What Do You Think 50

An exercise in thinking outside the box.

What do you think?

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There is obvious Wenckebach AV block.

Some people believe that sinus rhythm holds the sole rights to Wenckebach AV block! However, it can occur with any supraventricular rhythm and in this case an ectopic atrial rhythm.The P waves are inverted (blue arrows) and there is a progressive increase in the PR interval with a non-conducted beat (stippled blue arrow). You can also call this low atrial rhythm. However, on Holter tracings the leads are not defined, and it is more correct to call the rhythm, ectopic atrial.

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Here is another non-sinus P wave generated sequence.

Sinus rhythm with a short run of supraventricular tachycardia (yellow highlight). Within the run is a Wenckebach AV sequence (blue arrows) and a non-conducted P wave (blue stippled arrow). The P waves are similar but not identical to the sinus generated P waves. This is because the ectopic focus is close to the sinus node and probably in the crista-terminalis, which is a fibromuscular ridge in the right atrium immediately below the sinus node and a common site for atrial ectopy.

Is there anything else with this tracing?

Following the pause, sinus rhythm emerges for two cycles only (red arrows).          

The first sinus P wave is premature! Why?

Simple physiology. As there isa ventricular pause, sympathetic drive as well as other volume and pressure sensors increase the underlying sinus rhythm faster than the ectopic atrial rhythm. This is called ventriculo-phasic sinus arrhythmia and often overlooked when reporting ECGs. It is simply variations in the sinus rate, depending on whether there are QRS complexes during the P-P intervals and is seen in about 50% of cases of second degree or complete AV block.

To summarize:

The P-P interval with the embedded QRS complex is shorter than the P-P interval without the QRS complex.

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Wenckebach AV block.

2:1 AV block.

Complete heart block.

Anything more?

There are three pacemaker foci in the tracing.

• Ectopic atrial (blue highlight).
• Conducted sinus rhythm (red highlight).
• Junctional escape beat (yellow highlight) in the PR interval of the first non-conducting sinus P wave (red stippled arrow).

By definition this is a wandering atrial pacemaker.

When two or more atrial ectopics are present in a rhythm strip together with sinus rhythm, it is called a wandering atrial pacemaker or chaotic atrial mechanism.  When the rate exceeds 100 bpm, then the rhythm is referred to as a multifocal atrial tachycardia. There are often fusion beats as well.

Sinus rhythm (red highlight, red arrows) with atrial ectopics from two sites (yellow  and blue highlight, blue arrows). Two of the beats are atrial fusion (green highlight) where the timing results in the P waves being isoelectric or sinusoidal.

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Harry Mond