Atrial premature complexes (APCs)

An atrial premature complex originates from a focus that is “ectopic” (i.e. external) to the normal pacemaker system of the heart. It is not a physiological but rather a pathological event, triggered not by normal myocardial tissue that specializes in pace making but rather by myocardial tissue that specializes in mechanical contraction and conduction. Contracting cardiomyocytes are only supposed to respond to such pacing activity by “conveying” it forward towards more distal areas in the heart, but not to “proactively” initiate it. Its premature timing, by definition, precedes the point when the natural pacemaker should have initiated the next cycle, which typically results in a hemodynamically inefficient heartbeat.

Because such a premature QRS complex comes from a supraventricular focus, such as the atrial myocardium, its electrocardiographic morphology (which reflects the depolarization of the ventricular myocardium below) is identical to that of all other, normal QRS complexes, coming from the sino-atrial (SA) node. The reason for this morphological identity is that both normal and premature types of supraventricular complexes similarly travel across the AV-node, through the normal and fast-conducting system distal to it, towards and into the ventricles.

Therefore, and unlike ventricular premature complexes (VPCs), where the QRS morphology does differ from that of normal sinus complexes, atrial premature complexes (APCs) have a QRS that is normal-looking and should be morphologically identical to that of all sinus beats, which are related to as a “gold standard” for each patient. Other than their premature timing, nothing else helps identify them beyond doubt, and this is why they can be a challenging diagnosis to make, especially if they are not frequent.

Sometimes there is no P-wave preceding an APC, or the P that does precede it appears morphologically different from normal ones.

An example of such an APC is the sixth from the left in the time segment 08:16:02-08:16:06, when viewed at 50 mm/sec below, with an R-to-R interval of 432 ms. It has a P-wave that is very different from those of the preceding “standard” sinus complexes (originating in the sino-atrial node).

HR = 134 bpm, P = 55ms (wider than normal), 0.25 mv, PR = 90-105 ms, QRS = 55 ms, R = 1.1 mV, QT = 215 ms  | view the full report

However, this cannot be used as a pathognomonic finding because APCs often do occur with a preceding P that also happens to be morphologically normal and identical to other (“sinus”) P-waves. To see examples of normal-looking P-waves preceding APCs, skip to the following time sections while viewing them at 25 mm/sec, in the strip below:

• 18:03:47-18:03:55, the third complex from the left, with an R-to-R interval of 584 ms.
• 18:05:00-18:05:08, the seventh complex from the left, with an R-to-R interval of 488 ms.
• 18:05:34-18:05:42, the sixth complex from the right, with an R-to-R interval of 424 ms.

HR = 97-120 bpm, P = 40 ms, 0.4 mv, PR = 93 ms, QRS = 67 ms, R = 0.45 mV, QT = 200 ms  | view the full report

For these reasons, the morphology, the presence, or the absence of a P-wave preceding the suspected ectopic complex should not be used as rule-in or rule-out criteria for its prematurity.

When in doubt, carefully look at the same-suspected APC as seen in several (simultaneously inscribed) leads, rather than just at a single lead. 

In human beings, when the rhythm is highly regular, the time interval between the two beats flanking an APC is often only up to twice the normal beat-to-beat interval. This is opposed to the case with VPCs, where the combined interval between these two flanking complexes is often longer than twice the normal beat-to-beat interval, a phenomenon termed "a compensatory pause". This difference can sometimes be helpful in distinguishing APCs from VPCs based on auscultation only, even prior to having access to an ECG rhythm strip. However, this is much more challenging when sinus arrhythmia is present, as is commonly found in dogs.

The prematurity of an APC can be appreciated by combining auscultation with simultaneous palpation of arterial pulsation: when an ectopic atrial beat is premature, its matching pulse would be weaker than other pulses, or even totally absent. This is termed a "pulse deficit" and whether the palpated beat is weaker or absent would depend on exactly how early the premature heartbeat is. When extremely early, namely when the QRS-QRS interval is extremely short, diastole will be abbreviated enough to hardly allow for ventricular filling, rendering the following systolic contraction weaker enough than usual to generate no palpable pulsation at all. When diastole is a bit longer (but still shorter than usual) the resultant pulse will be palpable, but still weaker than usual. When auscultated, palpated, or merely visualized on an ECG tracing, a premature beat should take us by surprise, in that it appears at a time when it is not expected.

When the background rhythm is highly regular, this is a much easier task than when it is regularly irregular, as is typically found in many dogs with normal sinus arrhythmia. However, even in sinus arrhythmia, when a premature beat occurs just when the rhythm is in its slower phase, it should still draw our attention, as it should register as an unexpectedly premature beat. This is the reason for an increased "index of suspicion" which might often lead to a decision of documenting an ECG rhythm strip, to confirm such suspicions. In addition, during sinus arrhythmia, the cyclic change in heart rate is predictable and the transitions it generates between its relatively high and relatively low rate phases are typically gradual, rather than abrupt (as opposed to a "paroxysm" of a fast, pathological rhythm having a sudden onset, or a sudden offset, or both). This feature, too, should facilitate suspicion when one encounters an abruptly premature beat or complex in the midst of a gradually changing heart rate, whether during the low or the high rate phase of sinus arrhythmia.

In summary, premature (ectopic) beats are typically suspected first based on auscultation (which needs to be attentive and span a long enough section of time, to increase the likelihood of encountering premature beats). Usually, when they are suspected based on auscultation an ECG rhythm strip used to confirm their presence and their location of origin (as in APCs versus VPCs). To the inexperienced eye, APCs can sometimes be a bit hard to identify, especially when infrequent. Therefore, they should be very carefully sought, by screening each complex in the strip and its temporal relationships with its two flanking complexes, to seek prematurity rather than seeking a morphological difference, whether in QRS or in P-wave morphology.