It’s all in the timing

In young people, overnight Wenckebach AV block is a common finding. This is usually in the presence of sinus bradycardia/sinus slowing and thus following the dropped beat, there may be a junctional escape beat, terminating the sequence.

This combination of Wenckebach AV block and sinus slowing is an interplay between the vagal innervation of the sinus node (right) and AV node (left). The timing of the PR interval and the junctional escape beat can lead to bizarre tracings.

In this example, there is Wenckebach AV block without an apparent pause. The first complex of the sequence is conducted with a PR interval of 400 ms. Then follows an apparent AV block and junctional escape beat. However, this could be conducted and the concealed P wave (stippled red vertical arrow) is the blocked beat.             Or are both dropped?

Here is another example of a dropped beat followed by a junctional escape. In this example, the concealed sinus beat is in the T wave and conducts to the ventricle with first degree AV block. The first PR interval after a pause is always the shortest but not here. One of the many examples of atypical Wenckebach AV block.

Remember it is all in the timing!

Dr Harry Mond