Supraventricular tachycardia, abbreviated to SVT, includes a number of different fast heart beats that originate from the top chambers of the heart – the atria. The atria are located above the ventricles, which is why the term supraventricular is used. Tachycardia refers to a rapid heartbeat that is 100 beats per minute, or more. If the heart rate is 100 beats or more this is not always abnormal. For example, sinus tachycardia (the normal heart rhythm) could be normal during exercise.
Types of SVT
There are 3 main types of SVT, and can be divided in to the following groups:
- Atrioventricular Nodal Re-entrant Tachycardia (AVNRT) – AVNRT accounts for the majority of SVTs, and occurs due to a ‘short circuit’ within the atrioventricular node (AV node). The AV node forms part of the normal electrics of the heart. AVNRT occurs in around 1 in 500-700 people.
- Atrioventricular Re-entry Tachycardia (AVRT) – AVRT results from an additional electrical connection that runs between the top chamber (atrium) and lower chamber (ventricle) of the heart. AVRT occurs in around 1 in 1000 people.
- Atrial Tachycardia – There are different types of atrial tachycardia. In general they result from a small collection of cells in the atrium ‘firing off’ abnormal electrical impulses.
SVTs are a relatively common problem, and tend to be more common in younger people. In general, SVTs are a benign problem but can make people suddenly feel unwell when they occur.
What are the common symptoms?
Patients often describe the sudden onset of rapid palpitations. This can occur at any time, and is often not related to anything in particular. Some people find that if they drink caffeine or alcohol they might suffer with more frequent or longer episodes. Episodes of palpitations can last a few minutes, or in some instances may continue for several hours or until medical attention is sought. Symptoms of palpitations typically terminate very suddenly. Some patients discover that if they cough or strain (Valsalva) during episodes, they are able to terminate them.
Other symptoms that can accompany palpitations during episodes of SVT include dizziness, light headedness, shortness of breath, chest pain, or very rarely loss of consciousness can occur.
How is it diagnosed?
This can often be tricky. The longer the episodes of SVT, the easier it is to diagnose. Because palpitations are a very common symptom that people experience, it is necessary to record the heart rhythm while the symptoms are happening. Recording of the heart rhythm is typically done with an electrocardiogram (ECG), or heart monitor for 24 or 48 hours. The definitive method of diagnosing an SVT is to undertake an Electrophysiology Study (or EP Study). An EP Study is usually done under local anaesthetic to numb the top of the leg, followed by the insertion of some small tubes in to the femoral vein. Specialised wires are passed through these tubes, up towards the heart. We are then able to try and bring on the SVT by stimulating the heart with specially timed electrical impulses. Once the SVT has been initiated, we are able to identify the type of SVT and direct treatment towards the underlying cause. This is done by ablation, which is where a specialised catheter is used to cauterise the region that is responsible for causing the SVT.
How is it treated?
Treatment of SVTs can be with a variety of different medications. These can either be taken on a regular basis, or sometimes are only taken during episodes of SVT. The alternative to medication is to undergo an EP Study and ablation procedure. This is more definitive, and has a very high success rate. Some patients are able to terminate episodes of SVT by themselves by doing a manoeuvre called a Valsalva.