Interrelation of tissue temperature versus flow velocity in two different kinds of temperature controlled catheter radiofrequency energy applications.

Abstract

The influence of blood flow cooling down the energy delivering electrode during temperature controlled radiofrequency energy application is an important factor for ablation success. In this experimental in-vitro study, using tempered saline as blood equivalent, we observed a highly significant increase in tissue temperature, lesion depth and required energy amount with increasing flow velocity. Second, we found significant deeper lesions with use of pulsed radiofrequency energy application compared to continuous application. We conclude that, even with lower electrode temperatures, success can be achieved dependent on the local blood flow velocity, and deeper lesions can be created with the use of pulsed radiofrequency energy application. Success in temperature-controlled radiofrequency (RF) catheter ablation of arrhythmogenic areas in human hearts depend largely (among others) on the size of the electrode, developed pressure of electrode against tissue, as well as on the localization of the thermistor sensor within the electrode. In addition, the blood flow velocity at various sites of ablation is an important factor for the calculation of heat transport from the electrode, which obviously has not been given much consideration of in the past. The aim of the present in-vitro study, therefore, was to evaluate this important factor's influence on the temperature developed at the electrode and within the myocardial tissue. All experiments were carried out in a bath containing NaCl solution at 37 degrees C. Four different flow velocities were applied (0, 110, 180, 320 ml/cm2 *min). During and after temperature-controlled unipolar radiofrequency energy delivery (60 degrees C, 40 sec) the electrode temperature, the tissue temperature 5 mm in depth, and the total energy delivered were measured, as well as the actual depth of the lesion. The amount of energy applied to the electrode was regulated by the thermosensor in the electrode to obtain a maximum temperature of 60 degrees C. Two different kinds of radiofrequency energy delivery have been used: (1) continuous radiofrequency energy delivery as usual regarding clinical use, (2) pulsed radiofrequency energy delivery with a duty cycle length of 10 ms and a pause of at least the same duration during two consecutive duty cycles. At pulsed radiofrequency energy application, the energy for each duty cycle was held constant during delivery. The amount of pulses delivered to the electrode was regulated by the electrode's thermosensor. With both modes of radiofrequency energy delivery a uniform observation could be made. The more the flow velocity applied accelerated, the more the tissue temperature rose (R = 0.85; p < 0.00000001), and the lesion depth increased in spite of electrode temperature being held constant. The amount of the total energy delivered rose in proportion to the cooling down of the electrode dependent on the flow velocity (R = 0.69, p < 0.0000004). Steady-state temperatures had not been accomplished after 40 sec time. When energy was delivered at the pulsed mode, intramyocardial temperatures proved higher compared to the continuous mode with significant differences (p < 0.05) at comparable flow velocities applied between 180 and 320 ml/cm2*min and at same electrode temperatures. This resulted in significantly (p < 0.05) larger lesion depths in pulsed radiofrequency energy delivery. We suppose that this significant difference can be explained by a higher amount of total energy delivered at comparable electrode temperature in the pulsed mode as compared to the continuous mode.