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Predicting Steam Pops with Microwave Radiometry Background The disparity between catheter and tissue temperatures during irrigated RF ablation frustrates one's ability to predict steam pops. Microwave radiometry allows for “volumetric” temperature assessment—i.e., within a circumscribed volume around the catheter tip-permitting, direct assessment of temperature during ablation. The aim of this study was to examine (i) the ability of microwave radiometry to predict steam pops, and (ii) compare this to traditional parameters such as power, catheter temperature, and impedance. Methods and Results Irrigated RF ablation was performed in 8 sheep using the Tempasure ablation catheter in all chambers. Power, impedance, catheter tip, and volumetric temperature were continually monitored. Ablation was terminated after a pop or at 60 seconds. A pop was defined as an audible or visualized pop (intracardiac echocardiography). Predictors of pops were determined by univariate and multivariate GEE logistic regression modeling. A total of 48 pops occurred during 143 lesions applied at 20–50 W. There was no association between the chamber of the heart and the occurrence of pops. The rate of rise of volumetric temperature (greater than 1.5 °C/s) was the single best predictor of pops (OR: 88.8 [95% CI: 12–604], P < 0.0007). Pops only occurred above a maximum volumetric temperature threshold of 89 °C. Conclusions During irrigated RF ablation, steam pop occurrence can be predicted by both, the rate of rise and the maximum volumetric temperature measured by microwave radiometry.