EFFECTS OF HEATING ON IMPULSE PROPAGATION IN SUPERFUSED CANINE MYOCARDIUM

Objectives. The goal of the study was to quantify the response of myoc ardial impulse propagation to hyperthermia and identify the temperatur es required for transient and permanent block in conduction. Backgroun d. Although it is generally accepted that the effects of radiofrequenc y ablation are thermally mediated, the precise re sponse of myocardial impulse conduction to heating remains to be quantified. Methods. Twen ty-three preparations of ventricular myocardium from 10 beagle dogs we re superfused at 36.5 to 37.5 degrees C and paced at a cycle length of 600 ms. Heating was performed for 30 s at 5-min intervals by an indep endent flow of heated superfusate. A 16-electrode grid was used to rec ord extracellular electrograms directly before each heating episode (c ontrol value) and at 10, 20 and 30 s. Results. Between 38.5 and 45.4 d egrees C, conduction velocity was higher than that at the directly pre ceding control value (p < 0.05), reaching a maximum of 114% between 41 .5 and 42.5 degrees C. Above 45.4 degrees C, a gradual decrease occurr ed, with transient block (absence of impulse conduction for less than or equal to 5 min) after heating to 49.5 to 51.5 degrees C. This was f ollowed by tachycardia in 69% of all cases immediately after cessation of heating. Permanent block occurred after a significantly higher tem perature of 51.7 to 54.4 degrees C had been reached. Pacing at sites a llowing preferential conduction either parallel or perpendicular to fi ber orientation caused no difference in reaction to heating. Repeated heating of some preparations to 47.0 to 50.5 degrees C revealed no cum ulative effects on conduction velocity. Conclusions. Transient and per manent black in impulse conduction occurred at 49.5 to 51.5 degrees C and 51.7 to 54.4 degrees C, respectively, in superfused canine myocard ium, the former frequently being followed directly by tachycardia. Rea ction of conduction velocity to hyperthermia was independent of myocar dial fiber orientation and number of preceding heating episodes. Resul ts may contribute to a better understanding of electrophysiologic phen omena observed during radiofrequency ablation procedures.