What Do You Think 51

A lesson in timing.

This ECG has sat on my desktop for some time, waiting patiently for me to decide what is going on.

Maybe you can help!

What do you think?

When starting to interpret an ECG, look for the obvious.

Is there a P wave and is it sinus?

There are three conducted sinus P waves (red arrows) with narrow QRS complexes, one with a PR interval of 200 ms (red highlight), one with a prolonged PR interval of 240 ms (yellow highlight) and the third buried in the preceding T wave.

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A ventricular ectopic (green highlight) lies between two of the sinus beats. This is an interpolated ventricular ectopic which results in the next PR interval being extended.  Although unlikely this could be an atrial ectopic with aberrant ventricular conduction.  

Interpolation refers to a ventricular ectopic without a compensatory pause. Usually with a ventricular ectopic, there is a concealed non-conducted P wave buried in the QRS-T. With interpolation, there is no disruption of P wave conduction to the ventricle and for this to occur, there needs to be critical timing, together with the following criteria:

• No retrograde conduction.
• Appropriate prematurity of the ventricular ectopic.
• Sinus slowing.
• AV conduction beyond the absolute refractory period.
• Usually unmasking of a concealed sinus P wave.

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Let us summarize:

Within a conventional premature ventricular ectopic (red highlight), there is a concealed, non-conducted sinus P wave (red stippled arrow).

Occasionally, the non-conducted sinus P wave (red stippled arrow) becomes visible on the ECG.

As the sinus rate slows, the non-conducted P wave (red stippled arrow) may lie beyond the T wave.

In ECG reports, this is often called AV block. It may be more accurate to call it “pseudo AV block”.

Appropriate timing will allow AV conduction and thus interpolation.

Slow sinus rhythm (55bpm) and a ventricular ectopic (yellow highlight).  The sinus P wave lies beyond the ectopic T wave and conducts to the ventricle.

Partial AV refractoriness is very common with interpolation resulting in extension of thePR interval.

Despite the sinus cycles being regular (red arrows), the R-R interval of the QRS complexes with the embedded ventricular ectopic (yellow highlight) is extended by an amount equal to the PR extension (160 ms).

A short ventricular coupling interval (green stippled arrows) may also allow AV conduction and thus interpolation (yellow highlight).

So critical is the timing between AV conduction and non-conduction, that minute imperceptible changes in prematurity and sinus rate are enough to change the conduction status.

Let us now examine the latter half of the tracing.

There is another ventricular ectopic (blue highlight) from a different focus, which then sets up alternating ventricular ectopics from two foci with marginally different coupling intervals (560 ms and 640 ms) between the two ectopic forms.

This is a bidirectional rhythm and because the rate approximates 100bpm, it is called a bidirectional ventricular tachycardia.

There are many bidirectional rhythms(think ventricular bigeminy). Bidirectional ventricular tachycardia is a rare finding on the ECG demonstrating two usually broad alternating QRS morphologies with short coupling intervals. It was initially reported as an arrhythmia with digitalis toxicity. A number of QRS combinations have been reported including bundle branch and fascicular block configurations and even rarely, a narrow QRS. The most common appearance are two stable morphologically distinct foci or circuits in the distal His Purkinje system from both the left and right ventricles that alternate, thus creating what is called “ping pong in the His-Purkinje system” or the more scientific term QRS vector alternans.

The arrhythmia is also seen with the rare autosomal dominant inherited syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT) induced by adrenergic stress (exercise or emotion) and may degenerate into more serious, potentially lethal, ventricular tachyarrhythmias.

Sinus rhythm (red highlight) and the bidirectional ventricular tachycardia has a right(purple highlight) and left (grey highlight) bundle branch block configuration.

Complete heart block and bidirectional ventricular tachycardia.

Sinus rhythm with complete heart block (red stippled arrows) and an escape rhythm with a broad QRS (yellow highlight). Runs of bidirectional ventricular tachycardia with right (purple highlight) and left (grey highlight) bundle branch block configuration.

Because bidirectional ventricular tachycardia was most common with digitalis toxicity, the underlying rhythm was often atrial fibrillation. In this situation, it is important to differentiate the tachycardia from aberrant ventricular conduction.

Atrial fibrillation with a narrow QRS (yellow highlight). Run of bidirectional ventricular tachycardia with a right (purple highlight) and left (grey highlight) bundle branch block configuration. Although very unlikely, this could be a run of aberrant ventricular conduction with alternating right and left bundle branch block.

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My interpretation of the ECG is:

• Sinus rhythm.
• Interpolated ventricular ectopics.
• Bidirectional ventricular tachycardia.

Anyone with better suggestions?

Harry Mond