In the course of evaluating any electrocardiogram (ECG) or rhythm strip, it may become evident that, while a primary rhythm clearly exists, there are also additional beats that have morphology inconsistent with that underlying rhythm. These abnormal beats are referred to as ectopy, or ectopic beats. These ectopic electrical “beats” seen on the ECG or rhythm strip may or may not cause ventricular contraction or a mechanical beat. Because these ectopic beats occur between “normal” beats, however, they disrupt what may otherwise be a regular rhythm and may cause the pulse to feel irregular – at times, this irregularity in rhythm can be detected by the patient and thus become symptomatic.

Ectopy can arise from anywhere in the cardiac conduction system and from many segments of other cardiac tissues; ectopic beats are classified on the basis of where they originate: premature atrial contraction (PACs) with an atrial source, premature junctional contractions (PJCs) with a junctional or atrioventricular (AV) nodal source, and premature ventricular contractions (PVCs) with a ventricular source (Box 11.1). They are termed premature contractions because they appear between “normal” beats at a point earlier than one would expect the next beat. This earlier occurrence leads to a short R–R interval on the preceding beat and a long R–R interval on the following beat.

Premature Atrial Contractions

PACs are impulses generated in the atrial myocardial tissue. PACs are characterized by P waves of a different morphology and axis with a shorter than anticipated P–P interval when measured from the preceding beat. The PR interval in a PAC may be the same, shorter, or longer than that of the baseline rhythm, depending on the location of the ectopic atrial focus. Owing to the refractory period in the AV node and the His–Purkinje system, these impulses are not always conducted to the ventricles. When transmitted, however, the impulse follows the normal conduction pathway, resulting in a normal‐appearing, narrow QRS complex. PACs typically “reset” the SA node, resulting in a longer R–R interval immediately following the PAC (Figure 11.1).

Premature Junctional Contractions

PJCs arise from the AV node or the most proximal portion of the His–Purkinje system. The beat that follows proceeds through the normal conduction system and produces a narrow QRS complex that often lacks a preceding normal P wave and thus PR interval. Occasionally, however, P waves (often inverted) may be seen within the QRS complex, immediately preceding the QRS complex (with a very short P–R interval), or immediately following the QRS complex because of retrograde depolarization of the atria – producing the so‐called retrograde P wave. PJCs may or may not reset the SA node; therefore, the R–R interval immediately following the PJC may either be prolonged or may be consistent with the underlying rhythm (Figure 11.2).

Premature Ventricular Contractions

PVCs occur when an ectopic focus in the ventricular myocardium electrically discharges the ventricles prematurely, usually occurring during the relative refractory period of ventricular repolarization. This impulse gives rise to a QRS complex of unusual morphology, which is typically wide (>0.12 s) and lacking a preceding P wave.