Effects of heating on impulse propagation in superfused canine myocardium

Objectives.The goal of the study was to quantify the response of myocardial impulse propagation to hyperthermia and identify the temperatures required for transient and permanent block in conduction.Background.Although it is generally accepted that the effects of radiofrequency ablation are thermally mediated, the precise response of myocardial impulse conduction to heating remains to be quantified.Methods.Twenty-three preparations of ventricular myocardium from 10 beagle dogs were superfused at 36.5 to 37.5°C and paced at a cycle length of 600 ms. Heating was performed for 30 s at 5-min intervals by an independent flow of heated superfusate. A 16-electrode grid was used to record extracellular electrograms directly before each heating episode (control value) and at 10, 20 and 30 s.Results.Between 38.5 and 45.4°C, conduction velocity was higher than that at the directly preceding control value (p < 0.05), reaching a maximum of 114% between 41.5 and 42.5°C. Above 45.4°C, a gradual decrease occurred, with transient block (absence of impulse conduction for ≤5 min) after heating to 49.5 to 51.5°C. This was followed by tachycardia in 69% of all cases immediately after cessation of heating. Permanent block occurred after a significantly higher temperature of 51.7 to 54.4°C had been reached. Pacing at sites allowing preferential conduction either parallel or perpendicular to fiber orientation caused no difference in reaction to heating. Repeated heating of some preparations to 47.0 to 50.5°C revealed no cumulative effects on conduction velocity.Conclusions.Transient and permanent block in impulse conduction occurred at 49.5 to 51.5°C and 51.7 to 54.4°C, respectively, in superfused canine myocardium, the former frequently being followed directly by tachycardia. Reaction of conduction velocity to hyperthermia was independent of myocardial fiber orientation and number of preceding heating episodes. Results may contribute to a better understanding of electrophysiologic phenomena observed during radiofrequency ablation procedures