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Compensatory Versus Non-Compensatory Pauses: It’s All About the P Waves
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There is a great misunderstanding among those in the EP field regarding compensatory and non-compensatory pauses. This article aims to enlighten readers of any misconceptions.
The terms compensatory and non-compensatory refer to the post-ectopic pause that follows a premature atrial, junctional, or ventricular impulse. These terms have a bit of a confusing history. Some maintain that there is only a compensatory pause, which is either complete or incomplete, and the term non-compensatory is not used. I prefer using the terms compensatory and non-compensatory, as they are both self-evident and do not require further qualification. That is the terminology that will be used in this article.
Whether a post-ectopic pause is compensatory or non-compensatory depends on the effect that the ectopic beat has on the sinoatrial (SA) node. If the ectopic beat is able to reach the SA node — and finds it in a non-refractory state — the ectopic beat will discharge the SA node. If the ectopic impulse arrives while the SA node is refractory, nothing happens to the SA node, although the subsequent sinus impulse may be blocked by refractory tissue left in the wake of the ectopic beat.
What does “discharge the SA node” imply? Can we see the P wave produced by this “discharge”? To “discharge” the SA node means that the pacemaking cell within the SA node is rushed on through the depolarization and repolarization process by the arriving ectopic impulse. Following repolarization, the pacemaking cell then begins the process of spontaneous diastolic depolarization (SDD) once again. We will not see a P wave produced by this discharge of the SA node by the ectopic impulse, because the discharged impulse encounters absolute refractoriness in the surrounding tissue caused by the ectopic impulse. We must wait for the next spontaneous depolarization by the pacemaking cell to see a P wave. Therein lies the distinction between a compensatory pause and a non-compensatory pause.
Before we go any further, the concepts of (1) compensatory pauses, (2) non-compensatory pauses, and (3) interpolated beats (discussed later) are based on the actions of the SA node and consequently involve only P-P intervals. The QRS complexes are not part of this equation! Measurement of the R-R intervals can be very misleading due to the action of the ectopic beat on the AV node, which may cause a prolonged P-R interval (an example will be provided later). Again, all three concepts involve effects on the SA node only and are not based on the R-R intervals.
Let’s begin with the definition of a compensatory pause: a compensatory pause results when the ectopic impulse fails to enter the SA node. The SA node continues to produce impulses without any effect from the ectopic impulse. Each sinus P wave is normal and right on time. Occasionally, we can see every P wave produced, as in Figure 1.
In Figure 1, there is a sinus P wave immediately following the PVC. It falls during the refractory period of the AV node and/or His bundle, and fails to conduct to the ventricles. However, the point here is that the SA node was not affected by the PVC. Usually, the first post-ectopic P wave is not visible on the ECG strip because it is hidden in the repolarization of the ectopic beat — but if the pause is compensatory, it demonstrates that the P wave is really there and right on time.
Figure 2 contains two premature atrial complexes (PACs). It is a bit unusual in that one PAC is followed by a compensatory pause and the other by a non-compensatory pause. Contrary to popular belief, PACs followed by compensatory pauses are not rare.
The red line at the beginning of Figure 2 measures two normal P-P intervals. (When possible, always measure from/to the beginning of the deflection.) The two blue lines are the same line – just a different color. We begin by measuring the P-P interval (not the R-R interval) surrounding the first PAC, and easily see that it is exactly two P-P intervals in length. This represents a compensatory pause following a PAC. We cannot see the first post-ectopic P wave, because it is hidden in the S wave of the PAC.
Now let’s move on to the second PAC. The blue line in Figure 2 indicates that the visible P-P interval surrounding this PAC is less than two P-P intervals. This represents a non-compensatory pause. This is what happened to cause the non-compensatory pause: before the SA node could complete its depolarization process (which is relatively slow since it is based on slow calcium channels), the PAC entered the SA node and discharged it. The SA node then began its depolarization process once again. We call this process a reset. Because the time from the last normal P wave (before the reset occurred) to the reset itself is less than a full P-P interval, the P-P interval surrounding the ectopic beat is equal to one full P-P interval plus that partial P-P interval (ie, less than two complete, normal P-P intervals).
So, if the P-P interval surrounding an ectopic beat is equal to two normal P-P intervals, we call that a compensatory pause. And if the P-P interval surrounding an ectopic beat is less than two normal P-P intervals, we call that a non-compensatory pause. What do we call a P-P interval surrounding an ectopic beat that is longer than two normal P-P intervals? Answer: a non-compensatory pause! A non-compensatory pause can be less than or greater than two normal P-P intervals. Figure 3 is an example from my collection.
The two blue lines in Figure 3 are exactly the same length. The first line demonstrates two normal P-P intervals, while the second line indicates a non-compensatory pause that is longer than two normal P-P intervals. How does this happen? When the ectopic impulse enters the SA node, it usually just discharges it and resets it back to its normal resting membrane potential (around -60 mV), where it immediately resumes depolarization once again. In some instances, however, the ectopic impulse resets the SA pacemaking cell(s) to a much more negative membrane potential, which causes the pacemaking cell to take more time to reach its threshold potential. Therefore, the onset of the first SA nodal depolarization following an ectopic impulse can be significantly delayed.
If you have a very keen eye (or a set of calipers), you will have noticed that something else has happened in this snippet. The “normal” P-P intervals following the non-compensatory pause are now longer than the P-P intervals before the ectopic beat. The sinus rate has suddenly slowed. This occurs occasionally following non-compensatory pauses. (Why just non-compensatory pauses? Because ectopic beats with compensatory pauses have had no effect on the SA node!)
It is important that only the P-P intervals are measured. Every day there is a journal article or website that promotes measuring the R-R interval instead of the P-P interval. That is wrong! The R-R interval can be influenced by concealed conduction into the AV node causing an increase in the R-R interval that has nothing to do with compensatory or non-compensatory pauses. Do not measure the R-R intervals in these cases!
Figure 4 demonstrates an interpolated PVC. “Interpolated” means that: (1) the SA node was not affected — the PVC did not enter the SA node, and (2) the P wave that immediately follows the PVC (or other ectopic beat) was able to conduct. The bottom line is: just like the compensatory pause, the SA node was not affected and continued to produce normal P waves right on time. However, there is one issue.
Look at the P-R interval immediately before the PVC and the P-R interval that follows the PVC. The P-R interval that follows the PVC is longer. This is due to concealed conduction in the AV node. Concealed conduction refers to an impulse that has an effect on conduction of the following impulse — usually due to absolute or relative refractoriness. In this case, although the PVC did not enter the SA node, it did manage to enter the AV node where it was stopped. This left the AV node refractory. It was in a relatively refractory state when the sinus P wave arrived following the PVC and that sinus impulse was conducted more slowly. This resulted in a prolonged P-R interval, and therefore, an extended R-R interval.
In summary, do not measure R-R intervals when determining the presence of compensatory pauses, non-compensatory pauses, and interpolated ectopic beats. It’s all about the P waves!
Disclosures: The author has no conflicts of interest to report regarding the content herein.
