Ectopic complexes arise from areas other than the sinus node and, when manifest, disturb the normal sequence of automaticity and conduction in the heart. Premature complexes most commonly originate in the ventricles and atria but may also occur in the atrioventricular (AV) junction. Regarding terminology, ectopic does not refer to a mechanism but to the origin of the complex outside the sinus nodal area. Mechanisms include automaticity and reentry and are discussed in more detail in Chapter 5. These abnormal complexes are often referred to as beats, although technically a beat suggests a stroke or pulsation of the pulse or heart, whereas PVCs and PACs are actually electrical events and may not be coupled with contraction. Regardless, premature beats is a term so ingrained in the literature that it is not worth altering merely for semantics. This chapter discusses the electrocardiographic (ECG) consequences of premature complexes.
PREMATURE ATRIAL COMPLEXES
Effects on Sinus Nodal Function
Premature atrial complexes (PACs) can affect sinus nodal automaticity and conduction. The ECG events are a result of conduction from the PAC that is “concealed,” but the results of which are manifest.1-4 Figure 2-1 is a schematic illustrating some of these general principles. PACs may result in a compensatory pause, reset, or are interpolated (Figure 2-1).5,6 A compensatory pause occurs when an atrial complex fails to conduct retrogradely into the sinus node and does not allow the regular sinus impulse to manifest itself because of refractoriness of the tissue after the PAC. The subsequent sinus impulse is on time and conducts normally to the atrium (Figure 2-1, I). Thus, the interval from the spontaneous atrial complex (A1) preceding the PAC (A2) plus the time from the PAC to the subsequent sinus beat (A3) after the PAC (A1A2 + A2A3) equals twice the spontaneous sinus interval (2 × A1A1). Further, the return cycle length (R1), or A2A3, in this example is greater than the spontaneous sinus cycle length. The second return cycle (R2), or A3A4, will equal the basic (B) sinus cycle length if there has been no disturbance of sinus nodal automaticity and the sinus rate has minimal normal variability. An example of a compensatory pause produced at electrophysiologic testing is shown in Figure 2-2.
Schematic of effects of premature atrial complexes on sinus nodal automaticity. The ladder diagram represents conduction from the sinus node (S) to the atrium (A) showing conduction time by the diagonal line connecting S with A. Block is represented by a small horizontal line perpendicular to the normal conduction line. With interpolation, the third sinus impulse conducts without increased anterograde delay from the sinus node ...