVR (creating a negative QRS). In VT, activation often starts in the inferior/apical regions and moves TOWARD aVR, or is slowed by muscle-to-muscle conduction.

• Presence of an initial R wave: Highly specific for VT.
• Initial r or q wave width > 40 ms: Suggests slow muscle-to-muscle conduction diagnostic of VT.
• Notch on the descending limb of a negative QRS: Indicates fragmented ventricular activation.
• Vi/Vt ratio ≤ 1: Measures the velocity of initial activation (Vi) vs terminal activation (Vt) in the last 40ms. SVT is fast initially (Ratio > 1), VT is slow initially (Ratio ≤ 1).