The experience of an irregular heart rhythm, often felt as a flutter or a sudden thud, frequently leads people to search for information about specific heart conditions. Two common terms encountered are Premature Ventricular Contractions (PVCs) and Supraventricular Tachycardia (SVT). While both involve electrical disturbances in the heart’s normal rhythm, they represent distinct phenomena with different origins and patterns. Understanding these differences is essential for clarifying a diagnosis.
Understanding the Heart’s Electrical System
The heart’s rhythmic pumping action relies on a precisely timed electrical sequence. The sinoatrial (SA) node, located in the upper right chamber (atrium), acts as the natural pacemaker, generating the initial electrical impulse. This signal spreads across the atria, causing them to contract and push blood into the lower chambers (ventricles).
The electrical impulse then travels to the atrioventricular (AV) node, which momentarily delays the signal. This pause ensures the ventricles have time to fill with blood before contracting. The signal moves through the bundle of His and the Purkinje fibers, rapidly spreading excitation throughout the ventricles, triggering their contraction. This entire process, from the SA node impulse to the ventricular contraction, constitutes one normal heartbeat, called normal sinus rhythm.
Premature Ventricular Contractions (PVCs)
A Premature Ventricular Contraction is a single electrical event that disrupts the normal sequence of the heartbeat. This extra beat originates from an ectopic focus located within the ventricles, often in the Purkinje fibers. This ventricular focus spontaneously fires early, bypassing the normal electrical pathway from the SA node.
Because the impulse starts in the ventricles and bypasses the usual conduction system, the resulting contraction is often disorganized and less effective at pumping blood. The beat is premature because it occurs before the next expected normal beat, interrupting the heart’s regular rhythm. This early beat is typically followed by a pause, which can cause the subsequent normal beat to feel stronger or like a “thud” or “skipped beat.” PVCs are common and can be triggered by factors like stress, caffeine, or electrolyte imbalances.
Supraventricular Tachycardia (SVT)
Supraventricular Tachycardia describes a rapid, sustained heart rhythm that originates in the heart tissue located above the ventricles. This supraventricular area includes the atria and the atrioventricular (AV) node. Unlike the single, premature beat of a PVC, SVT involves a sustained run of fast heartbeats, typically ranging from 150 to 250 beats per minute.
The most common mechanism involves a re-entry circuit, an electrical short circuit that causes the impulse to loop continuously. For example, in Atrioventricular Nodal Reentrant Tachycardia (AVNRT), the circuit utilizes two pathways within the AV node. An early impulse, often a premature atrial contraction, gets caught in this loop, causing the rapid rhythm. This sustained high rate can feel like the heart is suddenly “racing” or “pounding” and may cause symptoms like dizziness or shortness of breath.
How PVCs and SVTs Differ
The fundamental difference between PVCs and SVTs lies in their origin, their pattern, and the resulting clinical picture. A PVC is an isolated extra beat originating from an ectopic focus in the ventricles (lower chambers). In contrast, SVT is a continuous, rapid rhythm originating from the upper chambers or the AV node (structures above the ventricles).
The rhythm pattern is also distinct: PVCs are characterized by a single premature beat that interrupts an otherwise normal rhythm. SVT is defined by a sustained, rapid heart rate, or tachycardia, that starts and stops abruptly. On an electrocardiogram (ECG), the electrical signal of a PVC appears wide and abnormal because it bypasses the normal ventricular conduction system.
SVT typically presents with a narrow electrical signal because the impulse often uses the normal His-Purkinje system to activate the ventricles. While frequent PVCs can sometimes lead to reduced heart function over time, they are generally considered less immediately destabilizing than SVT. SVT, due to its sustained high rate, can compromise the heart’s ability to fill properly and may require immediate intervention to break the re-entry circuit.