PATHOPHYSIOLOGY AND NATURAL HISTORY ARRHYHMII Differential electrophysiologic properties of decremental retrograde pathways in long RP' tachycardia

Long RP' supraventricular tachycardias (SVT) often demonstrate both slow and decremental conduction properties in the retrograde pathway of the reentrant circuit. The electrophysiologic properties of these pathways are poorly understood. We studied 10 patients with long RP' SVT (RP'/RR, 0.52 to 0.71); five had the unusual form of atrioventricular nodal reentry (fast-slow) and five patients had accessory AV pathways with slow, decremental retrograde conduction properties. During SVT, the effects of intravenous adenosine (37.5 to 150 ,g/kg), which increases potassium current (QK) in supraventricular tissue and hyperpolarizes membrane potential toward Ek (-90 mV), and the response to slow-inward channel blockade with verapamil (0.10 to 0.20 mg/kg iv) were evaluated. Adenosine and verapamil has similar effects in the presence of fast-slow AV nodal reentry since both agents terminated SVT by producing block in the retrograde slow AV nodal pathway. In contrast, adenosine and verapamil had differential effects on retrograde conduction in decremental accessory pathways. Adenosine terminated all episodes of SVT in the retrograde decremental pathway, whereas verapamil had a direct effect on this tissue in only two of five patients. Decremental retrograde accessory pathways can therefore demonstrate at least two types of electrophysiologic responses. Pathways that respond only to adenosine-induced hyperpolarizing K' current likely comprise depressed fast-Na+ channel tissue, i.e., partially depolarized (> -60 to -70 mV) atrial tissue. In contrast, decremental accessory pathways that respond to both modulation of the slow-inward calcium current and K' conductance have pharmacologic properties similar to those of the AV node and may represent more completely depolarized atrial fibers with resting membrane potentials of 60 mV or less. Circulation 76, No. 1, 21-31, 1987. FOR DIAGNOSTIC and therapeutic purposes, reentrant supraventricular tachycardia (SVT) is often classified on the basis of the RP' interval.1 In the absence of sinus node or intra-atrial reentry, long RP' SVT (RP'/RR >50%) is usually due to slow conduction in the retrograde limb of the reentrant circuit. Some of these slow retrograde pathways also manifest decremental conduction properties and are thought to be located either intranodally (atrioventricular [AV] nodFrom the Clinical Cardiac Electrophysiology Laboratory, Cardiology Section, Departments of Medicine and Pediatrics, University of Virginia School of Medicine, Charlottesville. Supported in part by a grant from the American Heart Association, Virginia Affiliate, and from the National Heart, Lung, and Blood Institute (1 ROI HL35272-01). Dr. Lerman is a recipient of the NIH New Investigator Award (HL-35860). Address for correspondence: Bruce B. Lerman, M.D., Cardiology Division, University of Virginia, Hospital, Box 158, Charlottesville, VA 22908. Received May 20, 1986; revision accepted April 9, 1987. al tissue) or extranodally (accessory AV connection). The intranodal type or the fast-slow form ofAV nodal reentry is thought to utilize the fast AV nodal pathway in the anterograde direction and the slow pathway for its retrograde limb,2" but this has not been shown unequivocally to occur.5'6 The electrophysiologic properties of decremental extranodal AV pathways, including posteroseptal connections associated with the permanent form of junctional reciprocating tachycardia (PJRT)7 8 and laterally located accessory AV pathways,9' 10 are also neither completely understood nor adequately defined. It has been postulated that these extranodal pathways may represent either an accessory AV nodal-like structure' 11,12 or an accessory AV pathway with decremental properties.13 Therapy of these tachycardias could be rationally designed if their underlying electrophysiologic properties were better defined. Vol. 76, No. 1, July 1987 21 by gest on A ril 4, 2017 http://ciajournals.org/ D ow nladed from