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SUMMARYElectrophysiological evidence suggestive of dual atrioventricular (A-V) nodal pathways is presented in two patients with normal P-R interval and reentrant paroxysmal supraventricular tachycardia (PSVT). His bundle recordings and atrial stimulation were used to obtain this electrophysiological evidence. Recently reported studies in man suggest that A-V nodal reentrance is a common mechanism of paroxysmal supraventricular tachycardia (PSVT) .6-8 A-V nodal reentrance implies either the presence of dual A-V nodal pathways or reentry by "reflection" in an area of depressed conduction.9' 10 The purpose of this report is to describe two patients with history of PSVT and normal P-R 549 by guest on May 7, 2018 http://circ.ahajournals.org/ Downloaded from

SUMMARYThe effects of pacing-induced changes in cycle length on the refractory periods of the atrium, A-V node and His-Purkinje system were studied in 24 patients using the extra stimulus technique. Refractory period determinations were made at two or more cycle lengths in all patients. Slopes relating cycle length and refractory periods were calculated using the least squares method.Both the effective and functional refractory periods (ERP and FRP)

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