Figure 5. Mechanisms for reentrant tachycardias involving the AV junction-paroxysmal supraventricular tachycardia (PSVT). A, Common and uncommon forms of AV node reentry. The AV node contains functionally fast (Fast) and slow (Slow) conducting pathways, with longer and shorter refractoriness, respectively. With the common form of AV node reentry, an atrial premature beat blocks anterogradely in the still-refractory fast pathway, but conducts to the ventricles via the no-longer refractory slow pathway. It returns through the no-longer refractory fast pathway to excite proximal tissues and initiate a narrow QRS PSVT. Retrograde P waves (arrows) are usually buried in the QRS or sometimes occur just after the QRS. With the uncommon form of AV node reentry, a ventricular premature beat penetrates the slow pathway and conducts retrogradely to excite proximal tissues. It can also initiate narrow QRS PSVT with retrograde P waves (arrows), but these precede the QRS with a long RP interval. B, Orthodromic and antidromic accessory pathway (AP) reentry. AP can be concealed or manifest during sinus rhythm (see text). With orthodromic AP reentry, anterograde conduction during is via the AV node. Therefore, QRS complexes during PSVT are narrow followed by retrograde P waves (arrows) with longer RP intervals than with the common form of AV node reentry. With antidromic AP reentry, QRS complexes are widened because of anterograde conduction via the AP and ventricular preexcitation (delta wave, shaded). Retrograde P waves (arrows) immediately precede the QRS.