Supraventricular tachycardia (SVT) is characterised by a rapid impulse formation, that emanates from the sinus node, from atrial tissue (focal or macro-reentrant atrial tachycardia (AT)), from the atrioventricular (AV) node, or from anomalous muscle fibres that connect the atrium with the ventricle (accessory pathways (APs)). The most frequently (90%) encountered SVTs are AV nodal reentrant tachycardia (AVNRT), AV reentrant tachycardia (AVRT) mediated by accessory pathways, and atrial flutter (AFL). The remaining SVTs are AT and non-paroxysmal, usually incessant, forms of SVT.

Paroxysmal forms of SVT (PSVT) are regular recurrent tachycardias with a sudden onset and termination. If terminated by vagal manoeuvres, a reentrant tachycardia involving the AV node is most likely. The ventricular rate during SVT is commonly between 140–250 beats/min (bpm). If vagal or pharmacologic manoeuvres (adenosine) during an SVT result in AV block with persistence of atrial tachycardia, the diagnosis is most likely AT. The A:V ratio is always 1:1 in AP mediated tachycardias. Non-paroxysmal forms of SVT are ongoing repetitive or permanent/incessant tachycardias, which if left untreated can result in systolic left ventricular dysfunction and dilation (tachycardiomyopathy). These forms of tachycardias—for example, incessant AT or AFL—may be of unknown duration and without significant symptoms. Inappropriate sinus tachycardia (IST) is another form of non-paroxysmal SVT.