Supraventricular Tachycardia

Supraventricular tachycardia (SVT), occurs when electrical impulses above the heart’s ventricles (supraventricular) start to produce an abnormally rapid heart rhythm. A normal heart rate is considered to be between 60 to 100 beats per minute (bpm) but during SVT, the heart rate is rapid and may be sustained way above 100 bpm, sometimes up to 200bpm, for several minutes or hours. The rapid heart rate typically starts abruptly and ends abruptly and prevents the heart muscle from contracting forcefully and can result in symptoms of palpitations, chest pain, difficulty in breathing, light-headedness, and dizziness. Although the symptoms can be distressing, SVT is not regarded as a dangerous or life-threatening heart rhythm abnormality.

 Types of SVT

The term SVT comprises of three different types of abnormalities. These are referred to as Atrioventricular Nodal Re-entrant Tachycardia (AVNRT), Atrioventricular Re-Entrant Tachycardia (AVRT) and Atrial Tachycardia (AT).

Atrioventricular Nodal Re-entrant Tachycardia (AVNRT)

Atrioventricular nodal re-entrant tachycardia (AVNRT) is the most common kind of SVT, causing an abnormally rapid heart rate, which can reach up to 220 beats per minute. It is caused by the presence of 2 distinct electrical pathways (rather than the usual single pathway present in most individuals) in the atrioventricular (AV) node (the electrical junction box between the top and bottom chambers of the heart) which in turn gives way to a re-entrant circuit within the AV node. Palpitations often start and end abruptly and occur repeatedly, sometimes with triggers, but often without. Episodes of AVNRT are more commonly seen in young females, but it can affect both men and women of any age group.

Atrioventricular Re-Entrant Tachycardia (AVRT)

Atrioventricular re-entrant tachycardia (AVRT) is a type of abnormal rapid heart rhythm that is commonly related to Wolff-Parkinson-White syndrome. This is a congenital disorder associated with an accessory pathway (an extra electrical connection between the top and bottom chambers of the heart) which permits electrical signals from the heart’s atria (top chambers) to pass to the ventricles (bottom chambers) earlier than normal leading to an unusual ECG appearance, referred to as a delta wave, or short-PR interval. The palpitations commence typically when there is an extra heartbeat (ectopic) beat which conducts down the AV node and back up the accessory pathway creating a continuous circuit of activation.

Atrial Tachycardia (AT)

Atrial tachycardia differs from AVNRT and AVRT in that there is no ‘short-circuit’ but is triggered by the presence of ‘excitable tissue’ within one of the atria (top chambers of the heart). This ‘excitable tissue’ or ‘hot spot’ can periodically activate and cause the heart to beat rapidly for prolonged periods. There are often no triggers and episodes can sometimes have a gradual onset and offset.

What causes SVT?

For most patients there are no obvious triggers for SVT but for some patients who experience SVT, triggers such as lack of sleep, exercise, changes in posture and psychological stress may increase the likelihood of them experiencing an episode. Listed below are some triggers for SVT:

• Alcohol
• Chronic lung disease
• Cocaine and methamphetamines
• Excessive caffeine consumption
• Exercise
• Emotional stress
• Pre-existing heart disease or heart failure
• Hyperthyroidism
• Over-the-counter drugs for colds or hay-fever, or medications for asthma

How is SVT diagnosed?

Supraventricular tachycardia (SVT) may start and stop abruptly, with stretches of normal heart rates in between. Symptoms may last anywhere from a few minutes to a few hours and may cause symptoms such as palpitations, chest pain, dizziness or lightheadedness, fainting (syncope), shortness of breath and sweating.

Supraventricular tachycardia becomes a problem when it occurs frequently and is ongoing, particularly if there are coexisting medical problems, which may make patients more susceptible to the effects of rapid heart rates. The uncertainty of not knowing when the next episode may occur and the possible need to present to the accident and emergency department to treat episodes can be a real nuisance.

The key to diagnosing SVT is capturing an ECG during an episode. In addition, other baseline investigations may help to identify or exclude other associated abnormalities.

• Blood tests – to exclude co-existing conditions
• Electrocardiogram (ECG) – an electrical trace of the heart
• Echocardiogram – an ultrasound scan of the heart
• A heart monitor (for between 24-hours and up to two weeks) to wear at home to pick up episodes of SVT

How is SVT treated?

Acute episodes of SVT – Tips and Tricks to stop an episode

1. Valsalva manoeuvre – this is a manoeuvre designed to increase pressure, typically by breath-holding – imagine trying to “unblock your ears” whilst descending on an aeroplane, or “pushing hard – or bearing down hard” when constipated. This action is performed for approximately 5 seconds, before “releasing” the breath suddenly. On release, there is a chance that the SVT will terminate. One practical tip is to blow into the tip of a syringe to try and move the plunger.
2. Drinking a big gulp of ice-cold fluids
3. Splashing cold water on your face
4. For patients under the age of 30, rubbing the neck (carotid sinus massage), with a rotational firm pressure on the carotid sinus for 10 seconds (typically on the side of the neck at the level of the chin)
5. Gentle pressure on closed eyes (termed orbital pressure). This can be tried on each eye for 5 seconds.

If these simple manoeuvres fail to stop palpitations, patients may need to present to the accident and emergency department, where the following options for treatment may be available:

1. Drugs to terminate rhythm. The most commonly used drug is adenosine. This is usually administered intravenously this through a drip line in the vein, and this usually stops palpitations within 30 seconds of being given. Other options include beta-blockers, such as metoprolol, which can be given IV or orally, and calcium blockers, such as verapamil.
2. Electrical Cardioversion – This involves delivering a small electrical shock through the heart, with a view to stunning the heart back into a normal rhythm. This is done using pads applied to the chest under a light general anaesthetic. The procedure is very quick and safe and takes a few minutes.

Longer-term treatment strategies for SVT

Catheter ablation.

An ablation procedure is a minimally invasive surgical procedure which is now strongly recommended as the treatment of choice for SVT and can be curative for most patients (>95%).

The procedure involves passing very fine catheters (wires) through a vein at the top of the leg into the heart. Using electrical navigation (3D mapping systems or X-ray), it is possible to position these catheters precisely within the heart to cauterise (radiofrequency energy) the ‘short-circuit’ or ‘excitable tissue’, permanently destroying it and thereby preventing a recurrence. Catheter ablation can be performed under local anaesthetic with sedation.