CONDUCTIVE PROPERTIES OF THE REENTRANT PATHWAY OF VENTRICULAR-TACHYCARDIA DURING ENTRAINMENT FROM OUTSIDE AND WITHIN THE ZONE OF SLOW CONDUCTION

Ventricular tachycardia (VT) was entrained with rapid ventricular paci ng outside and within the zone of slow conduction (SCZ), and the condu ctive properties of the reentrant pathway were compared between the tw o pacing sites. Underlying heart diseases were old myocardial infarcti on (n = 2), postoperative tetralogy of Fallot (n = 1) or double outlet of the right ventricle (n = 1), dilated cardiomyopathy (n = 1), and p ulmonary regurgitation of unknown cause (n = 1). Rapid pacing was cont inued for 5-10 seconds, and the time interval from paced stimulus to t he entrained electrogram at the exit from SCZ (St-Ex) or to the QRS co mplex (St-QRS) was measured. Rapid pacing was performed at three or mo re cycle lengths after a decrement in steps of 10 msec. During rapid p acing outside of SCZ and entrainment of VT, constant fusion and progre ssive fusion were observed, and St-Ex and St-QRS showed the same respo nse pattern: either a frequency dependent prolongation in 4 of 7 VTs o r a constant time interval in the other 3 VTs. When rapid pacing was a ttempted within SCZ, the response of the time intervals from paced sit e to the QRS (St-QRS) was the same as those observed during pacing out side SCZ except for one VT. In VT with repaired tetralogy of Fallot, t he frequency dependent prolongation was observed during pacing from ou tside of SCZ but not within SCZ. Diseased myocardium extending widely into the outflow tract of the right ventricle may be responsible for t he frequency dependent prolongation of St-Ex. In conclusion, the condu ctive property of the reentrant pathway might be assessed by observing the response patterns of St-Ex or St-QRS interval during transient en trainment of VT outside of SCZ, but exceptions may exist.