ELECTROPHYSIOLOGICAL MECHANISMS OF SPONTANEOUS TERMINATION OF SUSTAINED MONOMORPHIC REENTRANT VENTRICULAR-TACHYCARDIA IN THE CANINE POSTINFARCTION HEART

Background The electrophysiological mechanisms of spontaneous terminat ion of sustained monomorphic ventricular tachycardia (SMVT), in the po stinfarction heart, generally considered secondary to a reentrant mech anism, have not been fully investigated. Method and Results Epicardial activation maps of spontaneous termination of 20 different episodes o f SMVT (lasting 30 seconds to 10 minutes) from 8 dogs, 4 to 5 days aft er one-stage ligation of the left anterior descending coronary artery, were analyzed with the use of 254 bipolar electrode recordings with h igh density (2.5 to 2.8 mm between bipolar electrodes) in the ischemic zone. All ventricular tachycardias (VTs) were due to circus movement reentry with a characteristic figure-8 configuration. Termination alwa ys occurred when the two circulating wave fronts blocked in the centra l common pathway (CCP). Two basic mechanisms of spontaneous terminatio n were observed. (1) In 15 episodes, acceleration of conduction occurr ed in parts of the reentrant circuit and was associated with slowing o f conduction and finally conduction block in the CCP. Acceleration of conduction occurred in the last few cycles of VT both at the outer bor der of the arcs of functional conduction block in the ''normal'' myoca rdial zone and at the pivot points to the entrance to the CCP. When ac celeration of conduction was compensated on a beat-to-beat basis by an equal degree of slowing in the CCP, there was no discernible change i n the cycle length of the VT in the EGG. In some episodes, the termina tion of the original reentrant circuit was followed by the development of a different, slower reentrant pathway that lasted for one or a few cycles prior to termination. (2) In 5 VT episodes, the activation wav e front in the CCP abruptly broke across a stable are of functional co nduction block, resulting in premature activation of the CCP and condu ction block. Conclusions Distinct electrophysiological changes always preceded spontaneous termination of stable SMVT. The electrophysiologi cal basis for acceleration of conduction in parts of the reentrant cir cuit during the last few beats prior to termination and of the abrupt reactivation across a stable are of block remains to be determined.