Tachycardia and tachyarrhythmias are classified using several overlapping systems: by where in the heart they originate, by the width of the electrical signal on an ECG, by rhythm regularity, by the underlying cellular mechanism, and by how well the body tolerates them. A heart rate above 100 beats per minute in adults defines tachycardia, and the term “tachyarrhythmia” specifies that the fast rate stems from an abnormal electrical rhythm rather than a normal response to exercise or stress.
Classification by Anatomical Origin
The most fundamental division separates tachyarrhythmias into two groups based on where the abnormal electrical activity starts. The dividing line is a structure called the His bundle, a small cluster of specialized fibers that sits between the upper and lower chambers of the heart and acts as the electrical bridge between them.
Supraventricular tachycardias (SVTs) originate at or above the His bundle, meaning they arise in the atria or the junction between the atria and ventricles. This group includes atrial fibrillation, atrial flutter, and several types of reentrant circuits that loop through the upper heart. Ventricular tachycardias (VTs) originate below the His bundle, in the muscular walls of the ventricles themselves. This distinction matters because ventricular arrhythmias are generally more dangerous, since the ventricles are responsible for pumping blood to the lungs and the rest of the body.
Classification by QRS Width
On an ECG tracing, each heartbeat produces a spike called the QRS complex, which represents the ventricles contracting. The width of that spike is one of the fastest ways to sort a fast rhythm into categories. A QRS duration under 120 milliseconds is considered narrow, and a duration of 120 milliseconds or more is considered wide.
Narrow-complex tachycardias almost always originate above the ventricles. The electrical signal travels down through the heart’s normal wiring, so the ventricles contract in a coordinated, efficient way that produces a slim QRS. Wide-complex tachycardias are trickier. They can be ventricular in origin, meaning the signal spreads slowly through muscle tissue rather than the specialized conduction system. But they can also be supraventricular rhythms that conduct abnormally, a situation called “SVT with aberrancy.” Telling the two apart is one of the most important challenges in emergency cardiology.
One widely used approach for distinguishing them is the Brugada algorithm, which applies four steps in sequence. If no RS complex appears in any of the chest leads (V1 through V6), the rhythm is ventricular tachycardia. If the interval from the start of the R wave to the deepest point of the S wave exceeds 100 milliseconds in any chest lead, it’s VT. If the atria and ventricles are beating independently of each other (AV dissociation), it’s VT. Only if none of those criteria are met does the algorithm consider specific shape patterns in leads V1 and V6 before concluding the rhythm is supraventricular.
Classification by Rhythm Regularity
Fast rhythms can also be sorted by whether the heartbeat is regular or irregular, which narrows the diagnostic possibilities considerably.
Regular tachycardias include ventricular tachycardia, atrial flutter, and several types of reentrant SVT where the electrical impulse follows the same loop on every beat. In atrial flutter, the upper chambers may fire at rates of 250 to 350 beats per minute, though the ventricles typically respond at a fraction of that rate because the conduction system blocks some of the signals.
Irregular tachycardias are a smaller group. Atrial fibrillation, the most common arrhythmia overall, is characteristically irregular because the atria fire chaotically rather than in an organized circuit. The ventricles respond to these signals unpredictably, producing an uneven pulse. Polymorphic ventricular tachycardia, where the shape of the QRS complex changes from beat to beat, also produces an irregular pattern.
Classification by Cellular Mechanism
At the cellular level, three mechanisms produce virtually all tachyarrhythmias: reentry, enhanced automaticity, and triggered activity.
Reentry is the most common mechanism and drives the majority of both supraventricular and ventricular tachycardias. It happens when an electrical impulse doesn’t die out after activating the heart muscle. Instead, it loops back around an obstacle, whether that’s a patch of scar tissue or an extra electrical pathway, and re-excites tissue that has already recovered. The impulse essentially chases its own tail in a continuous circuit. This is the mechanism behind atrial flutter, many SVTs, and most sustained ventricular tachycardias.
Enhanced automaticity occurs when heart cells that normally wait for an electrical signal begin firing on their own at an abnormally fast rate. Certain cells in the atria, the junction, and the ventricles have the ability to generate impulses spontaneously, and when they do so too quickly, they can override the heart’s natural pacemaker.
Triggered activity falls between the other two mechanisms. It happens when leftover electrical fluctuations at the end of a heartbeat are strong enough to trigger an extra beat. These aftershocks can cascade into a sustained arrhythmia. Triggered activity is the mechanism behind Torsades de Pointes, a distinctive form of polymorphic ventricular tachycardia associated with a prolonged QT interval.
Ventricular Tachycardia Subtypes
Ventricular tachycardia gets its own layer of classification based on the shape of the QRS complex. Monomorphic VT shows a single, stable QRS shape with no variation from beat to beat. This consistency usually means the electrical signal follows the same abnormal path each time, often circling around a scar from a previous heart attack.
Polymorphic VT shows a QRS complex that shifts in shape and size from one beat to the next, indicating that the electrical pathway is unstable. The most recognizable form is Torsades de Pointes, where the QRS amplitude gradually grows and shrinks in a pattern that looks like the tracing is twisting around the baseline. The name literally means “twisting of the points.” Torsades occurs specifically in the setting of a prolonged QT interval and requires different treatment than other forms of VT, which is why the distinction matters.
Atrial Fibrillation Subtypes
Because atrial fibrillation is so common, it has its own classification system based on how long episodes last and whether they resolve on their own. Paroxysmal atrial fibrillation describes episodes that come and go, with no single episode lasting seven consecutive days. Persistent atrial fibrillation means the abnormal rhythm sustains itself for at least seven consecutive days, or it requires medical intervention to restore normal rhythm. Long-standing persistent atrial fibrillation has continued for more than 12 months. Permanent atrial fibrillation is the term used when both the patient and physician have decided to stop pursuing rhythm control and accept the ongoing arrhythmia, managing it with rate control instead.
Classification by Hemodynamic Stability
Regardless of what type of tachyarrhythmia is present, a critical real-time classification is whether the fast rhythm is causing the body to fail. A hemodynamically unstable tachycardia means the heart is beating too fast or too inefficiently to maintain adequate blood flow. The signs include low blood pressure, altered mental status or confusion, chest pain, difficulty breathing, low oxygen levels, and other markers of poor organ perfusion or shock.
This classification takes priority over all others in an acute setting. A patient with an unstable tachyarrhythmia needs immediate treatment to restore a viable rhythm, regardless of whether the underlying cause is supraventricular or ventricular, narrow or wide, regular or irregular. The mechanistic and anatomical classifications become more relevant once the patient is stable and the clinical team can analyze the ECG in detail to guide longer-term management.
Pediatric Thresholds
The standard heart rate threshold of 100 beats per minute applies to adults, but children have higher resting heart rates that vary by age. In newborns and infants, supraventricular tachycardia is typically defined by a heart rate exceeding 220 beats per minute. In older children, the threshold drops to around 180 beats per minute. These higher cutoffs reflect the fact that a resting heart rate of 120 or even 150 can be perfectly normal in a healthy infant.