Effect of electrode orientation on lesion sizes produced by irrigated radiofrequency ablation catheters.

BACKGROUND: Irrigated radiofrequency (RF) ablation catheters may produce different lesion sizes dependent upon the electrode orientation to the tissue. This study examined the effect of irrigated electrode orientation on the lesion size and explores a potential mechanism for this effect. METHODS AND RESULTS: Lesions were created in isolated porcine myocardium using an open irrigation, closed irrigation, and nonirrigated RF catheter (all 3.5-4 mm tips). Lesions were created with the electrodes with all permutations of electrode orientation (vertical or horizontal), contact pressure (6 or 20 g), and saline superfusate flow (0.2 or 0.4 m/sec) over tissue interface. The effect of electrode irrigation without RF delivery on tissue temperature was assessed with intramyocardial temperature probes and infrared thermal imaging. For both irrigated catheters, the horizontal orientation produced 25-30% smaller lesion volumes than the vertical orientation despite equal or greater power deliveries. The horizontal orientation produced larger lesion volumes for the nonirrigated catheter. Higher superfusate flow rates were associated with decreased lesion volumes for the irrigated catheters but greater lesion volumes for the nonirrigated catheter. Catheter irrigation alone without RF delivery reduced intramyocardial temperatures up to 4.9 degrees C and the horizontal orientation produced a 2-fold greater area of tissue cooling than the vertical orientation. CONCLUSION: Horizontal electrode orientations reduce lesion volumes for irrigated RF catheters. This effect may be in part due to greater areas of active tissue cooling in the horizontal orientation.

Time to electrode rewarming after cryoablation predicts lesion size.

INTRODUCTION: There are no methods in clinical use to assess tissue cooling during catheter cryoablation. Cryoablation electrode temperature may be a poor predictor of lesion size. The purpose of this study was to determine whether the time necessary for the cryoablation electrode to cool to target temperature or to rewarm after cryoablation can predict lesion size. METHODS AND RESULTS: Cryoablation was performed on live porcine left ventricle in a saline bath (37 degrees C) using 8-mm-tip catheter. Cryoablation was given for 300 seconds under all permutations of the following conditions: electrode orientation vertical or horizontal, contact pressure 6 or 20 g, superfusate flow over electrode-tissue interface at 0.2 or 0.4 m/s (N = 10 each condition set, total 80 experiments). The time intervals necessary to cool the electrode to the target temperature of -75 degrees C and to rewarm to + 30 degrees C after termination of cryoablation were recorded. Lesion volume was predicted best by the time necessary to rewarm the electrode to +30 degrees C (r2 = 0.65, P < 0.0001), followed by electrode temperature (r2 = 0.28, P < 0.0001) and time to cool the electrode to -75 degrees C (r2 = 0.24, P < 0.0001). Time to +30 degrees C and time to -75 degrees C were associated with superfusate flow rate, contact pressure, and electrode orientation (r2 = 0.80 and 0.61, respectively, both P < 0.0001). Superfusate flow rate, contact pressure, and orientation were also highly predictive of lesion volume (r2 = 0.93, P < 0.0001). CONCLUSIONS: Time to cryoablation electrode rewarming is a better predictor of cryoablation lesion size than is electrode temperature. Time to cryoablation electrode rewarming reflects important determinants of cryoablation lesion formation--convective warming, contact pressure, and electrode orientation--that are not ascertainable during clinical ablation procedures.

Determinants of lesion sizes and tissue temperatures during catheter cryoablation.

BACKGROUND: Factors which influence lesion size from catheter-based cryoablation have not been well described. This study describes factors which influence lesion size during catheter cryoablation. METHODS AND RESULTS: Cryoablation was delivered to porcine left ventricular myocardium in a saline bath using 4- or 8-mm electrode catheters. Ablation was delivered with the electrodes either vertical or horizontal to the tissue and both with and without superfusate flow over the electrode. The effect of electrode contact pressure was tested. Lesion dimensions were measured. All experiments were duplicated to measure tissue temperatures at 1-, 2-, 3-, and 5-mm deep to the ablation electrode. The 8-mm electrode produced lower tissue temperatures and larger lesion volumes when compared with the 4-mm electrode (all P < 0.05). Superfusate flow slowed the rate of tissue cooling, markedly warmed tissue temperatures, and reduced lesion volume when compared with no flow conditions. By linear regression modeling, lesion sizes and tissue temperatures were related to the presence of superfusate flow, electrode orientation, contact pressure and electrode size, or catheter refrigerant flow rate (r2 for models = 0.90-0.96, all P < 0.001). Electrode temperature predicted lesion size or tissue temperatures only when analyzed independent of electrode size or refrigerant flow rate. CONCLUSIONS: Lesion sizes and tissue temperatures during catheter cryoablation are related to convective warming, electrode orientation, electrode contact pressure, and any of the following: electrode size, catheter refrigerant flow rate or electrode temperature. However, electrode temperature may be a poor predictor of lesion size and tissue temperature for a given catheter size.