An electrocardiogram (EKG) records the heart’s electrical activity using electrodes placed on the skin. This test translates the small electrical changes that occur with each heartbeat into a visual tracing, allowing healthcare providers to analyze the heart’s rhythm and function. A Premature Ventricular Contraction (PVC) is a common irregular heartbeat where an extra beat originates in the lower chambers (the ventricles) before the next normal beat is expected. This early contraction disrupts the steady heart rhythm, and this article details the specific visual features a PVC leaves on the EKG tracing.
Establishing the Normal Heartbeat on an EKG
A normal heart rhythm (sinus rhythm) is initiated by the heart’s natural pacemaker, the sinoatrial (SA) node, resulting in a characteristic sequence of waves on the EKG. The first deflection is the P wave, representing the electrical activation (depolarization) of the upper chambers (the atria). This is typically a small, rounded, upright wave.
The signal pauses briefly at the atrioventricular (AV) node before rapidly traveling to the lower chambers. The subsequent large deflection is the QRS complex, which signifies the depolarization of the ventricles. In a normal heart, this complex is narrow and sharp, typically lasting between 0.06 and 0.10 seconds. Following the QRS complex is the T wave, representing the electrical recovery (repolarization) of the ventricles. In a regular sinus rhythm, a positive P wave precedes every narrow QRS complex, establishing the baseline against which abnormalities are measured.
The Mechanics of a PVC
A Premature Ventricular Contraction differs from a normal beat because its electrical impulse does not originate in the SA node. Instead, a PVC arises from an “ectopic focus,” an irritable site located within the ventricular muscle tissue. This ectopic site generates an impulse prematurely, meaning the beat occurs earlier than the SA node’s expected signal.
Because the impulse starts in the ventricle, it bypasses the heart’s normal, high-speed electrical wiring system (including the bundle branches and Purkinje fibers). The electrical current must instead spread slowly, muscle cell by muscle cell, throughout the ventricular tissue. This slow, inefficient cell-to-cell spread of electricity is the physiological reason for the dramatic change in the EKG’s visual appearance.
The mechanisms causing this ectopic firing can involve increased automaticity (where a ventricular cell spontaneously depolarizes) or triggered activity. The result is always a depolarization that is out of sync and travels through the heart’s muscle much slower than a normal signal.
Identifying the PVC: Specific Visual Characteristics
The most noticeable feature of a PVC on an EKG is the wide and bizarre QRS complex. Because the electrical signal travels through muscle tissue rather than the fast conduction system, depolarization takes significantly longer, prolonging the QRS complex to 0.12 seconds or more. This complex also appears unusually shaped, often notched or slurred, reflecting the abnormal impulse pathway.
A second defining characteristic is the absent P wave preceding the abnormal QRS complex. Since the impulse originated deep within the ventricles, it bypasses the normal sequence of atrial activation, resulting in no associated P wave. The T wave following the wide QRS complex is typically large and often points in the opposite direction (discordant) from the main QRS direction.
The third major feature is the full compensatory pause that usually follows the PVC. The premature beat does not reset the heart’s normal pacemaker timing. Although the SA node fires its next impulse on schedule, the ventricles are electrically refractory from the PVC and cannot respond. This delay results in a pause before the next normal beat is conducted, making the total time surrounding the PVC equal to the time of two normal heartbeats.
Patterns and Terminology for PVC Groupings
When PVCs occur frequently, they fall into recognizable arrangements identified by specific terminology.
Grouping Patterns
If a PVC follows every normal beat, the pattern is called bigeminy (Normal-PVC-Normal-PVC). If the abnormal beat occurs after every two normal beats, the pattern is known as trigeminy (Normal-Normal-PVC). When multiple PVCs occur consecutively, they are classified by number. Two PVCs in succession are termed a couplet, and three PVCs in a row are called a triplet. A triplet is sometimes referred to as a short run of non-sustained ventricular tachycardia.
Morphology
The shape, or morphology, of the PVCs indicates the number of ectopic sites. If all PVCs on the rhythm strip look exactly the same, they are classified as unifocal, suggesting they originate from a single spot. If the PVCs display two or more different shapes, they are called multifocal, indicating the impulses arise from various locations within the ventricles.