The dropped beat

In recent months, I have had a considerable number questions about Holter and ECG reports where the diagnosis of Wenckebach second degree AV block and non-conducted atrial ectopics has been raised. Each has its own distinctive footprints and only rarely is the distinction and thus the diagnosis, confusing.

The ECG footprints of a typical Wenckebach sequence

1        Progressive prolongation of the PR interval culminating in a non-conducted P wave

•      AV node progressively fatigues until block

•      AV node rests and conduction returns

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2        The PR interval is longest immediately before the dropped beat.

3        The PR interval is shortest immediately after the dropped beat.

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4        Greatest increase in PR interval is typically between the first and second beat with the “increment increase” of each conducted beat becoming shorter.

5         The R-R interval progressively shortens with each beat of the sequence.

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Footprints 4 and 5 occur only with typical sequences, which is actually uncommon. An explanation can be found in the enclosed reference. Atypical Wenckebach sequences are the norm and in particular occur overnight in the young and are vagal mediated.

Here are a few examples:

The Wenckebach sequence has the greatest increment in the PR interval prior to the dropped beat

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In this example the timings almost obliterate the pause from the dropped beat.

The most common scenario, however, is the combination of sinus slowing (right vagus) with Wenckebach block (left vagus) which typically occurs in the young.

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The vagal stimulation may also result in sinus arrest.

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Or a junctional escape beat following the pause.

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A Wenckebach AV sequence can also occur with a low atrial rhythm.

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What about non-conducted atrial ectopics?

Non-conducted atrial ectopics The P wave of a non-conducted atrial ectopic is premature and is the commonest cause of a pause. In this example the the P wave is similar to the sinus beat; originates from the crista-terminalis adjacent to the sinus node.

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However, the atrial site can be anywhere in the atrium or junctional tissue.

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Non-conducted atrial ectopics can occur in bigeminy

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They mimic 2:1 AV block. The ectopic P waves differ frrom the sinus beats.

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The differential diagnosis is ventriculophasic sinus arrhythmia (look it up!).

Frequently the ectopic P waves are concealed creating a pause without a cause!

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Not infrequently a Wenckebach sequence is terminated by a non-conducted atrial ectopic.

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To confuse the issue; an atrial bigeminy can also have a Wenckebach sequence.

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Finally here is a test case: Called a non-conducted atrial ectopic.

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The answer: This is a Wenckebach sequence, but the PR prolongation is so marked that the next P wave looks like a non-conducted atrial ectopic. There is also a junctional escape beat (or is it conducted??).

So when looking at a dropped P wave:

·         Is it premature?

·         Does it look sinus?

·         Is there gradual PR prolongation?

·         Is the PR interval before the dropped beat long and following it short?

Chelsea Cunningham