SVT vs Atrial Fibrillation: How to Tell the Difference — and Why It Matters

June 22, 2026Dr Boon LimUncategorized

Two conditions, one symptom — and very different implications

‘My heart was racing at 180 beats per minute for forty minutes and then stopped as suddenly as it started. Is that AF?’ This question, in some form, reaches me in clinic several times a week. Not all fast heart rhythms are atrial fibrillation — and the distinction matters more than many patients realise. Supraventricular tachycardia, or SVT, is a separate family of rhythm disorders that can feel remarkably similar to AF during an episode, but arises from entirely different mechanisms, carries different risks, and requires different treatment.

I want to explain the distinction clearly — both for patients trying to make sense of their symptoms, and for GPs who may want a reference point when reviewing patients with palpitations.

What is SVT?
SVT is an umbrella term covering any fast heart rhythm that originates above the ventricles. The term encompasses several distinct arrhythmias, of which the most common are:

• AVNRT (atrioventricular nodal reentrant tachycardia): A short-circuit within the AV node — the most common cause of SVT in adults, often occurring in younger patients
• AVRT (atrioventricular reentrant tachycardia): Uses an accessory electrical pathway between the atria and ventricles. Wolff-Parkinson-White syndrome involves a specific type of this pathway
• Atrial tachycardia: A rapid rhythm arising from a focal area of abnormal electrical activity within the atrium

Each of these produces a fast but typically regular rhythm, in contrast to the chaotic, irregularly irregular rhythm of AF.

The Key Differences Between SVT and AF

SVT characteristically produces a fast but regular heartbeat — typically 150–250 beats per minute — with an abrupt onset and offset that patients often describe as a switch being flipped. The rhythm is regular: a steady, relentless racing. Many patients can terminate an episode using a Valsalva manoeuvre — bearing down, as if straining, or immersing the face in cold water. This vagal manoeuvre slows conduction through the AV node and can interrupt the reentrant circuit.

AF, by contrast, produces an irregularly irregular rhythm. The rate is variable — sometimes fast, sometimes slower — and the heart feels chaotic rather than mechanically fast. Episodes tend to be less abruptly terminated by vagal manoeuvres, though adenosine can unmask the underlying rhythm. If you feel your pulse during AF, the irregularity is usually quite apparent.

On an ECG, SVT shows narrow complex tachycardia with a regular rate, often with retrograde P waves. AF shows absent P waves, a fibrillatory baseline, and an irregularly irregular ventricular response. This distinction is often immediately apparent to a trained eye.

Stroke Risk: A Critical Distinction

This is perhaps the most clinically important difference. Typical SVT — AVNRT and AVRT — does not carry the same stroke risk as AF. The mechanism of AF-related stroke depends on blood stasis in the left atrial appendage, which occurs because the atria are not contracting properly. In SVT, the atria continue to contract, and this stasis and clot-formation mechanism does not apply in the same way.

As a result, routine anticoagulation is generally not recommended for pure SVT unless there are additional risk factors. This is in stark contrast to AF, where anticoagulation decisions are paramount. If a patient presents with what appears to be SVT on a device recording, but there is any doubt about whether the rhythm is truly regular — or whether there are periods of AF interleaved — this distinction must be clarified before making anticoagulation decisions.

Treatment: SVT is Often Curative

Here is where the news is very good. SVT — particularly AVNRT and AVRT — is one of the most curable arrhythmias in all of cardiology. Catheter ablation achieves cure rates of 95–98% for the most common forms, with very low recurrence rates. For AVNRT, we perform a selective modification of the slow pathway within the AV node — a remarkably elegant procedure that resolves the arrhythmia in a single session. For AVRT, we ablate the accessory pathway.

I perform SVT ablation at One Welbeck Heart Health and Hammersmith Hospital as a day-case procedure, typically under conscious sedation or brief general anaesthesia, with patients going home the same day. The procedure takes approximately 60–90 minutes, and for most patients it represents a definitive end to years of unpredictable, disruptive episodes. Many describe it as life-changing — not because SVT is immediately life-threatening, but because the psychological burden of living with unpredictable palpitations is substantial, and its sudden removal is profound.

What Should You Do If You Think You Have SVT?

If you are experiencing episodes of sudden-onset rapid regular palpitations — particularly if they start and stop abruptly, and especially if they began earlier in life — it is worth a specialist cardiology review. A 12-lead ECG during an episode is the gold standard; if you own a Kardia Mobile device, recording your rhythm during an episode can provide valuable information before a formal consultation.

If episodes are infrequent, a prolonged ECG monitor — 7-day Holter or an insertable loop recorder — may be needed to capture the rhythm. Once the arrhythmia is documented and the mechanism understood, the decision about ablation versus medication can be made on a fully informed basis.

To discuss SVT or palpitations with Dr Boon Lim, book a consultation at OneWelbeck Heart Health — drboonlim.co.uk or call 0208 187 9888