Supraventricular Tachycardia

Supraventricular tachycardia (SVT) is any rapid heart rhythm originating at or above the atrioventricular node. Supraventricular tachycardias can be contrasted with the potentially more dangerous ventricular tachycardias - rapid rhythms that originate within the ventricular tissue.

Although "SVT" can be due to any supraventricular cause, the term is most often used to refer to one specific example, Paroxysmal supraventricular tachycardia (PSVT). Two common types of SVT are atrioventricular reciprocating tachycardia and AV nodal reentrant tachycardia. However, in the older adult population atrial fibrillation becomes one of the most common types of supraventricular arrhythmias - though this is typically considered separately. SVT is generally not life threatening, though it may cause worsening heart function if sustained for hours.

In general SVT is caused by one of two separate mechanisms. The first is re-entry, the second is automaticity. SVT due to re-entry (such as AV nodal reentrant tachycardia and atrioventricular reciprocating tachycardia) often presents with an almost immediate onset with sudden increase in heart rate. A person experiencing PSVT may feel the heart rate accelerate from 60 to 200 beats per minute or more. Typically, there is also a sudden reversion to normal rhythm.

The main pumping chamber, the ventricle, is to a certain extent protected against excessively high rates arising from the supraventricular areas by a 'gating mechanism' at the atrioventricular node, which only allows a proportion of the fast impulses to pass through to the ventricles. In a condition called Wolff-Parkinson-White Syndrome, a 'bypass tract' avoids the node and this protection and the fast rate may be directly transmitted to the ventricles. This situation has characteristic findings on ECG.

In automatic types of SVT (Atrial Ectopic Tachycardia, junctional ectopic tachycardia), there is more typically a gradual increase and decrease in the heart rate. These are due to an area in the heart that generates its own signal.