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.