Alternating bundle branch block during atrial bigeminy

Alternate patterns of ventricular excitation may occur with any type of supraventricular bigeminy. It is a physiologic phenomenon that depends on the rate dependency of bundle-branch refractoriness and the linking phenomenon related to concealed retrograde penetration of aberrantly conducted beats into their own blocked bundle branch [1–3]. An alternate pattern of ventricular activation can only occurs if the coupling intervals of the early atrial premature complexes, the refractory periods of the bundle branches, and the anterograde and retrograde conduction times within the bundles present a favorable combination of factors critical for alternation of ventricular activation. Figure 1 is an ECG of lead V1 showing atrial bigeminy with alternating bundle branch block in a 20 year-old woman without heart disease who complained of a skipping sensation in her chest. The coupling intervals of all the atrial extrasystoles were identical. The QRS complex during right bundle branch block (RBBB) is slightly longer than 0.12 s with careful scrutinity of the initial forces best seen in the last complex [4]. Discussion