Alternate-Beat Wenckebach Caused by VEBs

Report:

Sinus rhythm 92/min

VEBs, couplets and triplets

R-on-T phenomenon

Wenckebach second degree AV block for alternate P waves

Acute or recent inferior infarction

Comment:

The first two consecutively conducted P waves show slight but definite PR interval prolongation, similar to the other pair of P waves, toward the end of the strip. The next P is blocked by a VEB couplet, but its successor is conducted, with further PR prolongation. The next one is again blocked by a VEB couplet, but the following one is conducted with even longer PR interval. Then another VEB couplet blocks the P wave, its successor being conducted with the longest interval in this strip. After that comes a triplet, blocking two P waves. Thus, every second P wave is conducted with a longer PR interval until two P waves get blocked. The term “blocked” is used here to illustrate the analogy with alternate-beat Wenckebaching discussed below; of course these two P waves had no opportunity of conduction in the first place.

Next, the second pair of consecutively conducted P waves shows the expected PR prolongation; the next P is blocked by a single VEB. Now the subsequent PR interval has recovered – there is no more Wenckebaching in the alternate beats.

Typically, in the reported cases of alternate-beat Wenckebach, the proximal block is Wenckebach in type and the distal is fixed 2:1 block (opposite of what commonly happens in atrial tachycardia or flutter); the pauses contain two blocked P waves. The situation here is analogous to such a bi-level AV block, the VEB couplets ant the triplet playing the role of the (obviously) distal block74. An experiment of nature.

Below (Fig 115a) is another interesting interaction – or, rather, lack thereof – between Wenckebach block and a VEB: the sequence continues regardless of the VEB and produces what might be called postponed compensatory pause, populated by a blocked P wave and a sterile pacing spike.