An electrocardiogram (ECG) traces the electrical activity of the heart, visualizing the sequence of depolarization and repolarization that drives cardiac muscle contraction. This non-invasive tracing allows assessment of the heart’s rhythm and function. Premature Ventricular Contractions (PVCs) are a common rhythm disturbance, appearing as an extra, early heartbeat originating from an abnormal electrical site within the lower chambers. Analyzing the changes a PVC introduces helps identify the source of this abnormal rhythm.
Components of a Normal ECG Rhythm
The normal cardiac cycle, initiated by the sinoatrial (SA) node, produces three distinct deflections on the ECG tracing. These three waves represent the orderly progression of the electrical impulse through the heart’s four chambers. The first wave seen is the P wave, which corresponds to the electrical activation, or depolarization, of the upper chambers, the atria. This electrical signal spreads across both atria, causing them to contract and pump blood into the ventricles.
Following the P wave, the electrical impulse pauses briefly at the atrioventricular (AV) node before rapidly spreading into the lower chambers. This rapid spread through the ventricles creates the large, sharp deflection known as the QRS complex. The QRS complex represents the electrical depolarization and subsequent contraction of the ventricles, the heart’s main pumping chambers.
The final component is the T wave, a broader, rounded wave that signifies ventricular repolarization, the electrical recovery period where the ventricular muscle cells reset their electrical charge. This repolarization allows the ventricles to relax and prepare for the next incoming impulse. A normal heart rhythm involves a consistent, predictable sequence of P wave, QRS complex, and T wave.
The Electrical Origin of a PVC
A Premature Ventricular Contraction disrupts this normal sequence when an irritable focus, known as an ectopic pacemaker, arises within the ventricular muscle tissue. This abnormal site fires an electrical impulse earlier than the SA node’s scheduled signal, causing a premature beat. Because the impulse does not follow the normal, rapid His-Purkinje conduction system, it spreads slowly and inefficiently through the surrounding ventricular muscle cells.
This slow, disorganized spread of electricity results in a QRS complex that looks significantly different from a normal beat. The QRS complex of a PVC is characteristically wide and often appears bizarre or distorted in shape compared to the narrow, sharp QRS of a normal beat. The duration of a PVC’s QRS complex is typically prolonged, lasting 0.12 seconds or longer, reflecting the slower muscle-to-muscle conduction.
The abnormal path of the electrical impulse also leads to an altered recovery phase, which is visible in the T wave. The T wave associated with the PVC often points in the opposite direction, or is discordant, to the main deflection of the preceding wide QRS complex. This distinct morphology of the wide QRS and discordant T wave confirms the impulse originated low in the ventricles rather than following the standard electrical pathway.
Why the P Wave Disappears
When a PVC appears on the ECG, the wave often not seen immediately preceding the wide QRS complex is the P wave. The P wave, which signals normal atrial activation from the SA node, is absent because the impulse originates prematurely in the ventricles, bypassing the atria entirely. The ventricular impulse activates the ventricles before the normal, scheduled atrial impulse can be generated or conducted.
The SA node, the heart’s natural pacemaker, continues to fire on schedule, but its signal does not cause the PVC. In some cases, the ventricular impulse travels backward (retrogradely) toward the atria, activating them abnormally and producing an inverted P wave. This retrograde conduction can activate the atria abnormally.
If this retrograde P wave occurs, it is frequently obscured by the large, wide QRS complex of the PVC. The sheer size and amplitude of the ventricular electrical event effectively masks the smaller atrial signal, meaning the P wave’s electrical signature is hidden within the visually dominant QRS complex.
Visualizing the PVC and the Compensatory Pause
A PVC interrupting the regular rhythm is typically followed by a distinct interval of silence, known as a full compensatory pause, before the next normal heartbeat resumes. This pause is a visual hallmark of a PVC.
The compensatory pause occurs because the SA node’s normal timing is undisturbed by the premature ventricular impulse. The SA node sends its next scheduled impulse on time, but the ventricles are still in their refractory period, the time needed to recover electrically after the PVC. This scheduled SA node impulse is blocked from activating the ventricles, creating the pause.
The next successful beat arrives exactly on the normal schedule of the SA node. This results in the interval between the normal beat preceding the PVC and the normal beat following the pause being equal to the length of two normal cardiac cycles. The full compensatory pause confirms the SA node was not reset, indicating a ventricular origin.