Not a Sinus Bradycardia

Report:

Sinus rhythm

Sinus arrest

Junctional escape beats

Retrograde conduction

Non-phasic aberrant conduction

Escape-capture bigeminy

Comment:

The phenomenon of escape-capture bigeminy depends on the coincidental arithmetic relationship between the effective rate of the escape pacemaker and the effective rate of the capture beats (in, e.g., 2:1 AV block, the effective sinus rate is one half the actual rate). It also presupposes absence of retrograde conduction, which would, necessarily, reset the rate of the capturing pacemaker or suppress it altogether49.

Here, the retrograde conduction should reset the sinus rate, but the escape-capture sequence is preserved due to sinus node’s failure to continue discharging at the rate corresponding to the P’-P cycle. This is not a sinus bradycardia, as the sinus P waves follow the retrograde P waves by only 0.92”, corresponding to a rate of 67/min. One has to assume sinus arrest (or exit block) which allows the next escape beat to appear. This is the only example of this variant of escape-capture bigeminy I have seen (or heard of) so far. Even more exotic would be a sino-atrial entry block, but that, too, could explain the escape-capture bigeminy (if that is what this is).

It is possible that the rhythm consists of two junctional pacemakers, or a ventricular one with junctional bigeminy. In that case, the second beat in each pair is somehow caused by the first, or else has an exit block analogous to that postulated for sinus beats. The pause is longer than two P-P cycles.

The morphology of the escape beats is similar to sinus beats; junctional beats are often slightly different. This is called non-phasic aberration, to distinguish it from phasic (long-short) aberration. It may be due to altered activation front from a junctional focus. The sinus beats themselves, on the other hand, may have a soupçon of phasic aberrancy, exaggerating their difference from the junctional beats. Diminished S wave is the earliest sign of RBBB aberrancy. This is confirmed by comparing the sinus complexes to those produced by atrial pacing below (Fig 75a): S wave is bigger and the R’ is gone.