What is SVT?
Supraventricular tachycardia (SVT) affects up to one in 400 people. It is a common type of rapid heart racing, which is due to an abnormal electrical circuit travelling rapidly around your heart – usually at a rate of 150 to 250 beats per minute. These circuits generally involve the normal electrical tissue or fibres (the atrio-ventricular or AV node) and one or more abnormal fibres. Individuals with this type of tachycardia are usually born with these extra fibres (or pathways), even though the rapid heart racing may not become evident until the teen years or later in life.
Several different types of pathways exist. An electrocardiogram (ECG) obtained when your heart is racing, as well as an ECG obtained at normal times, may give a hint as to the type of abnormal electrical pathways you have. Most individuals who have these abnormal electrical pathways within their hearts have otherwise normal hearts and do not have problems with valves or coronary arteries.
The most common types of abnormal electrical pathways leading to these electrical circuits and tachycardia include:
A common condition where the normal electrical pathway (the AV node) has an extra fibre, and an electrical circuit may race around between the usual pathway and the extra fibre.
The circuit forms from electrical impulses travelling via the AV node and an abnormal extra pathway.
A rare condition, this is a special variant of AV re-entry where the extra electrical pathway that bridges the upper (atria) and the lower (ventricle) chamber of the heart has particular electrical properties that often make it possible to be seen on the ECG when the heart is in a normal rhythm.
Why perform catheter ablation?
Catheter ablation for both AV re-entry and AV node re-entry tachycardia is the treatment of choice for individuals with frequent arrhythmias, particularly those for whom drugs do not work or are poorly tolerated. The advent of catheter ablation as treatment for arrhythmias is useful for two reasons. Firstly, many individuals with supraventricular tachycardia require “lifelong” drug therapy to control attacks. With that comes the risk of side effects. Secondly, the procedure has a very high cure rate (approximately 95%) with a low risk of complications.
In order to cure the arrhythmias, special catheters (small, flexible wires with metal tips) are used. These catheters are placed into the heart under X-ray guidance. The catheters stimulate the heart and record electrical activity from within the heart.
In most cases, it is necessary to trigger an episode of tachycardia to determine its type and source. If the area of the heart responsible for the tachycardia can be identified, one of the catheters can be manipulated so the tip electrode is in contact with the abnormal tissue. Locating the target tissue is called mapping. During mapping, the ablation catheter is moved from spot to spot until the appropriate area is found. Mapping may take only minutes, but on occasions can take several hours.
Once the catheter is in the appropriate area, radiofrequency (RF) energy (also known as modified surgical cautery) is delivered via the ablation catheter. This heats and damages the tissue surrounding the catheter, potentially curing the tachycardia. Frequently, more than one area needs to be cauterised to fully eliminate the tachycardia. Following each RF delivery, retesting is performed to detect whether or not the tachycardia has been completely eliminated.
In experienced hands, 90 to 95% of cases of SVT can be cured with the procedure lasting an hour or two. In a small proportion of cases (approximately 5%), the cauterised area can recover, and may require and a second ablation procedure. In another small proportion of people, the area to be cauterised cannot be identified or is not accessible.
What should I expect?
Generally, patients are admitted to the hospital on the morning of the procedure. You will be taken to an Electrophysiology Laboratory (EP Lab) for the procedure. Medications to control your heart rhythm should generally be stopped two to four days prior to the procedure. Your cardiologist will let you know if this is the case. Since X-ray is required for the procedure, you must alert your doctor if you think you may be pregnant.
You will be given intravenous sedation or general anaesthetic. Local anaesthetic is placed in the skin using a small needle. Catheters may be placed in the right and/or left groin.
Through sheaths and under X-ray guidance the flexible EP catheters are advanced up the veins to the heart. There is very little sensation associated with the placement of these catheters. During the delivery of the RF energy there may be mild chest discomfort. Occasionally, multiple applications of electrical current may be required to destroy the abnormal fibre.
At the end of the procedure, all the catheters and sheaths will be removed. You will stay in bed for three to five hours to prevent bleeding at sites where the catheters were inserted. You will usually be discharged the next day.
The team performing the procedure includes the GenesisCare cardiologist, three catheter laboratory nurses, an anaesthetist and anaesthetic nurse, a radiographer, a cardiac scientist and a 3D mapping cardiac scientist.
Risks
The amount of tissue damaged in a typical ablation procedure is insignificant for overall heart pumping function. The scar tissue seems to remain stable over time and not cause problems even years later. Nonetheless, things can go awry during the procedure and occasionally complications can occur.
Possible complications include:
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Application of radiofrequency current close to the normal conduction system of the heart could produce heart block and necessitate implantation of a permanent pacemaker (<1% risk). This risk only applies if the abnormal pathway is close to your normal electrical conduction system (i.e. within a centimetre or less).
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A blood clot may form on the catheter that carries the radiofrequency current to the heart. The clot may dislodge and obstruct a blood vessel, which can cause a stroke, heart attack or injury to another organ (approx. one in 1000 to one in 5000 risk). This is usually only a risk if ablation is being performed on the left (arterial) side of the heart.
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In rare cases, the catheter could damage the heart valves or a coronary artery.
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Other rare complications reported in the literature include cardiac perforation (producing a hole in the heart wall), haemo- or pneumothorax (blood or air in the chest wall requiring tube drainage), damage to the femoral artery (one in 10000) and damage to the phrenic nerve (the nerve supplying the diaphragm).
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