Atrioventricular nodal re-entrant tachycardia, abbreviated AVNRT, is one of the most common heart rhythm disorders. It is noticed as the heart suddenly beating disproportionately fast, often 200 beats a minute or more. The rate varies with age and with how activated the nervous system is when the episode starts.
The arrhythmia is rarely dangerous, but can be very troublesome. It is not linked to other forms of heart disease and often occurs in otherwise healthy and fit people of all ages. If it occurs on top of other heart disease, it can be particularly burdensome.
What happens in the heart?
AVNRT comes from the AV node, where the electrical signals are conducted from the atria to the ventricles (see normal heart rhythm). Many people are born with both a fast and a slow pathway in the AV node. For the arrhythmia to start, conditions in the autonomic nervous system – the part of the nervous system that controls blood pressure and heart rate – must also be right.
The arrhythmia arises when the signals go round in a circle. Most often they go down the slow pathway in the AV node and up the fast one. We call this typical AVNRT. Some people have several slow pathways. Most are located in or near the AV node, but on rare occasions we find them at the mitral valve.
The nervous system and the AV node
The AV node receives many fibres from the autonomic nervous system, and its activity changes with what we do and are exposed to. During sleep, activity is high in the part that slows the heart (the parasympathetic system). During mental or physical stress, activity increases in the part that raises heart rate and blood pressure (the sympathetic system). Read more about this under What is HRV?.
Although the autonomic nervous system mainly regulates itself, it is possible to influence it. Parasympathetic activity can be increased by taking a deep breath, holding it and straining (the Valsalva manoeuvre). It also increases when you strain on the toilet.
Most commonly the arrhythmia is triggered by physical exertion. Others get it with strong emotions. It is not unusual to hear about major events, good or bad, just before the arrhythmia appeared for the first time. It may also be elite athletes who have changed their training, or who notice it for the first time during a decisive competition.
ECG
Treatment
Some learn to stop the episodes themselves with Valsalva-like manoeuvres. For most this is not enough. Many respond well to medication, both beta blockers such as metoprolol and centrally acting calcium antagonists such as verapamil. If the arrhythmia recurs, or the medicines do not work, many prefer ablation. Others choose ablation because they would rather not take medication.
Ablation
Ablation is done in specially equipped laboratories at the regional hospitals. Before the procedure the patient is received by a doctor and nurse, who explain the procedure, its purpose and possible complications. Blood tests are taken.
The procedure starts with local anaesthesia around the blood vessels in both groins. In our unit the veins are punctured in four places, and usually four catheters are passed up to the heart. Guided by X-ray and the electrical signals from the catheter tips, they are placed at specific sites in the heart. The examination then starts, and we try to induce the arrhythmia that causes the symptoms.
ECGs recorded during previous episodes give an indication, but we cannot be certain. It often turns out that what we thought was AVNRT is something quite different, for example atrial tachycardia or WPW syndrome. If AVNRT is confirmed, we treat the slow part of the circuit.
In adults it is most common to deliver radiofrequency energy into the tissue to weaken or eliminate the slow pathway. In children, and in certain cases in adults, we freeze it instead. The reason is that there is always a small risk of destroying the connection between the atria and the ventricles. If that happens, a pacemaker must be implanted. The risk is lower with freezing, but the chance of recurrence is somewhat higher. With radiofrequency the chance of success is well over 90 per cent in typical AVNRT.
With radiofrequency the risk of destroying the connection between the atria and ventricles is about 1 per cent, and with freezing even lower. Other serious complications, such as bleeding into the pericardium or severe bleeding from the puncture site, are even rarer.
After the procedure
You must stay in bed for about 3 hours. Some are discharged the same afternoon, others stay overnight. You should avoid lifting more than 4–5 kilos for the first two weeks to prevent bleeding from the puncture sites in the groin. Apart from that there are usually no restrictions, and follow-up afterwards is only needed in exceptional cases.