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Notes: RF Ablation and Creatine Kinase. Introduction: The primary mechanism of myocardial injury during radiofrequency (RF) catheter ablation in the heart is presumed to be thermal. Creatine Kinase has been measured in serum to assess the volume of myocardial injury after ablation. However, its thermal inactivation by RF ablation could lead to underestimation of the true volume of injury. Methods and Results: Serial RF lesions were created in 10 canine left ventricles in vivo, and serial serum Creatine Kinase activities were measured and compared to lesion volume. To assess the stability of myocardial Creatine Kinase during RF catheter ablation, 29 RF ablations were made on the epicardial surface of porcine left ventricle in vivo and a 2-mm core biopsy was rapidly removed. The cores were rapidly frozen, sectioned longitudinally in 1-mm slices, and homogenized in 0.3 M Tris buffer solution containing EDTA and dithiothreitol for subsequent analysis of Creatine Kinase activity. An additional 19 tissue cores from RF lesions were stained and used to determine mean lesion depth. Normal tissue biopsies were exposed to 60 seconds of hyperthermia (37° to 85°C, n = 190), or high-density RF current at 50X (0 to 100 mA/mm2, n = 50), and tissue Creatine Kinase activity was measured. There was no evidence of Creatine Kinase washout within the first 2 hours, and peak values were measured 5 to 7 hours postablation. Tissue Creatine Kinase activity in the first mm depth of RF lesions averaged 10% of control values and increased over the first 5 mm of lesion depth. The mean Creatine Kinase activity within the hemisphere of ablated myocardium was calculated to be 31% of control. Creatine Kinase activity declined significantly at temperatures above 65°C, but no difference in tissue Creatine Kinase activity was observed among differing levels of RF current exposure in the absence of significant heating. Conclusions: Creatine Kinase activity in myocardial tissue is significantly diminished within the RF lesion. Creatine Kinase activity is not stable at temperatures above 65°C, which are routinely achieved within the central zone of RF ablation, and is unaffected by RF current in the absence of hyperthermia. Measurements of serum Creatine Kinase activity after RF catheter ablation may significantly underestimate the volume of myocardial injury.