Irrigation of radiofrequency current (RF) ablation reduces the risk of thrombus formation. The aim of this study was to investigate the impact of different irrigation catheter flow rates and contact pressures from the catheter on the development of lesion dimension and thrombus formation. A thigh muscle preparation was achieved in six sheep to create a cradle that was filled and perfused with heparinized blood (250 mL/min, 37 C degrees). RF ablation (30 s, 30 W) was initially performed with three different irrigation flow rates (5 mL/min, 10 mL/min, and 20 mL/min) and a perpendicular position (0.1 N contact pressure) of the irrigated ablation catheter ("Sprinklr," Medtronic, Inc., Minneapolis, MN, USA). The next lesions were induced with constant contact pressure of 0.05 Newton (N); 0.1 N; 0.3 and 0.5 N and a parallel or perpendicular orientation of the catheter, respectively. A constant irrigation flow of 10 mL/min was maintained during these RF applications. Cross sections of the lesions were investigated with regard to maximal depth and maximal diameter at and below the surface. During high flow irrigation (20 mL/min) the surface diameter was significantly smaller (0.63 +/- 0.1 cm) compared to irrigation flowrates of 5 mL/min (0.88 +/- 0.2 cm) and 10 mL/min (1 +/- 0.1 cm). Thrombus formation was not observed during any RF application. Only in perpendicular catheter orientations with a contact pressure of 0.5 N were significantly deeper lesions (0.85 +/- 0.12 cm) induced compared to 0.05 N (0.55 +/- 0.02 cm), 0.1 N (0.7 +/- 0.01 cm) and 0.3 N (0.67 +/- 0.01 cm) contact pressure. There was no significant difference in lesion depth with different flow rates. Irrigated RF ablation even with low flow rates and high catheter contact pressure prevented thrombus formation at the electrode. Smaller lesion diameters have been created with high irrigation flow rates. The deeper lesion created with high catheter contact pressure might be caused by a greater power transmission to the tissue.

During RF catheter ablation, local temperature elevation can result in coagulum formation on the ablation electrode, resulting in impedance rise. A recent study has also demonstrated the formation of a so-called soft thrombus during experimental ablations. This deposit poorly adhered to the catheter tip and did not cause an impedance rise. The mechanism of soft thrombus formation and the role of the natural coagulation system are unknown. The formation of a soft thrombus was investigated experimentally by temperature-controlled RF delivery in heparinized blood at different heparin concentrations and in serum. After 60 seconds of RF delivery in blood with an electrode target temperature of 80 degrees C, a semisolidified mass had formed around the ablation electrode at all heparin concentrations. A smaller but structurally similar deposit had formed after RF delivery in serum. Scanning electron microscopy analysis revealed that these deposits consist of denaturized and aggregated proteins, and not of a classical thrombus. The formation of the so-called soft thrombus resultsfrom heat induced protein denaturation and aggregation and occurs independent of heparin concentration and also in serum. The formation of such deposits may occur at temperatures well below 100 degrees C, which may have important consequences for further development of ablation technologies.

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.

During radiofrequency energy delivery, the catheter tip temperature can be significantly lower than the tissue temperature. The authors performed tissue temperature-controlled radiofrequency ablation in vitro and evaluated the effects of cooling, electrode to tissue contact, and target tissue temperature on lesion size. Pieces of porcine ventricle were immersed in a bath of isotonic saline solution at 37 degrees C. Radiofrequency energy was controlled by the tissue temperature as measured with a thermocouple needle placed 2 mm beneath the ablation electrode. Radiofrequency power was delivered for 30 seconds and limited to 50 W. A total of 81 radiofrequency ablations was performed with different electrode to tissue contact forces (0.04 N, 0.36 N, and 0.67 N) and target tissue temperatures (50 degrees C, 60 degrees C, and 70 degrees C) using an irrigated (27 ablations, 20 mL/min irrigation flow rate) or a nonirrigated ablation catheter. Twenty-seven nonirrigated applications were performed with fluid flow maintained by the pump of the thermostat and another 27 applications without flow. Every combination was applied three times and the average values were used for evaluation. For tissue target temperatures of 50 degrees C, 60 degrees C, and 70 degrees C, the lesion volume for nonirrigated ablations was on average 21 +/- 8 mm3, 45 +/- 23 mm3, and 109 +/- 45 mm3, respectively, and for irrigated ablations 12 +/- 7 mm3, 37 +/- 20 mm3, and 92 +/- 30 mm3, respectively. In both application groups the lesion size did not correlate with the electrode to tissue contact force. In the nonirrigated ablation group there was no difference in lesion size between the group with fluid flow and those without. Lesion size during tissue temperature-controlled radiofrequency delivery increases with increasing target tissue temperature and becomes independent of flow and electrode to tissue contact.

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