usually the AV node, and the other, an abnormal accessory pathway (muscular connection) from the atria to the ventricle. Wolff-Parkinson-White syndrome (WPW) is a relatively common abnormality with an accessory pathway, the bundle of Kent crossing the AV valvular ring. In orthodromic AVRT, atrial impulses are conducted down through the AV node and retrogradely re-enter the atrium via the accessory pathway. A distinguishing characteristic of orthodromic AVRT can therefore be an inverted P-wave (relative to a sinus P wave) that follows each of its regular, narrow QRS complexes, due to retrograde conduction. In antidromic AVRT, atrial impulses are conducted down through the accessory pathway and re-enter the atrium retrogradely via the AV node. Because the accessory pathway initiates conduction in the ventricles outside of the bundle of His, the QRS complex in antidromic AVRT is wider than usual. A delta wave is an initial slurred deflection seen in the initial part of an otherwise narrow QRS of a patient at risk for WPW and is an indicator of the presence of an accessory pathway. These beats are a fusion between the conduction down the accessory pathway and the slightly delayed but then-dominant conduction via the AV node. Once an antidromic AVRT tachycardia is initiated, it is no longer delta waves but rather a wide complex (>120 ms) tachycardia that is seen. Junctional ectopic tachycardia (JET) is a rare tachycardia caused by increased automaticity of the AV node itself initiating frequent heartbeats. On the ECG, junctional tachycardia often presents with abnormal morphology P-waves that may fall anywhere in relation to a regular, narrow QRS complex. It is often due to drug toxicity. Classification The following types of supraventricular tachycardias are more precisely classified by their specific site of origin. While each belongs to the broad classification of SVT, the specific term/diagnosis is preferred when possible: Sinoatrial origin: Sinoatrial nodal reentrant tachycardia (SNRT) Atrial origin: Ectopic (unifocal) atrial tachycardia (EAT) Multifocal atrial tachycardia (MAT) Atrial fibrillation with rapid ventricular response Atrial flutter with rapid ventricular response (Without rapid ventricular response, fibrillation and flutter are usually not classified as SVT) Atrioventricular origin: AV nodal reentrant tachycardia (AVNRT) or junctional reciprocating tachycardia (JRT) AV reciprocating tachycardia (AVRT) – visible or concealed (including Wolff-Parkinson-White syndrome) Permanent (or persistent) junctional reciprocating tachycardia (PJRT), a form of SVT that involves a slow retrograde conduction over accessory pathway – occurs predominantly in infants and children but can occasionally