PATHOPHYSIOLOGY AND NATURAL HISTORY ELECTROPHYSIOLOGY Further observations on transient entrainment: importance of pacing site and properties of the components of the reentry circuit

Transient entrainment of circus-movement tachycardia utilizing an atrioventricular (AV) bypass pathway was studied in 13 patients (nine with the orthodromic form, two with the antidromic form, and two with both the orthodromic and antidromic forms). All patients had a leftsided AV bypass pathway. Pacing at selected rates faster than the spontaneous rate was performed during the tachycardia at a site proximal or distal to the AV node, an area of slow conduction within the reentry loop. Rapid pacing from a site proximal to the AV node (from the right atrium during the orthodromic form of the arrhythmia or the right ventricle during the antidromic form of the arrhythmia) always demonstrated at least one of the three entrainment criteria: constant fusion beats except for the last captured beat, which was entrained but not fused (first criterion); progressive fusion (second criterion); localized conduction block to a site(s) for 1 paced beat associated with interruption of the tachycardia followed by activation of that site(s) by the next paced beat from a different direction and with a shorter conduction time (third criterion). In contrast, rapid pacing from a site distal to the AV node (from the right ventricle during the orthodromic form of the arrhythmia, or the right atrium during the antidromic form of the arrhythmia) transiently entrained the tachycardia, but never demonstrated any entrainment criteria because the antidromic wave front from the pacing impulse always blocked in the AV node (concealed entrainment). We conclude that the location of the pacing site relative to the components of a reentry loop is critical to the demonstration of the criteria of transient entrainment; i.e., if it is proximal to an area of slow conduction and/or unidirectional block within a reentry loop, transient entrainment should be demonstrable, but if it is distal, it will not be demonstrable. Circulation 72, No. 6, 1293-1307, 1985. TRANSIENT ENTRAINMENT of a tachycardia during rapid pacing is an increase in rate of the tachycardia to the faster pacing rate, with resumption of the intrinsic rate of the tachycardia on either termination of pacing or slowing of the pacing rate below the intrinsic rate of the tachycardia. '-'° We, and subsequently other investigators, have presented evidence that this is best explained by continuous early entrance of the wave front from the pacing impulse into the excitable gap of a tachycardia's reentry loop. -8. 10 Furthermore, we'-' have suggested three criteria, any one of which, if present, establishes transient entrainment of a tachycardia. These criteria are as follows: (1) The demonstration of constant fusion beats in the electrocardioFrom the Department of Medicine and the Cardiovascular Research and Training Center, University of Alabama at Birmingham. Supported in part by U.S. Public Health Service NHLBI SCOR in Ischemic Heart Disease grant 1P17HL17667 and NHLBI grant 1RO I HL2938 1-01 A2. Address for correspondence: Albert L. Waldo, M.D., UAB Medical Center, Room 336LHR, University Station, Birmingham, AL 35294. Received Feb. 7, 1985; revision accepted Aug. 22, 1985. Vol. 72, No. 6, December 1985 gram during pacing at a constant rate faster than the rate of the tachycardia, except for the last captured beat, which is entrained but not fused. (2) The demonstration of progressive fusion, i.e., constant fusion beats in the electrocardiogram during rapid pacing at any constant rate faster than the rate of the tachycardia, but different degrees of constant fusion beats at different rapid pacing rates. (3) The interruption of a tachycardia during pacing at a rate faster than the rate of the tachycardia associated with localized conduction block to a site or sites for 1 beat followed by activation of that site or sites from a different direction and with a shorter conduction time by the next pacing impulse. We have, however, emphasized that transient entrainment may be present despite the inability to demonstrate any of these criteria,3 4 and recently two studies'0 " have suggested the importance of the pacing site for the demonstration of the entrainment criteria. In this article, using the model of circus-movement tachycardia utilizing an atrioventricular (AV) bypass pathway, a rhythm well accepted to be due to reentry, 1293 by gest on M ay 0, 2017 http://ciajournals.org/ D ow nladed from