When I describe Wenckebach AV sequences occurring from a non-sinus focus, I often see facial bewilderment. Remember, most Wenckebach AV sequences occur at the AV node, so it is not surprising that other supraventricular foci can also result in typical Wenckebach AV block sequences.Let us look at some examples with relevant nocturnal times in blue highlight.Nocturnal low atrial rhythm:

Accelerated low atrial rhythm with Wenckebach AV block (red highlight). The effects of low atrial slowing and vagal hypertonia are seen.With low atrial rhythm (yellow highlight), Wenckebach AV block sequence termination during vagal hypertonia may allow sinus rhythm to re-emerge (green highlight).

Accelerated low atrial run with Wenckebach AV block.

The background nocturnal rhythm is sinus (red highlight). Upon termination of the sequence, the rhythm returns to sinus.Similar appearances occur with other supraventricular runs:

The rate is about 95 bpm. There is a Wenckebach AV block sequence (arrows and red highlight), terminating in two dropped beats (blue arrows). A junctional escape beat occurs before the next P wave can emerge (red stippled arrow).An atrial run may terminate before a beat is dropped.

The atrial run has an increasing PR interval (red highlight) but terminates before a beat is droppedOnce the atrial rate increases, the tracings become more difficult to interpret:

Short run of atrial tachycardia rate 170 bpm. There is 3:2 (red highlight) and 2:1 (yellow highlight) Wenckebach AV block.The near impossible!

Atrial flutter with block. Short length sequences (red and yellow highlight) which show an increasing PR interval (red arrows) and a dropped beat (blue solid arrows). There are also alternating P waves, which do not conduct (blue stippled arrows). Hence, two dropped beats terminate a sequence, and the next sequence commences, but without a preceding “alternating P wave” blocked beat. The red highlight is a 5:4 sequence (5 atrial:4 ventricular), whereas the yellow highlight is 3:2.This complex appearance is called alternating Wenckebach AV block.Here is an example of sinus rhythm with 3:2 alternating Wenckebach AV block sequence (red highlight):

Two blocks; one at the AV node and the other lower, with the combination referred to as a multi-level block:

• Typical Wenckebach AV block (red and blue solid arrows) alternating with
• Complete AV block (blue stippled arrows).
• The next sequence commences without an intervening distal blocked beat.

This can also present with a bundle branch block, syncope, and sudden death.I just heard someone say, “this must be very rare?”.I don’t think so. It is just that we don’t look for it!Another atrial flutter example of 4:3 alternating Wenckebach AV block (red highlight).

Just when you thought it couldn’t get any more complex!

Atrial flutter with 3:2 alternating Wenckebach AV block sequence (red highlight). However, the alternating complete block extends for two P waves (blue stippled arrows). The dropped Wenckebach P wave may be fused with the distal block (green arrow)?There may be more than one explanation?What about ectopic atrial groupings:Nocturnal sinus rhythm and first degree AV block (red arrows) with an atrial couplet demonstrating 2:1 Wenckebach AV block (blue arrows).

Here is another which is difficult to explain!

The lower Holter tracing shows Wenckebach AV block (red highlight). The above tracing shows a Wenckebach AV sequence within a run of atrial bigeminy (yellow highlight).Because atrial pacing (AAI) conducts to the ventricle, it is not surprising that physiologic Wenckebach AV block can occur overnight (red highlight).

Many observers would regard this as an indication to upgrade to a dual chamber pacemaker. However, if asymptomatic it should be regarded as physiologic similar to nocturnal Wenckebach.Let us finish with an nocturnal “accelerated junctional rhythm” (yellow highlight) with a Wenckebach AV sequence.

Careful inspection reveals that this is actually sinus rhythm (red arrows) with long PR intervals (red highlight) and termination of the sequence with a dropped beat (blue arrow) which allows the normal sinus PR interval to be seen. There is no alteration in the sinus rhythm.Now take an aspirin and lie down!Until next timeHarry Mond