Popping phenomena in temperature-controlled radiofrequency ablation: when and why do they occur?

During temperature-controlled radiofrequency (RF) ablation a popping sound sometimes occurs. This popping phenomenon is known to be associated with unwanted effects like blood boiling, endocardial rupture, catheter dislocation, and impedance rise. The present in vitro study determined the influence of cooling, electrode contact, and tip temperature on the occurrence of popping phenomena. Pieces of porcine ventricle were immersed in a bath of saline solution at 37 degrees C. Forty-two RF ablations were performed with different electrode-tissue contact forces (i.e., 0.0-0.44 N) in a temperature-controlled mode (70 degrees C setpoint, 30 s, 50 W maximum power output, 4-mm tip, thermocouple). Half of the 42 ablations were performed with fluid flow (0.1 m/s, group I), the other half without flow (group II). In group I, mean tip temperature and power were 55.6 +/- 8.5 degrees C and 36.2 +/- 13.8 W, resulting in a lesion volume of 121 +/- 57 mm3. In group II, the respective values were 67.3 +/- 1.5 degrees C and 9.9 +/- 5.2 W resulting in a volume of 42 +/- 18 mm3. The differences between groups were statistically significant. Overall, ten popping phenomena occurred in group I and none in group II. Pops occurred significantly more often when the contact force was < 0.1 N (8/10) and the tip temperature was < 60 degrees C (8/10). Two endocardial ruptures occurred, both were associated with a popping phenomenon. Using temperature control, the probability of pops is significantly higher when the ablation electrode and the endocardial tissue surface are exposed to fluid flow and the electrode-tissue contact is poor. Under these conditions the tissue temperature can be much higher than the temperature measured at the tip electrode and can potentially reach 100 degrees C causing intramyocardial steam formation and a popping phenomenon.