SVT vs Atrial Fibrillation: What Is the Difference?

By Dr Nabila Laskar — published September 2026.

SVT and atrial fibrillation both cause a racing heart, but they are very different conditions with different implications. Dr Nabila Laskar explains how to tell them apart.

Two of the most common arrhythmias I see in clinic are SVT (supraventricular tachycardia) and atrial fibrillation (AFib). Both cause a rapid or irregular heartbeat. Both can feel alarming. But they are fundamentally different conditions, with different mechanisms, different risk profiles, and different management. Understanding the distinction is important if you have been told you have one or the other, or if you are trying to work out what your episodes might be.

The key difference in how they feel

SVT typically starts and stops abruptly, which patients often describe as “switching on and off like a light.” The heart rate during SVT is usually fast and regular, often 150 to 230 beats per minute. Atrial fibrillation, by contrast, causes an irregularly irregular rhythm. Patients often describe it as the heart “flip-flopping,” “quivering,” or feeling “chaotic” rather than racing in a regular pattern. The difference in regularity is the most useful distinguishing feature from the patient’s perspective.

The mechanism

SVT arises from abnormal electrical circuits or firing within or near the AV node, causing rapid but organised electrical activity. The heart beats fast but in a coordinated rhythm. In atrial fibrillation, the atria fire chaotically from multiple points at very high rates (300 to 600 impulses per minute), and only some of those impulses conduct through to the ventricles, creating the characteristically irregular pulse.

The risk profile

This is where the distinction matters most. SVT in a structurally normal heart carries very low risk of serious complications. It is frightening during an episode but rarely dangerous. Atrial fibrillation carries a significantly increased risk of stroke due to clot formation in the left atrial appendage during the chaotic atrial activity. This is why AFib, once diagnosed, requires formal stroke risk assessment and often anticoagulation. SVT does not.

How they are diagnosed

Both require an ECG recorded during an episode for definitive diagnosis. A resting ECG between episodes is frequently normal for both conditions. A 7-day Holter monitor is the most practical way to capture an episode for either arrhythmia if symptoms are intermittent. During an SVT episode, the ECG typically shows a narrow-complex regular tachycardia. During AFib, it shows the characteristic irregularly irregular narrow complexes without distinct P waves.

Treatment differences

SVT can often be terminated by vagal manoeuvres and, where episodes are frequent or distressing, catheter ablation is curative in a high proportion of patients. AFib management focuses on rate or rhythm control, stroke prevention, and identifying and managing underlying triggers such as hypertension, sleep apnoea, alcohol, and thyroid disease.

If you have episodes of rapid heartbeat and are unsure which condition you have, the most useful next step is prolonged ambulatory monitoring to capture an episode and characterise the rhythm precisely.

Medically reviewed by Dr Nabila Laskar, Consultant Cardiologist, GMC 7040901.