Paroxysmal Atrial Tachycardia
Supraventricular tachycardia (SVT) is a 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 a specific example, paroxysmal supraventricular tachycardia (PSVT), two common types being atrioventricular reciprocating tachycardia and AV nodal re-entrant tachycardia. In the older adult population, atrial fibrillation becomes a common type of supraventricular arrhythmia – though it is typically considered separately. SVT is generally not life threatening, though it may cause worsening heart function if prolonged.
In general, SVT is caused by one of two mechanisms. The first is re-entry, the second is automaticity. Re-entry (such as AV nodal re-entrant tachycardia and atrioventricular reciprocating tachycardia) often presents with an almost immediate onset with sudden increase in heart rate. A person experiencing this type of PSVT may feel the heart rate accelerate from 60 to 200 beats per minute or more. Typically, when it reverts to normal rhythm, this is also sudden.
The main pumping chamber, the ventricle, is protected (to a certain extent) 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 this node and its 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 tachycardia, junctional ectopic tachycardia), typically there is a more gradual increase and decrease in the heart rate. These are due to an area in the heart that generates its own electrical signal.