Distinct Activation Patterns of Idioventricular Rhythms and Sympathetically-Induced Ventricular Tachycardias in Dogs with Atrioventricular Block RENE CARDINAL,* BENJAMIN J. SCHERLAG,** MICHEL VERMEULEN,* and J. ANDREW ARMOURS From the "Research Center. Hopital du Sacr6-Coeur de Montreal and Department of Pharmacology. Universite de Montreal, Montreal, Canada; the **Department of Medicine, University of Oklahoma, Oklahoma City. Oklahoma; and the ^Department of Physiology and Biophysics. Dalhousie University. Halifax, Canada CARDINAL, R., ET AL.; Distinct Activation Patterns of Idioventricular Rhythms and Sympathetically- Induced Ventricular Tachycardias in Dogs with Atrioventricular Block. To investigate mechanisms of ventricular impulse formation in response to sympathetic stimuiation in the healthy canine heart in situ, we compared the patterns of ventricuJar activation during the idioventricuJar rhythms arising after complete atrioventricuJar (AV) block and ventricular tachycardias induced by RSG or LSG stimulation. Isochronal maps were generated hy computer from 116-127 unipolar electrograms recorded from the entire ventricular epicardium in 15 open chest, anesthetized dogs. In eight of these, bipolar electrograms were recorded with plunge electrodes/rom 11 selected endocardial sites located below epicardial break- through areas. Intracardiac recordings from the His-Purkinje system were made with electrode catheters. After electrograms were recorded during sinus rhythm, complete AV block was induced by injecting formaldehyde into the AV node and idioventricular rhythms occurred spontaneously at a rate of 37 ± 12 beats/min (mean ± SD, n = 25}. During idioventricular rhythms, endocardial activation preceded the earliest epicardial breakthrough, which occurred in either the right anterior paraseptal region, antero- apical left ventricle, or postero-apicaJ left ventricle. These sites were consistent with a focal origin in the subendocardial His-Purkinje system. Total epicardial activation times lasted for 47 ± 13 msec (n = 40}. Idioventricular rhythms were suppressed by overdrive pacing (intermittent trains of ten beats with decremental cycle length from 500 to 200 msec) or by intravenous calcium infusion (to plasma levels of 10.1-15.2 mMJ. Right or left stellate ganglion stimulation increased idioventricular rhythm rates (to 52 ± 13 beats/min, n = 28} and also induced, in all preparations, ventricular tachycardias that had signifi- cantly faster rates (189 ± 55 beats/min,n = 27, P < 0.005J. Ventricular fibrillation was induced after brief runs of ventricular tachycardia in five of the preparations. During ventricular tachycardias, epicardial activation occurred on the right ventricular outflow tract or the postero-lateral wall of the left ventricle, and preceded endocardial activation in 50% of cases. Total epicardial activation times (103 ± 29 beats/ minj were significantly longer than during idioventricular rhythms (P < 0.005}. Ventricular tachycardias displayed overdrive excitation at critical pacing cycle lengths (360-280 msec) and were not suppressed by calcium infusion. Thus, differential mechanisms of impulse formation with distinct localizations can be elicited from healthy ventricular myocardium. (PACE, Vol. 15, September 1992) activation mapping, idioventricular rhythm, ventricular tachycardia, sympathetic nervous system, atrio- ventricular block ship from the North American Society for Pacing and Electro- This work was supported by grants from the Medical Research physiology. Council of Canada and the Quebec and Nova Scotia Heart , i i f . . . , Foundations. Rene Cardinal, Ph.D., was a Scholar of the Fends Address for reprints; Rene Cardina , Ph.D., Research Center, de la recherche en Sante du Quebec. Benjamin |. Scherlag, 5^°?'*^^ du Sacre-Coeur de Montreal, 5400 Couin Blvd. West, Ph.D.. was the recipient of a Visiting Scientist Award from the Montreal (Quebec) H4J 1C5 Canada. 1-ax: (514) 338-2694. Medical Research Council of Canada and a Travelling Fellow- Received February 20. 1992; accepted March 23. 1992. 1300 September 1992 PACE. Vol. 15