Background Radiofrequency (RF) catheter ablation lesion size has been limited by the small volume of tissue directly heated by the RF electrode.Microwave (MW) energy has been proposed as an alternative energy source to generate larger lesions because of its increased volume of direct tissue heating. To further characterize MW ablation of myocardium, we studied the temperature-versus-distance profiles during MW ablation in an in vitro model of perfused and superfused porcine right ventricular free wall.Methods and Results Radial tissue temperatures in 19 isolated porcine right ventricles were measured and recorded with four fluoroptic thermometry probes placed within the myocardium at 2.5-mm radial increments from the catheter. The MW antenna catheters used were monopolar and helical-coil antennas resonating at 915 and 2450 MHz. Durations of energy delivery for a 915-MHz MW monopolar antenna (60 to 600 seconds) and a 4-mm-tip RF electrode (60 and 300 seconds) were varied to compare time courses of lesion formation. For each lesion, the temperature at the lesion border zone (the isotherm of irreversible tissue injury) was determined. Similar lesion size and temperature profiles were observed for 915-versus 2450-MHz MW antennas and monopolar versus helical-coil MW antennas. Lesion depth for the 915-MHz monopolar antenna increased monoexponentially with a half-time of 170 seconds. The isotherms for all MW antenna designs were not significantly different. The mean isotherm of irreversible tissue injury for MW lesions was not significantly different from the mean isotherm for RF lesions (54.4 degrees C versus 53.6 degrees C, respectively).Conclusions Microwave ablation has the potential to directly heat a greater volume of tissue than RF ablation but only with efficient MW antennas.The primary mechanism of tissue injury for both MW and RF ablation appears to be thermal. (Circulation. 1994;89:2390-2395.)