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Figure 1 Response to PAC Delivered When Junction Is Refractory

Response to premature atrial complex (PAC) delivered when junction is refractory (local atrial activation from PAC occurs at or after His activation). (A) Response in junctional tachycardia: a PAC delivered at a time the junction focus has already depolarized blocks at the atrioventricular node (AVN) and is unable to influence the immediate or the next junction beat. Solid circles represent junction focus. Black lines show conduction through AVN, His (H), and atrium (A). (B) Response in atrioventricular node re-entry tachycardia (AVNRT): a similarly timed PAC can influence the next beat of AVNRT by early engagement of the slow pathway. Black lines show conduction through AVN, His (H), and atrium (A), and red lines show PAC and its response. Although this figure shows advancement of the next beat (x-n), delay of the next beat or termination of tachycardia are also specific to AVNRT. Red arrow indicates PAC. FP = fast atrioventricular node pathway; SP = slow atrioventricular node pathway; x and x-n = H-H intervals.