Medically Reviewed By: Dr Paul Lim – MBBS, M.Med (Int Med), MRCP (UK), FAMS (Cardiology)
Supraventricular Tachycardia (SVT, AVNRT & WPW)
SVT is a short circuit in the heart’s wiring that sends the rate from normal to around 200 in a single beat, and it is alarming to experience without usually being dangerous. Heart racing out of nowhere? Sit down with Dr Paul Lim, who is easy to talk to, and find out which circuit is behind it and whether it can be removed for good.
Dr Paul Lim Chun Yih
Senior Consultant Cardiologist & Electrophysiologist
Clinical Experience
Treated by Dr Paul Lim
Senior Consultant Cardiologist & Cardiac Electrophysiologist
SVT sits at the centre of Dr Paul Lim’s subspecialty. Cardiac electrophysiology is the study of the heart’s electrical circuits, and treating re-entry circuits by catheter ablation is the work it was built around. He maps the circuit during an electrophysiology study and treats it in the same session. He completed advanced fellowship training at Barts Heart Centre, London under Singapore’s HMDP award, and is a Fellow of the Heart Rhythm Society (FHRS).
What Is Supraventricular Tachycardia?
Tachycardia means a fast heart rate, above 100 beats per minute. Supraventricular means it starts above the ventricles, in the upper chambers or at the junction between them. Put together, SVT is a fast rhythm driven from the top of the heart rather than the bottom, and that distinction decides almost everything about how it is treated and how seriously it is taken.
Nearly all of it works the same way. Somewhere in the heart sits an extra electrical route that should not be there, usually present since birth and causing nothing for decades. If a stray early beat arrives at precisely the moment that route is ready and the normal one is not, the signal sets off around a loop and keeps circling. Each lap drives another heartbeat, and because the lap time is fixed, the rate is fast and metronomically steady. The loop runs until something breaks it.
That mechanism explains the thing people describe most consistently. The onset is not gradual. The heart goes from 70 to 190 between one beat and the next, and when it stops, it stops dead, often mid-conversation. Episodes that come and go like this are called paroxysmal supraventricular tachycardia, often shortened to PSVT. Episodes that begin and end suddenly, lasting from seconds to hours, are the pattern that distinguishes SVT from a heart simply beating fast for an ordinary reason.
Why SVT is so often put down to anxiety
A heart at 190 produces adrenaline, breathlessness and a strong sense that something is badly wrong, which is a reasonable description of a panic attack. By the time an episode is assessed it has usually stopped, and a normal ECG afterwards shows nothing. Many people go years being treated for anxiety before an episode is caught on a recording. The comparison further down this page sets out the features that tend to separate the two.
What SVT Looks Like on an ECG
Three features identify it: the complexes are narrow, the rate is fast and regular, and the episode starts and stops abruptly. The fourth strip shows the one clue that can be seen between episodes.
Caught an episode on a watch or a monitor? Bring the trace to Dr Paul Lim and have it identified.
Symptoms of SVT
Episodes range from a few seconds to several hours, and their character is more diagnostic than their length.
During an episode
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A racing heart that arrives instantlyNo build-up. People often remember exactly what they were doing when it started.
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Fast, but perfectly steadyToo quick to count easily at the wrist, and it does not stumble.
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Pounding in the neck or throatCharacteristic of AVNRT, because the upper and lower chambers contract against each other.
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BreathlessnessThe chambers have little time to fill between beats.
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Light-headednessOccasionally with a feeling of being about to faint.
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Chest discomfort or tightnessCommon, and it does not usually mean the arteries are involved.
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A strong sense of alarmThis follows the heart rate rather than causing it, which is the distinction that matters.
Afterwards
- It stops as abruptly as it began, often with a single thud as normal rhythm takes over.
- A pressing need to pass urine within half an hour. The stretched atria release a hormone that prompts it, and it is a genuinely useful pointer towards SVT.
- Tiredness for a few hours after a long episode.
- A normal ECG if you reach a clinic after it has stopped, which is the usual source of the delay in diagnosis.
Get urgent help during an episode if
Call 995 or go to the nearest A&E if a racing heart comes with chest pain that is severe or lasts more than a few minutes, with fainting or a blackout, with severe breathlessness, or if the episode will not stop and you feel increasingly unwell. An episode that has run for more than about half an hour and is not responding to anything is also a reason to be seen the same day rather than waiting it out.
Types of SVT
SVT is an umbrella term. Which circuit you have decides where the ablation catheter goes, so identifying it is the point of the investigation.
AVNRT (AV nodal re-entrant tachycardia)
Also written AV nodal reentrant tachycardia, this is the commonest form, making up roughly 60 per cent of SVT. The AV node is the relay station between the upper and lower chambers, and some people are born with two functionally separate routes through it: one that conducts quickly but recovers slowly, and one that conducts slowly but recovers quickly. In everyday life this arrangement does nothing at all.
Trouble starts when an extra beat arrives at the one moment the fast route is still recovering. The signal goes down the slow route instead, finds the fast route ready at the bottom, and travels back up it, then down the slow route again. The loop is self-sustaining, and very small, which is why AVNRT runs fast. It is more common in women and often first appears between the twenties and forties.
Because the loop sits inside the AV node, the upper and lower chambers contract almost simultaneously and the atria squeeze against closed valves. That produces the hammering sensation in the neck that people with AVNRT describe and people with other SVTs often do not.
AVRT (AV re-entrant tachycardia)
Here the extra route lies outside the AV node: a strand of conducting muscle called an accessory pathway, bridging the atria and ventricles directly. The heart normally keeps the upper and lower chambers electrically separate except at the AV node, so an accessory pathway is an unintended second doorway, present from birth.
The circuit runs down one route and back up the other, forming a much larger loop than AVNRT. Where the pathway also conducts forwards during normal rhythm it shows on a resting ECG, and the condition is called Wolff-Parkinson-White, covered in full below: it is the one form of SVT with a genuine, if small, safety concern attached.
Atrial tachycardia
A less common form in which the problem is not a loop at all. A single site in an atrial wall starts firing rapidly and overrides the sinus node. Because it does not depend on the AV node, it may not respond to vagal manoeuvres or adenosine the way the re-entry tachycardias do, and it can be more persistent. It is also the form most often associated with an underlying problem in the atrium, so it prompts a closer look at the heart’s structure.
Atrial flutter and atrial fibrillation also begin above the ventricles and are technically supraventricular, but they behave differently enough, and carry a stroke risk that the rhythms on this page do not, that each has its own page.
Wolff-Parkinson-White Syndrome
The one form of SVT where the finding itself, rather than just the symptoms, changes what gets recommended.
In Wolff-Parkinson-White the accessory pathway conducts forwards as well as backwards, so part of each normal beat reaches the ventricle through the shortcut, arriving before the signal that took the proper route through the AV node. The ventricle depolarises early, and a resting ECG shows two linked findings: a short PR interval, because the signal arrived sooner than it should have, and a delta wave, the slurred ramp at the start of the QRS complex where the early signal spreads slowly through muscle instead of racing down the normal conducting fibres. That is the fourth strip above.
Two terms are used, and the difference between them is not pedantry. A WPW pattern means the ECG findings are present but the person has never had an arrhythmia. WPW syndrome means the pathway has actually caused one. Most people with the pattern never develop the syndrome.
Why WPW is followed more closely than other SVT
The AV node has a useful safety property: it conducts slowly, which limits how fast the ventricles can be driven from above. An accessory pathway has no such brake. If someone with WPW develops atrial fibrillation, where the atria fire chaotically at very high rates, those signals can pass straight down the pathway to the ventricles at a rate the heart cannot sustain. This is uncommon and it is the reason WPW is taken seriously: it is the mechanism behind the rare cases of sudden cardiac arrest associated with the condition.
It is also the reason some drugs used for atrial fibrillation are avoided in WPW, because by slowing the AV node they can push more of the signal down the unprotected pathway. If you have been told you have a delta wave or pre-excitation, mention it to any doctor treating you for a fast heart rhythm.
Assessment turns on how readily the pathway conducts. An electrophysiology study measures that directly, and a treadmill stress test gives a useful indication: a delta wave that disappears abruptly as the heart rate climbs suggests a pathway poor at conducting fast, which is reassuring. Catheter ablation of an accessory pathway has a high success rate and removes the risk along with the symptoms, which is why it is often recommended in WPW syndrome even where episodes have been infrequent, and discussed in certain occupations and in athletes with the pattern alone. A small minority of accessory pathways run in families, so mention any relative with WPW, or a family history of sudden cardiac death at a young age, at your consultation.
SVT, Sinus Tachycardia and Panic Attacks
All three produce a fast heart and a frightened person. These are the features that tend to separate them, though none is conclusive on its own.
| Feature | SVT | Sinus tachycardia | Panic attack |
|---|---|---|---|
| How it starts | Instantly, from one beat to the next | Gradually, in proportion to the cause | Over a minute or two, building |
| Usual rate | 150–250, and steady | 100–150, varying with activity | Usually under 150 |
| How it ends | Abruptly, often mid-sentence | Slowly, as the cause resolves | Gradually, over many minutes |
| Order of events | The heart races first, the fear follows | The cause comes first | The fear and the racing rise together |
| Trigger | Often none that you can identify | Exertion, fever, pain, stress, anaemia | Often a situation or a thought |
| Response to vagal manoeuvres | May stop it outright | Slows it briefly, then it returns | No specific effect on the rate |
| Afterwards | A need to pass urine is common | Nothing particular | Exhaustion, lingering unease |
| On an ECG during it | Narrow, fast, regular, P waves hidden | Normal shape, simply faster | Sinus tachycardia |
The two are not mutually exclusive, which is worth saying plainly. Living with unpredictable episodes of a heart at 200 is a reasonable thing to become anxious about, and plenty of people with confirmed SVT have genuine anxiety as well. Identifying the rhythm does not dismiss the anxiety; it usually relieves a good deal of it.
How to Stop an SVT Episode
Because AVNRT and AVRT both run through the AV node, anything that briefly slows the node can break the loop. These are called vagal manoeuvres.
The modified Valsalva manoeuvre
The most effective self-administered technique. Sitting semi-upright, blow hard against a closed throat, or into a 10 ml syringe hard enough to just move the plunger, for about 15 seconds. Then lie flat at once while someone raises both your legs to roughly 45 degrees, for a further 15 seconds. The leg raise is what makes this version work better than straining alone.
Cold water to the face
Splashing the face with very cold water, or briefly immersing it in a basin of cold water, triggers the same reflex. It is the easier option when you are alone, and works for some people when straining does not.
If it does not work, go in
If two or three attempts have no effect, or you feel increasingly unwell, go to A&E. In hospital an episode is usually stopped with an injection of adenosine, which blocks the AV node for a few seconds and interrupts the circuit. It causes a brief, distinctly unpleasant flushing sensation that passes in seconds, worth knowing about in advance. Where that is unsuitable, electrical cardioversion restores normal rhythm under brief sedation.
Two things not to do
Do not press on your eyeballs. It was once taught and can damage the eye. Do not attempt carotid sinus massage on yourself. Pressing on the neck can dislodge deposits from the artery wall, so it is avoided altogether where a carotid artery is known to be narrowed, and it is done by a clinician who has first listened for a bruit over the artery, not at home. Have your cardiologist take you through the Valsalva technique in person before you rely on it.
Want the technique demonstrated properly, and the circuit identified? Book with Dr Paul Lim.
How SVT Is Diagnosed
The whole exercise comes down to one thing: capturing an episode on a recording. Everything else supports that.
The history
More is decided here than people expect. How episodes start and stop, how fast and how regular the pulse is, whether there is pounding in the neck, and whether you need to pass urine afterwards all point towards or away from SVT, and towards which circuit. Describing one episode in detail is more useful than describing ten in general terms.
Resting ECG
An SVT ECG taken between episodes is usually normal, and a normal one does not rule SVT out. It is still done every time, because it is the test that finds a delta wave, and finding pre-excitation changes the plan completely.
Ambulatory monitoring
A Holter monitor worn for 24 hours upwards catches an episode if they are frequent. Where they come weeks or months apart, a patch recorder, an event recorder or an implantable loop recorder covers a longer period. A good-quality smartwatch trace is genuinely helpful here, so bring one if you have managed to record an episode.
Echocardiogram and blood tests
A 2D echocardiogram confirms the heart is structurally normal, which it usually is, and which is part of why SVT is well tolerated. Thyroid function and a full blood count rule out an overactive thyroid or anaemia driving the rate.
Electrophysiology study
An electrophysiology study is the definitive test. Fine catheters are placed in the heart through a vein in the groin, the circuit is started under controlled conditions, and its mechanism and location are mapped. Because mapping and treatment use the same catheters, the study and the ablation are normally done in one session, discussed and consented to beforehand.
How SVT Is Treated
SVT is one of the few cardiac conditions where a lasting cure is a realistic goal rather than a matter of long-term control.
Watching, with a technique to fall back on
Where episodes are rare, brief and stop readily with a vagal manoeuvre, no further treatment may be needed. What changes things is frequency, duration, how unwell episodes make you, and whether you have stopped doing things because of them. Reducing caffeine, alcohol and sleep debt lowers the chance of the triggering beat, and is a sensible first step whatever else is decided.
Medication
Beta-blockers and calcium-channel blockers slow conduction through the AV node, making the circuit harder to start and slower if it does. They reduce how often episodes happen without abolishing them. Antiarrhythmic drugs are more effective and used where the simpler options have not worked, though they need monitoring and are a less attractive long-term proposition for someone in their thirties than a procedure that ends the problem. Medication choice differs in WPW, for the reason set out above.
Catheter ablation
Catheter ablation is the treatment that removes the problem rather than managing it. Catheters are passed to the heart through a vein in the groin, the circuit is mapped, and the critical part of it is then treated with heat or cold so it can no longer conduct. For AVNRT this means modifying the slow route through the AV node; for AVRT it means interrupting the accessory pathway.
Success rates for both are high, generally quoted above 90 per cent, with a modest chance of the circuit recurring and needing a second procedure. Atrial tachycardia is the exception: it has no single fixed circuit to interrupt, and published success rates for ablating it sit meaningfully lower, nearer three quarters. The main specific risk in AVNRT ablation is damage to normal conduction through the AV node, which would mean a pacemaker. This is uncommon, and the mapping that precedes the ablation reduces it. Dr Lim’s standard is an overnight stay, home the next day.
Ablation for SVT has a published physician fee benchmark of S$4,700 to S$9,400 before GST. That is the doctor’s fee alone. Hospital facility charges, anaesthesia and consumables are billed separately, and the actual fee varies with the complexity of the case. Our cardiology fees page lists consultation and test charges.
When to See a Cardiologist About a Racing Heart
Book an appointment if any of the following applies to you.
- Your heart races suddenly, without warning, and stops just as suddenly.
- Episodes are becoming more frequent or lasting longer than they used to.
- You have recorded a fast rhythm on a smartwatch and want to know what it was.
- An ECG has reported a delta wave, pre-excitation or WPW.
- You have been told it is anxiety, and the pattern does not fit what you experience.
- An episode has caused fainting or near-fainting.
- You are avoiding exercise, travel or being alone because of the episodes.
- You are considering ablation and want to know whether your circuit is one that responds well to it.
Both clinics carry out the ECG, Holter and echocardiogram on site, so the initial assessment is usually completed in one or two visits. More on when a cardiology referral is worthwhile.
Book a Consultation for SVT
See Dr Paul Lim about episodes of a sudden racing heartbeat: which circuit is responsible, and whether catheter ablation can end them.
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SVT FAQ
The questions people ask most often after a first episode, or after a report names a circuit they have never heard of.
What is supraventricular tachycardia (SVT)?
Supraventricular tachycardia is a fast heart rhythm starting above the ventricles, in the upper chambers or at the AV node. It usually runs between 150 and 250 beats per minute, stays regular, and shows narrow complexes on an ECG. Its defining feature is behaviour: an episode begins abruptly, from nothing, and ends just as abruptly, which is why it is also called paroxysmal SVT. The cause is a short circuit in the heart’s wiring rather than disease of the heart muscle, so it is common in young, otherwise healthy people.
Is SVT dangerous?
SVT is rarely life-threatening in a structurally normal heart. Episodes are frightening and disruptive, but the heart tolerates them and most people come to no harm. Two situations need closer attention: SVT in someone who already has heart disease, where a very fast rate is less well tolerated, and Wolff-Parkinson-White syndrome, where an accessory pathway can in rare cases conduct atrial fibrillation to the ventricles dangerously fast. That is why a resting ECG matters even between episodes.
What is the difference between SVT and a panic attack?
They overlap enough that SVT is commonly mistaken for anxiety for years before it is diagnosed. The useful distinction is the shape of the episode. In SVT the rate jumps instantly to a fast, steady rate, the fear follows the heartbeat, and it stops as suddenly as it started, often mid-sentence. In a panic attack the rate climbs over a minute or two, rarely holds above about 150, and settles gradually. An urge to pass urine afterwards also suggests SVT. None of it is conclusive, which is why the aim is to capture an episode on a recording.
How do I stop an SVT episode?
Vagal manoeuvres briefly increase vagal tone, which slows conduction through the AV node and can break the circuit. The modified Valsalva is the most effective: sitting semi-upright, blow hard against a closed throat or into a syringe for about 15 seconds, then lie flat at once while someone raises your legs to about 45 degrees for 15 seconds. Cold water on the face also works for some people. Have your cardiologist take you through it rather than relying on a description. Do not press on your eyeballs, and do not attempt carotid massage on yourself.
What is AVNRT?
AV nodal re-entrant tachycardia, also written AV nodal reentrant tachycardia, is the commonest form of SVT at roughly 60 per cent of cases. Some people are born with two functionally distinct routes through the AV node, one fast and one slow, which normally causes nothing. If a well-timed extra beat arrives while one route is recovering, the signal travels down one and back up the other, forming a loop that circles continuously. It is more common in women and often first appears between the twenties and forties.
What is Wolff-Parkinson-White syndrome?
Wolff-Parkinson-White syndrome means having an extra electrical connection between the upper and lower chambers, present from birth, and arrhythmia because of it. The pathway lets part of the signal bypass the AV node and reach the ventricle early, showing on a resting ECG as a short PR interval and a slurred upstroke into the QRS complex called a delta wave. Where the pattern is present but has never caused symptoms, it is a WPW pattern rather than the syndrome, and that distinction changes how closely it is followed up.
Can SVT be cured?
Catheter ablation offers a genuine cure for most forms of SVT, unlike medication, which only suppresses episodes. Because the problem is a defined circuit rather than diffuse disease, interrupting a small area of tissue can remove it permanently. Published success rates for AVNRT and accessory pathway ablation are high, generally quoted above 90 per cent, with a modest chance of the circuit recurring and needing a second procedure. Atrial tachycardia is the exception, with success rates nearer three quarters. Your own figure depends on which circuit you have and where it sits.
What triggers an SVT episode?
The circuit has to be started by a well-timed extra beat, so anything that makes extra beats more likely can set an episode off. Caffeine, alcohol, nicotine, poor sleep, dehydration, emotional stress and sudden exertion are the usual suspects, and some people find episodes cluster around hormonal changes or illness. Many episodes have no identifiable trigger at all. Keeping a short note of what preceded each one is worth doing, because it sometimes reveals a pattern that is straightforward to act on.
What is the difference between SVT and atrial fibrillation?
Both are fast rhythms from the upper part of the heart, and the key difference is regularity. SVT is regular, running at a steady fast rate driven by a fixed circuit. Atrial fibrillation is irregular, because the atria fire chaotically and only some signals get through. They also differ in consequence: atrial fibrillation carries a stroke risk that usually needs treating, while SVT does not. The two are told apart on an ECG, which is why capturing an episode matters.