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The purpose of these experiments was to determine whether overdrive suppression of conduction (OSC), i.e., transient worsening of conduction or development of atrioventricular block after cessation of rapid pacing, could be produced in the canine His-Purkinje system damaged by ligation of the anterior septal coronary artery and to investigate the responsible mechanism. We found that OSC occurred in vivo after rapid ventricular and His bundle pacing but not after atrial pacing, and that it occurred in vitro after rapid pacing from the left bundle branch but not after pacing from the proximal His bundle. OSC was related to the duration and cycle length of pacing. Lidocaine increased while verapamil reduced the duration of OSC in vivo. The mechanism responsible for the unidirectionality of OSC is not clear but is probably related to the geometry of the atrioventricular junction and the anterograde versus the retrograde activation sequence. Changes in regional myocardial blood flow, autonomic tone, hemodynamic variables, or ventricular function do not appear to be required to produce OSC, based on the demonstration of the phenomenon in vitro. The data suggest a time-and rate-dependent change in factors affecting conduction such as excitability or cell-to-cell coupling, possibly due to accumulation of intracellular cations such as calcium. Circulation 70, No. 3, 495-505, 1984. TRANSIENT worsening of atrioventricular conduction or development of block after cessation of rapid ventricular pacing has been noted by Langendorf and Pick' in one patient. Runge and Narula2 noted six similar cases and referred to this block as "fatigue phenomenon" in the His-Purkinje system. Atrial pacing also induced the "fatigue phenomenon," but was less effective than ventricular pacing. The duration and degree of atrioventricular block after cessation of ventricular pacing in these patients were related to the rate and duration of pacing, as also shown in a case report In a series of animal studies designed to investigate the pathophysiology of atrioventricular block after occlusion of the anterior septal artery,4' El-Sherif et al.6noted that rapid ventricular pacing but not rapid atrial pacing could induce paroxysmal atrioventricular block. They explained this observation on the basis of a fatigue phenomenon that resulted from repetitive depolarization of Purkinje cells in the lesion but did not pursue the mechanism further.Since the electrophysiologic mechanisms responsible for transient atrioventricular block after rapid pacing are not known, we will here refer to this phenomenon with the descriptive term "overdrive suppression of conduction" (OSC). The purpose of the present experiments was to investigate the time-and rate-dependent effects of anterograde and retrograde conduction on overdrive suppression of His-Purkinje conduction in the canine His-Purkinje system damaged by ligation of the anterior septal coronary artery. We also investigated the mechanism responsible for the block by determining the effects of verapamil...

SUMMARY Thirteen episodes of concealed junctional ectopic impulses (JEI) in ten patients are described. In nine patients the JEI manifested as isolated automatic impulses and in one as a parasystolic junctional tachycardia. In addition to the previously described unexpected prolongation of the P-R, Type I and Type II A-V block, the following phenomena were recorded: 1) marked, greater ATRIOVENTRICULAR (A-V) BLOCK due to concealed junctional ectopic impulses (JEI) was first described in 1947.' Not until 1962 was the next case of A-V block due to JEI reported.' To the best of our knowledge a total of 11 instances of spontaneous JEI concealed within the junctional tissue because of an antegrade and retrograde block have been reported.3 "' In each instance the presence of JEI was suspected by its effect on the behavior of the subsequent impulse. The assumption that the unexpected and "unphysiological" behavior of A-V conduction was due to concealed JEI proposed by Langendorf and Mehlman' was confirmed in man by direct recording from the His bundle by Rosen, Rahimtoola and Gunnar,5 in the dog by Damato, Lau and Bobb" and in the isolated rabbit A-V conduction system by Moore, Knoebel and Spear.12 The purpose of this communication is to present 13 episodes of concealed JEI recorded in ten patients. Manifestations, phenomena, not previously described include: 1) initiation of a marked, greater than 400 msec, than 400 msec, and persistent prolongation of the P-R interval, 2) striking changes in the duration of the P-R with an occasional sequence of R-P and P-R intervals simulating "supernormal" A-V conduction, 3) unexpected variation of the junctional escape interval explained by junctional parasystole with entrance block, 4) postponed conpensatory pause, 5) concealed junctional discharge with reciprocation.prolongation of the P-R interval; 2) persistence of this prolongation; 3) wide variation of P-R duration in the same record with occasional R-P, P-R relationship suggesting " supernormality" of A-V conduction; 4) variation of junctional escape interval; 5) postponed compensatory pause; and 6) reciprocation due to concealed junctional discharge, as opposed to the well recognized phenomenon of concealed reciprocation.

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