Hyperthermia causes significant changes in myocardial cellular electrophysiological properties that include membrane depolarization, reversible and irreversible loss of excitability, and abnormal automaticity. There appear to be specific temperature ranges for reversible and irreversible electrophysiological changes. These observations may have important implications for tissue temperature monitoring during radiofrequency catheter ablation.

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. We used validated three-dimensional finite element models to perform these analyses. We found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density decreased more slowly with distance from the electrode surface. We analyzed the effects of three tissue-electrode angles: 0, 45, and 90 degrees. More power was needed to reach a maximal tissue temperature of 95 degrees C after 120 s when the electrode-tissue angle was 45 degrees. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45 degrees than for 0 and 90 degrees. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.

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.

RF catheter ablation not only results in a marked reduction in blood flow within the acute pathological lesion but also causes reduced flow beyond the borders of the acute lesion because of microvascular endothelial cell injury. The progression or resolution of tissue injury within the region beyond the border of the pathological lesion may explain the late electrophysiological effects of RF ablation.

RF catheter ablation results in ultrastructural damage to the myocardium extending up to 6 mm beyond the acute pathologic RF lesion border as defined by NBT histochemical staining.