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  SVT | AVNRT

AVNRT or Atrio-ventricular nodal re-entry tachycardia is a supraventricular tachycardia (SVT); that is a fast heart rhythm originating from above the ventricles. It is the commonest regular SVT, occurring more often in women. It may occur in childhood but is more often seen in patients in their twenties and thirties. It produces a sensation of fast, regular palpitations which usually start and stop suddenly. In some people there may be a clear precipitant, such as anxiety, caffeine, alcohol etc. however this is by no means always the case. On occasion patients may also notice chest tightness, breathlessness or dizziness. After the palpitations pass off some patients may feel rather lethargic or may need to pass urine.

In the heart the sinus node drives the heart rate. Electricity spreads to the atrioventricular (AV) node and then into the ventricles. Electricity reaches the AV node by two pathways, a fast one from above and a slower one from below. In many people (about 80%) there is very little difference between these two pathways. However in about 20% the pathways conduct at speeds different enough to be detected at an electrical study; a slow pathway and a fast pathway are seen. In a small proportion of these people the speeds are different enough to allow a circuit to be formed in certain circumstances where electricity spreads down the slow pathway and back up the fast pathway, producing symptoms.

It is possible to stop the tachycardia when it is running by slowing conduction though the AV node; some patients find they can do this by drinking a cold glass of water, breath holding, straining etc. In Accident and Emergency we can stop it with various injections (adenosine, beta-blockers or verapamil for instance) which also slow conduction. Some patients carry around medicines which can be taken at the start of the palpitations to reduce their length, although this takes at least about half an hour to work.

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The frequency of attacks can be reduced, sometimes significantly, by medicines taken in the long term; this of course may need to be a lifelong enterprise. Avoiding precipitants, such as smoking, alcohol and caffeine can also reduce the frequency of attacks.

It is possible to burn away (ablate) AVNRT in most people leading to a permanent cure (see section on ablation). This, like any other ablation, involves passing a catheter whose end can be heated into the heart, usually via a vein at the top of the leg. The extra AV nodal (slow) pathway is then located and burnt away. Usually AVNRT can be cured at the first attempt in 90-95% of people. If it returns, more attempts can be made. As with any other invasive procedure there are risks although these are small. Fortunately complications only occur in about 1% of patients. The most important risk is that of damaging the AV node itself. In this situation electricity can not spread form the atria to the ventricles and the patient requires a pacemaker. Although this is not ideal, especially in a young person, it is by no means 'the end of the world'. Other risks, damaging the vein at the top of the leg through which the catheters are passed, blood leaks around the heart etc. are extremely rare.

After the ablation patients need to lie flat for a few hours to let the holes in the veins close up. Often patients will be watched overnight and if an ECG is fine the following day, discharged. Some patients notice occasional skips and jumps in the heart or a slightly faster pulse for the first few weeks after the procedure, however these always settle.

 


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