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 pause
There 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 pause
On 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?
This is ventricular bigeminy with second degree Wenckebach VA block
No, 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.
In 49+ years as a practicing cardiologist, Dr Harry Mond has published 260+ published manuscripts & books. A co-founder of CardioScan, he remains Medical Director and oversees 500K+ heart studies each year.
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