Supraventricular tachycardia (SVT)

What is supraventricular tachycardia?

Supraventricular tachycardia (SVT) is an abnormal heart rhythm (arrhythmia) that causes a sudden and rapid heartbeat. A normal resting heart rate is considered to be between 60 and 100 beats per minute (BPM). A BPM over 100 is classed as tachycardia (abnormally fast).

SVT occurs when there's an electrical abnormality in the middle of the heart or the upper chambers of the heart (atria). There are several types of supraventricular tachycardia (SVT) heart conditions. They can occur at any age from babies in the womb to adults in old age.

What are the types of supraventricular tachycardia (SVT)?

There are two main types of supraventricular tachycardia (SVT) that cause an irregular heart rhythm. 

Re-entrant SVT

This is the most common type of SVT. It occurs when the electrical signal gets stuck in a loop within the upper chambers of the heart. This causes your heart to contract too quickly. There are two different types of re-entrant SVT. 

• Atrioventricular nodal re-entrant tachycardia (AVNRT): The most common type of supraventricular tachycardia. AVNRT is caused by an extra pathway near the AV node, which controls the lower chambers of your heart (ventricles). In individuals with AVNRT, the electrical signal goes around the AV node in an abnormal loop. Young women are the most at risk for this type of SVT but it can affect anyone. 
• Atrioventricular reciprocating tachycardia (AVRT): AVRT is more prevalent in younger people and children. AVRT usually occurs when people are born with an extra electrical connection between the upper and lower chambers of the heart. This causes the electrical signals to travel through an accessory pathway, bypassing the normal AV node. This creates a re-entry circuit. AVRT is an underlying cause of Wolff-Parkinson-White syndrome.

Automatic SVT

Automatic SVT is less common than re-entrant SVT. It’s caused by abnormal functioning in the heart tissue which means the atria contract too quickly. There are multiple types: 

• Atrial tachycardia: In this type of SVT, the atria send abnormal electrical signals, which causes your heart to contract too quickly. This means your heart beats rapidly. This type of SVT is usually more common in people with heart failure or lung diseases. 
• Sinus nodal re-entrant tachycardia (SNRT): A rare form of supraventricular tachycardia, this condition is characterised by a heart rate of 100-150 BPM. It’s caused when an electrical signal gets stuck in a loop within the sinoatrial (SA) node in the right atrium. It's usually found in children and adults with structural heart disease but it's uncommon and doesn't usually present any obvious symptoms. Other types of supraventricular tachycardia include:
• Sinus tachycardia: Usually a stress response, sinus tachycardia causes a rapid heart rate that returns to normal once the stressor has passed. It's often caused by fear, exercise or dehydration.
• Inappropriate sinus tachycardia (IST): This clinical syndrome causes your heart rate to exceed 100 BPM at rest without any reasonable explanation. The underlying cause of IST is disputed. It's thought to be triggered by an abnormality in your heart's natural pacemaker, the sinus node. 
• Multifocal atrial tachycardia (MAT): MAT is usually associated with respiratory diseases (such as COPD) or congestive heart failure. It's not uncommon to experience MAT if you have cancer or renal disease. 
• Junctional ectopic tachycardia (JET): Often present in children and adolescents, JET is an abnormal heart rate caused by the AV node.

Atrial fibrillation and atrial flutter are also types of SVT. However, these are usually seen as separate conditions as they are linked with other risk factors. Find out more about atrial fibrillation and atrial flutters. 

What causes supraventricular tachycardia?

Some cases of supraventricular tachycardia can be put down to an obvious cause. For example, vigorous exercise, stress or a lack of sleep. For others, the cause won't be as obvious. 

Some of the most common causes of supraventricular tachycardia include:

• Lung disease
• Consuming too much caffeine or alcohol
• Drug use – or taking other stimulants that can cause your heart to beat faster
• Pregnancy
• Smoking
• Thyroid disease
• Medications – those designed for asthma and cold and allergy drugs
• Thyroid disease 

What increases your risk of getting SVT?

Supraventricular tachycardia is the most common type of heart arrhythmia in younger people. It's also more common in women – especially during pregnancy – but SVT can happen to anyone.

Some activities and conditions can increase your risk of getting SVT. These are:

• Ageing: Some types of SVT like atrial tachycardia are more common as you get older.
• Electrolyte imbalances: If your body is lacking in substances like potassium and magnesium, this can disrupt electrical conduction in the heart and trigger SVT.
• Heart diseases or past heart surgery: Coronary artery disease, heart failure, or damage to your heart during an operation can cause supraventricular tachycardia.
• Congenital heart disease: If you've been diagnosed with a heart condition since birth, it may result in a fast heartbeat.
• Thyroid disease: Both an underactive and overactive thyroid can affect your blood pressure and also alter how the sinus node operates.
• Sleep apnoea: High blood pressure and sleep apnoea share several underlying causes, with similar associated symptoms.
• Diabetes: People with uncontrolled diabetes are at risk of developing high blood pressure and heart disease.
• Smoking and drugs: Frequent use of nicotine and illegal drugs may raise your heart rate and exacerbate symptoms of SVT.

How is supraventricular tachycardia (SVT) diagnosed?

Your GP or consultant will start by asking you a few questions about your symptoms. They'll need to know how often you've felt signs of SVT and whether you think anything seems abnormal. Your consultant will ask about your family medical history too, as supraventricular tachycardia is sometimes hereditary. They'll then conduct a physical examination. 

If your consultant thinks you have SVT, there are several tests to identify it.

Non-invasive tests 

• Electrocardiogram (ECG): This records the rate, rhythm and electrical activity of your heart and will likely be your consultant's first port of call if you've experienced SVT symptoms.
• Ambulatory ECG: For this test, you'll need to wear a portable ECG device to monitor your heart for up to a week. It can show your consultant how your heart rate fluctuates as you go about your daily activities. 
• Exercise ECG: This type of ECG is carried out as you walk on a treadmill and shows how your heart rate responds to stress. 
• Echocardiogram: This ultrasound test of your heart allows your consultant to assess the structure and function of your heart for any damage that might be causing supraventricular tachycardia. 

Invasive tests

• Implantable loop recorder: This small device is placed under your skin and records your heart rate. This means it'll identify when you have an abnormally fast heart rate. It may be monitored for up to three years. 
• Blood tests: Your consultant will examine your blood to identify any underlying conditions that could be causing your SVT. For example, thyroid issues. 

How to check your heartbeat

A fast heart rhythm is the main symptom of SVT. If you think you might be suffering from supraventricular tachycardia, you can test your heart rate at home by taking your pulse. To do this, you need to:

1. Rest for five minutes or more and avoid caffeine. This is to ensure your reading is as accurate as possible. 
2. Press your index and middle finger onto the thumb side of your opposite wrist. If you can't find your pulse in your wrist, you can press your index and middle finger to the side of your Adam's apple in the soft, hollow area of your neck.  
3. Count the number of beats in 30 seconds. 
4. Multiply this number by two to find out your BPM. 

You may have supraventricular tachycardia (SVT) or another type of irregular heartbeat if your heart rate is over 100 BPM. 

Vagal manoeuvres

The first treatment option for supraventricular tachycardia is usually a vagal manoeuvre. These are techniques designed to stimulate the vagus nerve, which is known to lower your heart rate and can stop your SVT during an attack.

Your consultant might also try massaging the carotid artery in your neck or ask you to blow through a straw or cough. Straining as though you are trying to go to the toilet or lift a heavy weight and putting a cold towel on your face are other vagal manoeuvres.

Supraventricular tachycardia (SVT) medication  

You might be given a 'pill in the pocket' tablet to control the electrical pulses in your heart. You must take this when an attack starts. In more serious cases, you might require medication delivered intravenously.

Typical medication given as a pill in the pocket treatment for SVT include calcium channel blockers such as verapamil and beta blockers such as propranolol, bisoprolol or atenolol to slow your heart rate down and lower your blood pressure.

SVT therapy and surgery

Electrical cardioversion

This is a relatively simple medical procedure that shocks your heart back into a normal rhythm following an episode of supraventricular tachycardia. This may be used to relieve severe symptoms or if your condition is unstable.

• External cardioversion: During an external cardioversion, electrodes connected to a defibrillator are placed on your chest and back. These electrodes deliver an electric shock to help restore your heart to a normal BPM. You'll be under general anaesthetic so will be asleep during the process. 
• Internal cardioversion: Carried out under local anaesthetic, your consultant will insert a small tube into a vein at the top of your leg. A wire will then be passed through this tube until it reaches your heart. Once this is in place, a small electrical shock is delivered to restore a regular heartbeat. You can normally go home on the same day as this cardioversion but some people need to stay in hospital overnight for further monitoring. 

Catheter ablation 

During the procedure, your cardiologist will place a small catheter through a blood vessel in your groin until it reaches the heart. Once the area of the heart that's causing the problem is identified, your surgeon will burn the extra connections in the heart, destroying them completely. 

This can completely remove the root cause of SVT and prevent it from recurring. 

Pacemaker 

Occasionally, a pacemaker is needed to cure SVT. During this procedure, your surgeon will place a pacemaker under the skin near the collarbone. A wire will connect the pacemaker to your heart to control your heart rate.