Supra ventricular tachycardias (SVT) are those which require the atrium or the AV node for maintenance. In general they have a narrow QRS. But there can be wide QRS if there is a rate dependent aberrancy or a prexisting bundle branch block. Another reason for a wide QRS is involvement of an accessory pathway in the anterograde limb of the circuit. SVTs are twice as common in women compared to men. The incidence increases with age. While atrioventricular nodal re-entrant tachycardias are more common in adults, atrioventricular re-entrant tachycardias due to accessory pathways (like Wolff–Parkinson–White syndrome) are commoner in children.
SVT can be classified into atrial and atrioventricular depending on the region of re-entry. It can also be divided into regular or irregular, with atrial fibrillation being the commonest form of irregular tachycardia with a supra ventricular origin. The QRS complexes can be narrow or wide as discussed above. SVTs can also be divided into AV node dependent vs AV node independent. Another form of classification of SVT is into short RP (long PR) vs long RP (short PR) interval tachycardias.
Tachycardias with atrial circuit are sinus tachycardia, sinus node re-entrant tachycardia, atrial tachycardia, multifocal atrial tachycardia, atrial flutter and atrial fibrillation. The different types of atrial flutter are typical (counterclockwise), reverse-typical (clockwise), left atrial, atypical and scar related atrial flutter. Upper loop re-entry, lower loop re-entry and figure of eight re-entry are other descriptions of the circuits in atrial flutter.
Tachycardias with atrioventricular circuits are atrioventricular nodal re-entrant tachycardia (AVNRT), AV re-entrant tachycardia (AVRT) and junctional ectopic tachycardia (JET).
AVNRTs themselves can be of three types: typical, atypical and the slow-slow variety. In the typical variety, anterograde conduction is over the slow pathway and retrograde conduction is over the fast pathway. Hence the initiating premature atrial complex will have a long PR interval. The RP interval is short (short RP tachycardia) in this variety, with PR>RP interval. P wave is either within the QRS or at the terminal portion of the QRS. These P waves manifest as pseudo S waves in inferior leads and pseudo R waves in V1. In atypical AVNRT, the opposite pattern of activation is seen so that RP is more than the PR interval (long RP tachycardia). In the rare slow-slow pathway tachycardia, the re-entry occurs between two slow pathways.
AVRTs can be divided into orthodromic and antidromic. In orthodromic AVRT, the anterograde conduction is through the AV node and the QRS complex is narrow. Ortho dromic AVRTs contribute about 90% of the AVRTs. Antidromic AVRT has the reverse pattern and the QRS is wide and bizarre.
JET is due to enhanced automaticity of the AV node and can manifest AV dissociation in the ECG with a narrow QRS. It can be congenital as well as acquired. It can occur in the setting of cardiac surgery and responds to cooling (induction of hypothermia).