Discrepancies between catheter tip and tissue temperature in cooled-tip ablation: relevance to guiding left atrial ablation.

BACKGROUND It is not known whether catheter tip temperatures with a cooled-tip ablation can be reliably extrapolated to estimate actual tissue temperatures. The relationship between catheter tip temperatures, tissue temperatures, power, and microbubble formation is not known. METHODS AND RESULTS Nine dogs underwent 111 radiofrequency energy deliveries at the pulmonary vein ostia with a cooled-tip catheter. Catheter tip and tissue temperatures were markedly discrepant. Catheter tip temperature plateaus at 36 degrees C to 39 degrees C with increasing power, whereas tissue temperature increases to a mean of 75+/-3 degrees C at 45 W (maximum temperature >100 degrees C). Seventy-two energy deliveries were performed, titrating power to microbubble formation guided by intracardiac echocardiography. Type I and II microbubble formation occurred in 45 (63%) and 19 (26%) ablations, respectively. Type I microbubble emergence occurred at lower powers (21+/-8 versus 26+/-4 W; P=0.05), catheter tip temperatures (38+/-5 degrees C versus 48+/-10 degrees C; P=0.02), and tissue temperatures (65+/-19 degrees C versus 81+/-9 degrees C; P<0.001) than type II microbubble formation. Maximum impedance decreases during ablation before microbubble formation were less with type I microbubble (20+/-9 versus 37+/-11 Omega; P<0.001) compared with type II microbubbles. One quarter of type I microbubbles abruptly transitioned to type II microbubbles with significant changes in power or catheter tip temperature. No microbubbles were seen in 19 ablations (26%) despite powers up to 26+/-9 W and tissue temperatures up to 81+/-17 degrees C. CONCLUSIONS Catheter tip and tissue temperatures are markedly discrepant during cooled-tip ablation. Type I and II microbubble formation occurs at overlapping power and catheter tip and tissue temperature ranges. Neither the absence of microbubbles nor the presence of type I microbubble formation ensures against excessive tissue heating. The appearance of microbubbles may indicate possible tissue overheating and signal a need to decrease energy.