SUPRAVENTRICULAR TACHYCARDIA (SVT)

SVT usually presents with a sensation of clearly defined attacks of very rapid heart beat, usually >150 per minute. The onset and offest of attacks is clearly defined. Attacks may last seconds or minutes, in severe cases a number of hours, but almost never longer than this.

The attack may be stopped by change of posture (bending or lying down) or vagal manoeuvres (breath holding, Valsava, carotid sinus massage).

It is very important to obtain a 12 lead ECG if possible during an attack. This will identify the tachycardia and make planning treatment easier. An acute attack can usually be terminated with iv adenosine; if adenosine has beed used in the past with good effect, we can be pretty sure the arrhythmia was SVT.

SVT is the arrhythmia best treated by ablation. If attacks are very infrequent, it is acceptable to treat with a "pill in the pocket" approach. This means taking a dose of an anti-arrhythmic drug such as flecainide or a beta blocker hoping that this will terminate the attack. However, if this is becoming a frequent need, or if long-term regular anti-arrhythmic medication is being required, ablation should be considered.

Ablation for SVT is carried out as a day case with sedation and local anaesthesia. The success rate is around 90%+ for a first procedure and the risks are minimal. 

One problem used to be a 1% or so risk of accidental heart block resulting in the need for a permanent pacemaker. We have promoted a technique called cryoablation in Bristol where ablation can be achieved without risk of pacemaker. It seems that this form of energy is hardly ever capable of blocking the AV node. Ther is a slightly higher recurrence rate compared with RF ablation but the safety in our view makes this the desired approach in SVTs where ablation near the AV node is required.