Aberrant Reentry Beats of Junctional Origin

Report:

Sinus bradycardia 36/min (middle & end of bottom strip)

Sinus arrest, possibly exit block

Junctional escape rhythm

Reentry (echo) beats of junctional origin

Retrograde first degree VA block 0.26”

Antegrade first degree AV block 0.26”

Left bundle branch block aberrancy

Reciprocal rhythm

Bigeminy

Comment:

In the top strip it is difficult to make out the exact polarity of the atrial waves; the initial negative notch could be produced by superimposition of upright initial P wave on a descending limb of the T wave. Similarly, terminal negativity looks like increased P-terminal force. Fortunately, looking at the undisturbed T waves of the sinus beats in the bottom strips, one can see the baseline on which the atrial waves in the upper strip are superimposed. The atrial waves in the top strip are then seen as negative-positive, mirror image of their sinus counterparts. The rhythm in the top strip is thus a true reciprocal rhythm: each junctional beat is followed by a retrograde P wave and each retrograde P wave is followed by a ventricular complex. This is not an escape-capture bigeminy. The ventricular complex is conducted with LBBB aberrancy due to the colossal cycle length discrepancy (phasic aberrant conduction).

The retrograde and antegrade conduction times are usually similar in reciprocal rhythms; here they are identical. Both are considerably longer than the 0.20” of the sinus rhythm.

As always, there is a differential diagnosis: atrial premature beats in bigeminy during junctional rhythm. One would expect the bigeminy to continue during sinus bradycardia as well, but this is not always the case. The retrograde morphology of the P waves following the junctional beats is against it too, but not conclusively. For absolute proof, one would need more evidence. It is provided (not shown) when the patient was paced and the reentry beats remained “married” to the paced ones through a range of pacing rates.

Exit block is suggested by shortening of the P-P interval before the long pause in the middle strip. The true length of the pause, however, remains unknown, as the retrograde P wave appears before the next sinus P wave has a chance to capture the atria.