We have previously looked at the atrial ectopic compensatory pause.Reviewing the ventricular ectopic compensatory pause is a lesson in timing!

A ventricular ectopic is a premature wide QRS with no preceding P wave. Following and encompassing the QRS is a compensatory pause (red highlight) terminated by the next sinus cycle.It extends from the commencement of the QRS to the onset of the P wave.Why a pause?To understand this, we need to define two types of compensatory pauses: Full and partial.Full compensatory pauseThere is a concealed non-conducted sinus P wave (red stippled vertical arrow) buried in the ectopic QRS. We wait for the next sinus cycle and hence the full pause.

Partial compensatory pauseOn occasion, the timings allow a retrograde P wave (green vertical arrow) from the ventricular ectopic to depolarize the atrium. This obviously precedes the sinus P wave and inhibits the next sinus cycle (blue stippled vertical arrow).

In theory, the retrograde and sinus P waves may collide resulting in a fusion beat, which because it is concealed, would not be recognised.The pause that follows is referred to as a partial compensatory and may look longer than the full pause on the ECG and hence is very confusing.This is rarely discussed in the literature and I have never seen an explanation.So let me try!

Full compensatory: The two sinus cycles containing the ventricular ectopic (three P waves) are identical to two sinus cycles without an ectopic (1700 ms blue stippled horizontal arrows). Partial compensatory: The two sinus cycles containing the ventricular ectopic (1800 ms) is shorter than the two sinus cycles without an ectopic (2000 ms) by the distance from the retrograde P wave to the next predicted sinus cycle (~200 ms yellow highlight). The cycle commences at the retrograde P wave and continues to the next sinus P wave (purple stippled horizontal line). In reality, a partial compensatory pause may be only a few ms shorter than a full one.A poorly understood feature of ventricular ectopics is that the non-conducted sinus P wave may not be concealed within the ectopic QRS.

With appropriate timing (including sinus rate, ectopic prematurity) as well as QRS width and shape, the sinus P wave may emerge from the tail of the QRS or even the T wave.

In the above left ECG, the sinus rate is ~60 bpm (blue highlight) and the non-conducted sinus P wave (red highlight and red stippled vertical arrow) lies at the end of the T wave.The right ECG has a heart rate of ~50 bpm (blue highlight) and the non-conducted sinus P wave is later again (yellow highlight and red stippled vertical arrow), but still in the refractory period of the ectopic. This mimics second degree AV block. I call this pseudo AV block and remember it is physiologic and depends on the timing.Now that we know everything about the compensatory pause, how would you interpret this ECG?

• Sinus rhythm with normal PR interval
• Ventricular bigeminy
• P waves in the ST/T (red vertical arrows)
• Timing suggests retrograde conduction (blue horizontal arrows)
• The retrograde P wave is gradually progressing through the ST/T (red horizontal arrows) with increasing R-P interval (very subtle).

This is ventricular bigeminy with second degree Wenckebach VA blockNo, I don’t understand how this occurs, but cardiac conduction is truly amazing.We will consider the ventricular ectopic without a compensatory pause next time.Remember it is all in the timing.Harry Mond