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essence, a supraventricular rhythm (Table 5). The term ‘narrow QRS tachycardia’ indicates those with a QRS duration <_120 ms. A wide QRS tachycardia refers to one with a QRS duration >120 ms (Table 6). In clinical practice, SVT may present as narrow or wide QRS tachycardias, most of which, although not invariably, manifest as regular rhythms. These Guidelines do not cover AF, which is the subject of separate clinical Guidelines4 and various consensus documents.57

4 Electrophysiological

mechanisms of supraventricular tachycardia

Arrhythmia can originate from abnormal impulse initiation in an individual myocyte or, more realistically, in a close cluster of myocytes. This can occur in non-pacemaker cells through a mechanism similar to the physiological automaticity of pacemaker cells [sinus node and AV node (AVN)], and is thus named ‘abnormal’ or ‘enhanced automaticity’. An alternative form of abnormal impulse initiation involves oscillations of membrane potential, named early or delayed ‘after-depolarizations’. In such cases, the resulting arrhythmias take the name of ‘triggered activity’.8 Arrhythmias resulting from enhanced automaticity and triggered activity are defined as ‘non-re-entrant’. Arrhythmias can also arise when myocardial regions activated later in propagation re-excite regions that have already recovered excitability. This results from abnormal propagation of the excitation wavefront and/or of tissue refractoriness. This mechanism, named ‘re-entry’, is based on the syncytial nature of myocardial tissue and is thus radically different from focal impulse initiation.8 A detailed discussion and schematic representation of common SVT circuits is provided in the Supplementary Data.

Table 6 Differential diagnosis of narrow and wide QRS tachycardias

Narrow QRS (120 ms) tachycardias Regular • Physiological sinus tachycardia • Inappropriate sinus tachycardia • Sinus nodal re-entrant tachycardia • Focal AT • Atrial flutter with fixed AV conduction • AVNRT • JET (or other non-re-entrant variants) • Orthodromic AVRT • Idiopathic VT (especially high septal VT) Irregular • AF • Focal AT or atrial flutter with varying AV block • Multifocal AT Wide QRS (>120 ms) tachycardias Regular • VT/flutter • Ventricular paced rhythm • Antidromic AVRT • SVTs with aberration/BBB (pre-existing or rate-dependent during tachycardia) • Atrial or junctional tachycardia with pre-excitation/bystander AP • SVT with QRS widening due to electrolyte disturbance or antiarrhyth-

mic drugs Irregular • AF or atrial flutter or focal AT with varying block conducted with aberration • Antidromic AV re-entrant tachycardia due to a nodo-ventricular/fascicular AP with variable VA conduction • Pre-excited AF • Polymorphic VT • Torsade de pointes • Ventricular fibrillation Occasionally, AF with very fast ventricular response may apparently resemble a regular narrow QRS tachycardia. AF = atrial fibrillation; AP = accessory pathway; AT = atrial tachycardia; AV = atrioventricular; AVNRT = atrioventricular nodal re-entrant tachycardia; AVRT = atrioventricular re-entrant tachycardia; BBB = bundle branch block; JET = junctional ectopic tachycardia; SVT = supraventricular tachycardia; VA = ventriculoatrial; VT = ventricular tachycardia.

Table 5 Conventional classification of supraventricular tachycardias

Atrial tachycardias Sinus tachycardia • Physiological sinus tachycardia • Inappropriate sinus tachycardia • Sinus nodal re-entrant tachycardia Focal AT Multifocal AT

MRAT • Cavotricuspid isthmus-dependent MRAT

 Typical atrial flutter, counter-clockwise (common) or clockwise

(reverse)  Other cavotricuspid isthmus-dependent MRAT • Non-cavotricuspid isthmus-dependent MRAT

 RA MRAT  LA MRAT AF AV junctional tachycardias

Atrioventricular nodal re-entrant tachycardia (AVNRT) • Typical • Atypical

Non-re-entrant junctional tachycardia • JET (junctional ectopic or focal junctional tachycardia) • Other non-re-entrant variants Atrioventricular re-entrant tachycardia (AVRT) • Orthodromic (including PJRT) • Antidromic (with retrograde conduction through the AVN or, rarely,

over another pathway)

AF = atrial fibrillation; AT = atrial tachycardia; AV = atrioventricular; AVN = atrioventricular node; JET = junctional ectopic tachycardia; RA = right atrial; LA = left atrial; MRAT = macro-re-entrant atrial tachycardia; PJRT = permanent junctional reciprocating tachycardia; RA = right atrial. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026