Right bundle branch block on alternate beats during acute pulmonary embolism

Abstract

The electrocardiogram of a patient with acute pulmonary embolism showed right bundle branch block (RBBB) on alternate beats; following thrombolysis, the pattern evolved to persistent RBBB and eventually to normal conduction. Analysis of serial tracings suggested that the mechanism of RBBB alternans was tachycardia-dependent bidirectional bundle branch block, caused by prolongation of both anterograde and retrograde refractory periods (RPs) of the right bundle branch (RBB). The sinus impulse found the RBB refractory, and was conducted over the left bundle branch only, depolarizing the left ventricle and then attempting to penetrate retrogradely the RBB; at that time, however, the RBB was still refractory. When a QRS complex had a RBBB configuration, therefore, the RBB was not depolarized; the ensuing sinus impulse found the RBB fully responsive as a consequence of the long period intervening between two successive depolarizations, and resulted in normal intraventricular conduction. With right ventricular afterload decrease, the recovery of RBB anterograde and retrograde excitability was asynchronous, since the retrograde RP became normal earlier than the anterograde one. In accordance with the relatively short retrograde RP, the RBB was retrogradely invaded by the transseptal impulse coming from the left ventricle; this "shifted to the right" the anterograde RP of the RBB. The RBB, thus, was still refractory to the next sinus impulse, and RBBB again occurred; the RBB, thus, was once more depolarized retrogradely, and this led to perpetuation of RBBB. Finally, intraventricular conduction became normal owing to full normalization of RBB anterograde and retrograde refractoriness.

Figure 1

Serial tracings from the same patient (selected strips of leads I and V₁): (a) on admission (heart rate:135); (b) 6 hours later (heart rate:100); (c) shortly after successful thrombolysis (heart rate:103); and (d) on the discharge, 10 days later (heart rate:102).

Figure 2

Diagrammatic explanation of intraventricular conduction (strips of lead V₁ from panels b, c, and d of Figure 1). Horizontal bars depict the effective refractory periods (RPs) of the right bundle branch (RBB) (a, anterograde; r, retrograde); the RP of left bundle branch (LBB) is not illustrated. The vertical dashed lines represent conduction over the LBB. The horizontal dashed double lines indicate transseptal conduction. The vertical solid lines represent conduction (either anterograde or retrograde) over the RBB. TSCT: transseptal conduction time.
In the top panel, the RBB cycle is the interval from an anterograde successful RBB depolarization to the next attempted anterograde RBB activation. The RBB cycle is shorter than the anterograde RP, and the retrograde RP is longer than the sum of RBB cycle plus TSCT. This generates in the RBB a bidirectional block, causing a 2:1 RBBB.
In the middle panel, the RBB cycle is the interval from a retrograde successful depolarization to the next attempted anterograde activation of the RBB. Both anterograde and retrograde RPs of RBB are shorter than in the upper panel, but the retrograde RP shows a higher degree of shortening than the anterograde one. The RBB cycle is shorter than the anterograde RP, and the retrograde RP is shorter than the sum of RBB cycle plus TSCT. The block is unidirectional here, so that retrograde concealed activation of the blocked bundle branch (“linking”) occurs. This postpones the RPs of the RBB in each beat, resulting in repetitive block.
In the bottom panel, the RBB cycle corresponds to the time interval between two consecutive anterograde depolarizations of the RBB. The duration of both RBB RPs has returned to normal (anterograde RP <RBB cycle), and the RBBB has disappeared.