London Heart

Supraventricular Tachycardia

What is SVT?

An SVT is a fast, abnormal heart rhythm that originates from the top chambers of the heart (the atria). It is an umbrella term for a number of different abnormal heart rhythms. Some of these heart rhythms are intermittent and others are present all the time.

The normal heart beat

The way our heart is made to beat efficiently and at the right time is through small electric currents passing through the muscle and tissue of the heart. These small impulses originate from the heart’s “inbuilt pacemaker” which is known as the Sino-atrial Node (or SA node). Impulses from the SA node pass down an electrical pathway via a second relay station (AV node) to tell the bottom of the heart to beat. This system ensures that both the top chambers (the atria) and the bottom chambers (the ventricles) beat at the appropriate time to make sure that the blood is pumped efficiently around the body.

Are there different causes of SVT?

An SVT is caused generally in 3 ways:

  • A part or parts of the atria (not the SA node) gives off impulses which can then pass through the heart, overriding the SA node and causing the heart to beat faster (atrial tachycardia)
  • An extra bit of wiring exists within the AV node allowing a short ciruit and fast heart rhythm (AVNRT)
  • There is an extra bit of wiring present in the heart between the atria and ventricles or within the atria which can lead to a short circuit causing a fast heart rhythm (AVRT or Wolff-Parkinson-White (WPW) syndrome)

What are the symptoms of an SVT?

Most people find that they experience palpitations (feeling a fast fluttering of your heart), or shortness of breath. Some people are aware of their heart beating in their throat or ear. You may notice that you can’t do as much physical activity as you could before. In some cases, it can make you feel dizzy and light-headed. Sometimes patients have symptoms for many years and have been labelled as having panic attacks!

Is an SVT dangerous?

Even though the heart can beat very fast with an SVT, if the heart is structurally normal they are usually not dangerous. However certain SVTs caused by an extra pathway between the top and bottom of the heart may be dangerous although this is rare.

How is an SVT diagnosed?

The unpredictable nature of an svt can present a diagnostic challenge. It is often difficult to predict how long an episode will last and when it will terminate. A detailed account of symptoms will be taken as typically an SVT starts and stops abruptly and the history can be very informative. Generally, the way that most SVTs are diagnosed is on an ECG (heart rhythm trace). This is because an SVT will interrupt the normal heart rhythm and will be visible on an ECG.

In many patients the SVT may have stopped by the time an ECG is performed.

The ECG may however provide clues as to the cause but may also be completely normal. Therefore, your Cardiologist may want to perform a longer heart rhythm trace. This is called a Holter or ambulatory ECG monitor. This involves being attached to an ECG recorder or patch monitor for a prolonged period of time (usually 24-48 hours but it can last up to a week). Whilst you are wearing the monitor, you go about your day as you normally would and can record any symptoms you have which can then be matched up to what your heart rhythm is doing at that moment.

In patients with infrequent episodes, a small device known as an implantable loop recorder can be implanted under the skin (Reveal LinQ monitor). This device can remain in place for up to 3 years. This is implanted in the outpatient setting under local anaesthetic and the procedure only takes several minutes. Wireless technology means your cardiologist can be kept informed of your heart rhythm without you having to make frequent trips to hospital.

In addition blood tests will be performed to ensure that there is not another reason for your heart to be racing (sometimes, your blood salt levels and hormone levels can affect the electrical system of your heart)

Your Cardiologist may also perform an ultrasound scan of the heart (an echo) to see if there are any structural abnormalities and also to obtain an idea of how well the heart is pumping.

Are there any triggers for SVT?

Episodes are commonly triggered by caffeine, alcohol, sleep deprivation and stress. However often there is no particular trigger and episodes can start spontaneously.

How is an SVT treated?

This depends on what is causing the SVT and also depends on the impact it is having on your life (i.e. how often is it happening and how badly does it affect you when it does). Some people find that the SVT does not affect them very much and episodes are infrequent

Essentially there are three types of treatment:

Conservative measures:
These may include holding your breath in a particular way or using cold water. Whilst effective in some patients for stopping the SVT, they may not work.

If symptoms are intrusive or frequent your Cardiologist may offer you medications which can be used on an‘as required’ basis and taken when the SVT occurs to settle the heart rhythm back to normal. Alternatively medications can be used on a regular basis to try and prevent episodes from starting. However medications may not be effective in everyone and can cause side effects in some. Also some patients continue to experience episodes despite being on medications. Commonly used drugs include beta-blockers, verapamil , flecainide and sotalol.

Catheter ablation:
Nowadays first line treatment for SVT is Catheter Ablation. This is a minimally invasive procedure (not surgery) where fine wires are passed from the vein in the top of your leg all the way to the heart. Whilst they are there, your Cardiologist can do a detailed study of the electrical activity of your heart (an EP study) and can identify the exact part of the heart that is causing the abnormal electrical activity.

This is done using electrical signals and sometimes a computer generated ‘virtual’ map of your heart. During this procedure, attempts are made to trigger off the SVT, in a very controlled environment, which is useful in making the correct diagnosis. Once the diagnosis is made high-frequency energy is delivered to the tiny area in the heart causing the problem to destroy or modify the abnormal cells. This means that they cannot interfere with the normal electrical pathway of the heart anymore.

This procedure carries a success rate of well over 90% and can be performed under local anaesthetic with sedation or a general anaesthetic, usually as a day case. It is a very safe procedure and the risks of serious complications are low (1-2%). These include bleeding at the top of the leg, bleeding around the heart and very occasionally requirement of a permanent pacemaker.