There has been some difficulty understanding the different types of compensatory pauses following an atrial ectopic.A premature atrial ectopic will usually be propagated retrograde toward the sinus node. If it invades and depolarizes the sinus node, resetting of the sinus cycle will occur. Consequently, the cycle length after a premature atrial ectopic is longer than the basic sinus cycle, creating what is called a partial compensatory pause.

In this illustration, the sinus cycle is 1350 ms (44 bpm). The premature atrial ectopic, highlighted in red, has no obvious P wave, because it is concealed and resets the sinus cycle, so the next P wave does not occur (blue stippled arrow). The sinus cycles with the embedded atrial ectopic are only 2000 ms apart rather than the 2700 ms without an atrial ectopic; hence the term partial compensatory pause.If however, the sinus node is not depolarized, the next sinus cycle results in a concealed sinus P wave and a full compensatory pause.

The premature atrial ectopic, highlighted in red, has a P wave preceding it. The next sinus cycle falls in the refractory period of the conducting system (red stippled arrow) and is not conducted. Now there is a full compensatory pause.On occasion, resetting the sinus node results in a prolonged suppression of sinus activity (greater than a full compensatory pause). This is referred to as a long compensatory pause resulting from an abnormal or prolonged sinus node recovery time, indicative of sinus node dysfunction.

The absence of a compensatory pause is referred to as interpolation.

In the presence of long sinus cycles or sinus bradycardia, a premature atrial ectopic may conduct to the ventricle leaving enough time for the atrioventricular conduction to at least partially recover before the next sinus beat. This next sinus P wave will conduct to the ventricles, albeit usually with a prolonged PR interval or marked first degree AV block. Because of the prematurity, the atrial ectopic as well as the next sinus beat may be conducted with aberration.

The three ECGs above are from the same patient. The first (left) shows an atrial ectopic with a compensatory pause and no aberration. In the next two ECGs (right), the sinus rate is slower and the atrial ectopics are more premature with the P waves buried in the T waves of the previous sinus cycle. This results in interpolation with degrees of aberration. The next sinus complex has first degree AV block.Assoc Prof Harry Mond