My first thought about ventricular interpolation was “everyone knows about this topic and there is nothing to teach”. Be that as it may, let me try.Here is a “typical” interpolated ventricular ectopic. A ventricular ectopic without a compensatory pause (red highlight).

Before we try to interpret the physiologic mechanisms, let us review the footprints of the typical ventricular ectopic (red highlight).

Why the compensatory pause? We have already discussed the mechanisms of the atrial ectopic compensatory pause in fact sheet 2.We also touched on interpolation and the timings of the sinus cycle lengths.Here are the sinus cycles with a “typical” ventricular ectopic.

Note the stippled red vertical arrow is a sinus P wave concealed in the premature ectopic QRS and does not conduct, because the conducting system is refractory.Not all P waves associated with ventricular ectopics are concealed.

In these examples, sinus P waves can be seen beyond the ectopic T waves (red highlight) and are dependent on the sinus cycle length and the ectopic prematurity. In the latter case, the P wave is well beyond the ectopic T wave, but AV conduction is still refractory. In both these cases, there are compensatory pauses.In order to fulfil the footprints of interpolation, AV conduction following the sinus P wave must not be totally refractory.

To summarize:

• The rhythm is relatively slow
• The ectopic very premature
• The P wave is usually not concealed.
• Because conduction is partially refractory, the P wave is conducted with first degree AV block (yellow highlight).
• Although the P-P intervals remain constant (excluding sinus arrhythmia), the R-R interval with the embedded ventricular ectopic is longer by a factor equal to the PR extension.

R-R (embedded QRS) 1340 ms = R-R 1140 ms + (400-200 ms) Of course, there are always exceptions to these rules!Here the ventricular ectopic is very late, yet still interpolated.

Here the sinus R-R interval is longer than the one with the embedded ventricular ectopic.

Theoretically, in the absence of sinus arrhythmia, this shouldn’t occur.Think of another mechanism! I have labelled a probable inverted P wave, suggesting this is an echo beat rather than interpolation. Following the ventricular ectopic, there is retrograde conduction to the atrium, which then conducts anterograde creating an echo QRS.Some may suggest that the mechanisms creating this should also result in a longer “R-R interval with the embedded ventricular ectopic”, and you would be correct.

Not surprisingly, the sinus beat following the interpolated ventricular ectopic may be conducted with aberration.

This is an excellent example of how the QRS width with aberration varies between different leads and can be almost normal in some.With marked sinus bradycardia, the appearances begin to suggest second degree AV block (It’s all in the timing).

Ventricular ectopics in groups (ectopy), create strange patterns and combinations with both bigeminy and trigeminy (try and work these out!). They may not be all ventricular ectopics.

Quadrigeminy is easier.

The smart ones may call this parasystole, but it isn’t! I will discuss this another time.Interpolated ventricular couplets are even more complicated with different timings and interpretations.

The top example of an interpolated couplet is straightforward. There is one P wave (blue vertical), which although concealed, conducts to the ventricle.The bottom example is more complex. Is the second ectopic actually sinus and conducted with aberration from the concealed P wave (red vertical stippled)? There is still interpolation, but not necessarily a couplet.Can interpolation occur with atrial fibrillation? Remember it is just timing.

Can there be pseudo-ventricular interpolation?In this example, the artefact (red highlight) was called an interpolated ventricular ectopic and the real ventricular ectopic (yellow highlight) was called artefact.

I stated earlier that in order for interpolation to occur, there must be a relative bradycardia. With a tachycardia, the compensatory pause shortens and may give the impression of interpolation.

Now you know everything about interpolation!Remember, it’s all in the timing!Harry Mond