Temperature Measurement as a determinant of Tissue Heating During Radiofrequency Catheter ablation: An Examination of Electrode Thermistor Positioning for Measurement Accuracy

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Abstract
Temperature monitoring has been proposed as a control for lesion occurrence and dimension during radiofrequency transcatheter ablation. Effective temperature measurement depends on thermistor positioning relative to the heated cardiac tissue and the convective cooling effects of the circulation. But the accuracy of a single tip thermistor as a measure of peak electrode-tissue interface temperature is unknown. A standard 8-French, 4-mm electrode catheter with 5 thermistors (1 tip thermistor, 4 radial thermistors) was used to deliver radiofrequency energy in vitro to 3 porcine right ventricles and in vivo to 7 mongrel dogs. In vitro, the catheter orientation was varied. In vivo the catheter was positioned under fluoroscopy at a variety of atrial, tricuspid annular, and ventricular sites, with no attempt to adjust catheter orientation. In both cases varied discrete power levels were used so that a wide temperature range was attained. Lesions created in vivo with a standard, single thermistor tipped electrode were compared to those of a catheter with a thermistor extending 1 mm from the tip. Power was varied and tip thermistor temperatures recorded. All lesions were examined pathologically. Comparisons of radial thermistor temperature to tip thermistor temperature for 3 catheter orientations in vitro resulted in tip thermistor underestimation of peak electrode-tissue interface temperature by a median of 0.5 degrees C in 35% of the perpendicular orientations, 1.9 degrees C in 82% of the 45 orientations, and 5 degrees C in 83% of the parallel orientations. During in vivo trials, the tip thermistor underestimated the peak electrode-tissue interface temperature during 2 of 51 lesions by 1.2 degrees C and 7.6 degrees C. There was a sudden rise in electrical impedance in 17 of 51 radiofrequency energy deliveries. Only one case was observed where the peak electrode-tissue interface temperature was below 95 degrees C. The normal to extended tip thermistor configurations analysis showed similar relationships between lesion size and temperature. Accuracy of a single tip thermistor was found to be dependent upon catheter-tissue orientation. With routine catheter positioning in vivo, the tip thermistor was a good indicator of peak electrode-tissue interface temperature. Thus with power regulation to avoid temperatures greater than 90 degrees C, a single flush-mounted tip thermistor is probably adequate for temperature monitoring of lesion formation and avoidance of impedance rises.