Fascicular Wenckebach AV Sequences

This isn’t as hard as it sounds. It is, however, controversial and if you don’t question some of the tracings, then you are not trying hard enough!You may disagree with some of my diagnoses, but I am trying to present a concept.We are now familiar and comfortable with Wenckebach sequences from the AV node. If the block is distal to the node and in the bundle branches, they are still Wenckebach AV sequences, but appear very different!So what does an ECG with fascicular Wenckebach AV sequences look like?

Did I hear a loud gasp?Don’t give up and let me explain!Here is a portion of lead V1. The rhythm is sinus (red arrows) with inverted P waves, which are common in that lead.

There are sequences, each with three QRS complexes, and all different QRS widths; the first is narrow at 90 msec (red highlight), the second wider at 110 msec (blue highlight) and the third a full left bundle branch block at 140 ms (yellow highlight). This is a Wenckebach AV sequence in the left bundle branch. It is a transitional phase and seen with a developing or rate dependent bundle branch block.This is another example of a fascicular Wenckebach AV block:

Left panel, a sudden increase in the QRS width. Right panel, the transition is a sequence of increasing QRS widths: 100 msec (red highlight), 140 msec (blue highlight) and the third marginally wider at 160 msec (yellow highlight).This example shows a 2:1 fascicular Wenckebach AV block.

Above, left: The rhythm is sinus bradycardia and a narrow QRS (red highlight).Above, right: At maximum rate of 119 bpm, the QRS is wide (yellow highlight). Below: With sinus tachycardia 110 bpm (red arrows), alternate beats have a wide QRS, confirming 2:1 Wenckebach block in one bundle branch.This was first reported in both left and right bundle branches in 1976 (Peter et al. Heart and Lung 1976; 5: 747-54).I have also seen it combined with AV nodal Wenckebach sequences.

Sinus rhythm (red arrows) with a 3:2 Wenckebach AV block (red highlight). The QRS complexes in the sequence are alternatively narrow (yellow highlight) and wide (blue highlight) suggesting a fascicular Wenckebach in the left bundle branch (V1).The pathophysiology may be similar to alternate Wenckebach AV sequences, discussed previously.Here is another example of combined Wenckebach AV nodal and fascicular (left bundle branch) block.

The prematurity of atrial ectopy may also allow Wenckebach sequences in the fascicles.

Sinus rhythm (red arrows) with atrial bigeminy (blue arrows). There is aberrant conduction in the right bundle branch with increasing block; normal conduction (red highlight), incomplete right bundle branch block (blue highlight) and complete right bundle branch block (yellow highlight). The sequence pattern suggests Wenckebach AV fascicular block.Did you see the ventriculo-phasic sinus arrhythmia?It’s all in the timing!Harry Mond