SVT (supraventricular tachycardia) is a narrow complex rhythm. On an ECG, the QRS complex in SVT typically measures less than 120 milliseconds (0.12 seconds), which is the standard cutoff between narrow and wide. However, there are specific situations where SVT can produce a wide complex, and understanding why matters for getting the right diagnosis and treatment.
Why SVT Is Narrow Complex
The “narrow” in narrow complex refers to how quickly the electrical signal spreads through the ventricles. In SVT, the abnormal rhythm originates at or above the AV node, the electrical gateway between the upper and lower chambers of the heart. Even though the rhythm is abnormal, the signal still travels down through the heart’s normal fast-conducting wiring to reach the ventricles. Because the ventricles activate through this normal pathway, they contract in a coordinated, efficient way, and the QRS complex on the ECG stays short and narrow, under 120 milliseconds.
A wide QRS complex, by contrast, means the ventricles are being activated through a slower, less organized route. This is why ventricular tachycardia (VT), which starts in the ventricles themselves, produces a wide complex: the signal has to spread muscle cell to muscle cell instead of zipping through the specialized conduction system.
When SVT Can Look Wide
SVT doesn’t always stay narrow. There are three well-recognized situations where an SVT produces a QRS complex of 120 milliseconds or wider, making it look like ventricular tachycardia on a monitor.
Bundle Branch Block
If one of the two main electrical branches leading to the ventricles (the right or left bundle branch) is damaged or temporarily blocked, the signal has to take a detour. It travels down the working branch first, then spreads across to the other side. This detour widens the QRS. The underlying rhythm is still SVT, but the ECG pattern mimics a wide complex tachycardia. This can happen in people with a pre-existing bundle branch block, or it can develop on the fly during a fast heart rate, which is called “aberrant conduction.” When a bundle branch block develops during SVT and the rhythm slows down as a result, that’s actually a clue that an accessory pathway on the same side is involved in the circuit.
Accessory Pathway Conduction
Some people are born with an extra electrical connection between the atria and ventricles, called an accessory pathway. In the most common form of accessory pathway tachycardia (orthodromic AVRT), the signal goes down through the AV node normally and returns to the atria through the accessory pathway, producing a narrow complex. But in the less common antidromic form, the circuit runs in reverse: the signal travels down to the ventricles through the accessory pathway, bypassing the AV node entirely. Because the ventricles are activated through this abnormal shortcut instead of the normal conduction system, the QRS complex is wide. The 2025 American Heart Association guidelines list conduction through a bypass tract as one of four mechanisms that produce a wide QRS.
Ventricular Pacing
If someone has a pacemaker that stimulates the ventricles directly, the electrical activation starts from the pacemaker lead rather than from the normal conduction system. Even if the underlying rhythm triggering the pacemaker is supraventricular, the paced QRS will be wide.
Why the Distinction Matters
When a fast heart rhythm shows up as wide complex on a monitor, the critical question is whether it’s VT or SVT with one of the conditions above. This matters because VT and SVT can require different management. In emergency protocols, the first branch point in evaluating any fast rhythm is QRS width: narrow (under 0.12 seconds) or wide (0.12 seconds or more).
For narrow complex tachycardias, the approach often starts with vagal maneuvers (bearing down, applying a cold stimulus to the face) and, if needed, a rapid-acting medication called adenosine that briefly interrupts conduction through the AV node. These work well for SVT precisely because the AV node is part of the circuit.
For wide complex tachycardias, the picture is more complicated. If the patient is unstable, with low blood pressure, altered consciousness, or signs of shock, the treatment is synchronized cardioversion regardless of whether it’s VT or SVT. If the patient is stable, clinicians try to determine whether the wide rhythm is truly VT or SVT with aberrancy, because the best treatment options differ. Structured ECG criteria exist to help distinguish the two, though studies show these tools are far from perfect in real-world use. One widely used algorithm achieved 85 to 91 percent sensitivity for identifying VT when used by cardiologists, but specificity dropped to 55 to 60 percent, meaning a significant number of SVTs were misclassified.
How Common Is Wide Complex SVT?
The vast majority of SVT episodes present as narrow complex. Wide complex SVT is the exception, not the rule. Antidromic AVRT accounts for roughly 5 to 10 percent of all accessory pathway tachycardias, and pre-existing bundle branch block affects a minority of patients. So when you see “SVT” in a textbook or clinical context, the default assumption is narrow complex unless there’s a specific reason for it to be wide.
That said, any wide complex tachycardia of uncertain origin is generally treated as VT until proven otherwise, because VT is more dangerous and more common in that setting. This is a safety principle, not a reflection of how SVT typically behaves.
Reading Your Own ECG
If you’ve been diagnosed with SVT and are looking at your ECG strip, you’ll most likely see a fast, regular rhythm with narrow, tight QRS complexes and a heart rate somewhere between 150 and 220 beats per minute. In the most common subtype (AVNRT), the P waves are often invisible because the atria and ventricles fire almost simultaneously. If your QRS complexes appear wider than normal, that’s worth flagging with your cardiologist, as it could point to an accessory pathway or conduction abnormality that changes how your SVT is managed long term.