Tachyarrhythmias, abnormally fast heart rhythms, originate from two main regions of the heart: above the ventricles (supraventricular) or within the ventricles themselves. The dividing line is a structure called the His bundle, a cluster of specialized cells that relays electrical signals from the upper chambers to the lower chambers. Every tachyarrhythmia can be traced to a specific structure or tissue within one of these two zones, and the exact origin determines how the rhythm behaves, how it looks on an ECG, and how it’s treated.
The Two Major Zones of Origin
The heart’s electrical system runs from the top down: the sinus node fires in the right atrium, the signal travels to the atrioventricular (AV) node, passes through the His bundle, then splits into bundle branches that activate the ventricles. Tachyarrhythmias that start at or above the His bundle are called supraventricular tachycardias (SVTs). Those that start below it, in the ventricular muscle or lower conduction fibers, are ventricular tachycardias (VTs).
This distinction matters practically. SVTs generally produce a narrow, normal-looking heartbeat pattern on an ECG (QRS under 120 milliseconds), while VTs produce a wide, abnormal pattern. A narrow complex on a fast rhythm almost always means the problem started up top. A wide complex could be either, but several ECG clues, like the heart’s upper and lower chambers beating independently of each other (AV dissociation), strongly point to a ventricular origin.
Supraventricular Origins: Atria and AV Node
The most common supraventricular tachyarrhythmias in adults are atrial fibrillation and atrial flutter. Beyond those, the major types include AV nodal reentrant tachycardia (AVNRT), AV reciprocating tachycardia (AVRT), and focal atrial tachycardia. Each one traces back to a different structure.
Atrial Fibrillation and the Pulmonary Veins
Atrial fibrillation, the most common sustained arrhythmia worldwide, is typically triggered by rapid electrical firing from the pulmonary veins, the four vessels that carry blood from the lungs back into the left atrium. The left superior and left inferior pulmonary veins are particularly common culprits, though any of the four can be involved. Sleeves of atrial muscle tissue extend into these veins, and these cells can fire erratically, bombarding the atrium with chaotic electrical impulses. This is why catheter ablation for atrial fibrillation focuses on electrically isolating the pulmonary veins from the rest of the atrium.
Atrial Flutter and the Cavotricuspid Isthmus
Typical atrial flutter follows a very predictable loop. The electrical wave circles around the tricuspid valve in the right atrium, bounded on one side by the valve itself and on the other by a ridge of tissue called the crista terminalis. The critical bottleneck in this circuit is a narrow strip of tissue between the inferior vena cava and the tricuspid valve, known as the cavotricuspid isthmus. Because all the electrical activity funnels through this small gap, it’s an ideal target for ablation. A line of energy across that isthmus breaks the circuit permanently.
AVNRT and the Dual Pathway System
AVNRT is the most common type of SVT that causes sudden episodes of rapid heartbeat. It originates from a reentry circuit within or immediately next to the AV node. In people with this condition, the AV node area has two distinct electrical pathways instead of the usual one: a fast pathway that conducts quickly but recovers slowly, and a slow pathway that conducts slowly but recovers quickly. Under the right conditions, an electrical impulse can travel down one pathway and loop back up the other, creating a self-sustaining circuit that drives the heart rate to 150 to 250 beats per minute.
About 90% of AVNRT cases follow the “slow-fast” pattern, where the impulse travels down the slow pathway and returns up the fast one. The remaining cases use the reverse direction or involve two slow pathways.
Focal Atrial Tachycardia
Some tachyarrhythmias arise from a single irritable spot in the atrial tissue that fires faster than the sinus node. Common locations include the crista terminalis (a muscular ridge in the right atrium), the atrial appendages, and the junction where the superior vena cava meets the right atrium. In children, the atrial appendages are the most frequent source, which differs from adults. These focal tachycardias can be mapped precisely during an electrophysiology study and are often curable with ablation.
Accessory Pathways: Shortcuts Across the Valve Plane
In conditions like Wolff-Parkinson-White syndrome, an extra electrical connection bridges the atria and ventricles outside the normal AV node pathway. These accessory pathways can exist anywhere along the ring of tissue separating the upper and lower chambers, but they cluster in predictable spots. The most common location is along the left side of the mitral valve, accounting for 30% to 58% of cases. The posteroseptal region, near the back of the heart where the septum meets the valve rings, is the next most common at roughly 25% of cases.
These pathways create the conditions for AVRT, where an electrical impulse loops from the atria to the ventricles through one route (say, the normal AV node) and back up through the other (the accessory pathway), or vice versa. The location of the pathway affects both the ECG pattern and the success rate of ablation. Left lateral and posteroseptal pathways have ablation success rates above 90%, while pathways near the coronary sinus or in the midseptal region are harder to reach, with success rates dropping to 50% to 73%.
Ventricular Origins: Muscle and Conduction Fibers
Ventricular tachycardias start below the His bundle, either in the thick ventricular muscle or in the specialized conduction fibers (Purkinje fibers and bundle branches) that distribute electrical signals across the ventricles. The mechanism and location depend heavily on whether the heart is structurally normal or diseased.
Scar-Related Ventricular Tachycardia
In people with prior heart attacks or other forms of heart disease, VT most often arises from the border zone of scar tissue in the ventricular wall. Healthy heart muscle conducts electricity smoothly, but scar tissue creates a patchwork of conducting and non-conducting zones. Electrical impulses weave through surviving channels within the scar, slowing down enough to loop back and re-excite tissue that has already recovered. This reentry mechanism is the most common cause of sustained VT in the setting of heart disease.
The Purkinje-Muscle Junction
Research published in JCI Insight has identified a specific origin point for certain ventricular arrhythmias: the junction where Purkinje fibers meet the working heart muscle in the inner (subendocardial) layer of the ventricle. In conditions involving abnormal calcium handling within heart cells, small electrical disturbances in the ventricular muscle cells, too weak to cause a full beat on their own, can trigger the adjacent Purkinje fibers to fire a complete impulse. This mechanism appears relevant not only in inherited arrhythmia syndromes but potentially in heart failure as well. Cells farther from this junction, such as those in the outer (epicardial) layer, do not appear to trigger these arrhythmias even when they carry the same underlying defect.
Outflow Tract Tachycardias
In people with structurally normal hearts, the single most common site of origin for VT is the right ventricular outflow tract (RVOT), the muscular funnel that channels blood from the right ventricle into the pulmonary artery. These tachycardias arise from abnormal automatic firing in the RVOT, particularly along the septum (the wall between the ventricles) and the free wall. The septum and free wall produce subtly different ECG patterns. Septal sites generate taller, narrower beats in the inferior ECG leads and show earlier transition across the chest leads, while free-wall sites tend to produce notched, broader beats. These distinctions allow doctors to pinpoint the exact origin before ablation, with ECG-based localization proving accurate in about 90% of cases.
Fascicular Tachycardia
The left bundle branch splits into three fascicles: the left anterior, left posterior, and left septal (middle) fascicle. Tachycardia can originate within any of these, producing a distinct ECG pattern for each. The left posterior fascicle is the most commonly involved, producing a characteristic pattern that responds to the calcium channel blocker verapamil rather than to drugs that work on most other VTs. This unusual drug sensitivity is one of the hallmarks that helps identify fascicular VT. Left anterior fascicular VT and left septal fascicular VT are less common but follow the same general reentry mechanism within their respective conduction fibers.
How Location Shapes the Heartbeat Pattern
The origin of a tachyarrhythmia directly determines the shape of each heartbeat on an ECG. When a rhythm starts high in the conduction system, the electrical impulse still travels through the normal highways of the His bundle and bundle branches, producing a narrow, efficient-looking QRS complex. When a rhythm starts in the ventricular muscle itself, the impulse has to spread cell by cell through tissue not designed for rapid conduction, producing a wide, distorted QRS.
Within ventricular tachycardias, the specific origin also shapes the QRS. A rhythm from the right ventricle produces a pattern resembling left bundle branch block, because the right ventricle activates first and the impulse then spreads leftward. A rhythm from the left ventricle produces the opposite: a right bundle branch block pattern. The axis of the heartbeat in the limb leads further narrows the location, with inferior-axis patterns pointing to origins near the base of the heart and superior-axis patterns pointing to origins near the apex. These ECG signatures allow clinicians to map the origin of a tachyarrhythmia from a simple 12-lead tracing, often before any invasive testing is done.