Supraventricular Arrhythmias Reentrant Tachycardias Reentry SVT (Fig 22.8) requires an anatomic substrate for abnormal conduction and the appropriate electrophysiologic environment Reentrant tachycardias, such as AVRT and AVNRT, require a difference in the electrophysiologic properties of two AV pathways to initiate tachycardia; this allows the occurrence of unidirectional block in one pathway while maintaining conduction in the other Patients presenting with a reentrant arrhythmia often report symptoms consistent with an abrupt onset and termination They may go so far as to recognize that coughing or holding their breath (vagal maneuvers) will terminate the tachycardia Both AVRT and AVNRT are typically narrow QRS tachycardias with a 1 : 1 relationship between the atrium and ventricles One exception to this rule is patients who have an underlying bundle-branch block (i.e., tetralogy of Fallot), as the tachycardia will have the same underlying wide QRS morphology as seen in sinus rhythm In this regard it is always helpful to review old ECGs, if available, in patients presenting with palpitations (Figs 22.9A and B) Both tachycardias use the AV node as the anterograde limb of the tachycardia circuit and thus will terminate with vagal maneuvers and AV nodal blocking agents such as adenosine FIG 22.8 A 12-lead electrocardiogram demonstrating short RP′ narrow complex supraventricular tachycardia with a normal QRS axis FIG 22.9 (A) A 22-year-old patient with tetralogy of Fallot underwent repair and subsequent transcatheter pulmonary valve baseline electrocardiography that showed sinus rhythm and right-bundle-branch block with a QRS duration of 200 ms The same patient (B) presenting with a wide complex tachycardia with an identical QRS morphology in all leads and no evidence of atrioventricular dissociation This electrocardiogram represents atrial tachycardia with an underlying right-bundle-branch block Atrioventricular Reentry Tachycardia AVRT is usually due to a reentrant circuit that involves anterograde conduction over the AV node and retrograde conduction over an accessory AV pathway (orthodromic reentrant SVT) The accessory pathway or bypass tract is a microscopic muscular connection that traverses the mitral or tricuspid annulus, allowing electrical continuity between the atrium and the ventricles Some accessory pathways allow conduction from the atrium to the ventricle anterograde; thus there is a pattern of manifest preexcitation, a delta wave, on the surface ECG Some pathways can conduct only in the retrograde (ventricleto-atrium) direction and are thus concealed, without evidence of a delta wave on the surface ECG Finally, some pathways can conduct in both directions There is an association between AVRT and structural heart disease, especially the Ebstein malformation of the tricuspid valve and congenitally corrected transposition of the great arteries (AV and ventriculoatrial [VA] discordance),