Aims Photodynamic therapy (PDT) is based on non-thermal injury mediated by singlet oxygen species and is used clinically in cancer therapy. In our continuing efforts to apply this technology to cardiac catheter ablation, we clarified the optimal condition for creating PDT-mediated lesions using a laser catheter. Methods and results In a total of 35 canines, we applied a laser directly to the epicardium of the beating heart during open-chest surgery at 15 min after administration of a photosensitizer, talaporfin sodium. We evaluated the lesion size (depth and width) using hematoxylin-eosin staining under varying conditions as follows: laser output (5, 10, 20 W/cm<sup>2</sup>), irradiation time (0-60 s), photosensitizer concentration (0, 2.5, 5 mg/kg), blood oxygen concentration (103.5 ± 2.1 vs. 548.0 ± 18.4 torr), and contact force applied during irradiations (low: <20 g, high: >20 g). A laser irradiation at 20 W/cm<sup>2</sup> for 60 s under 5 mg/kg (29 μg/mL) of photosensitizer induced a lesion 8.7 ± 0.8 mm deep and 5.2 ± 0.2 mm wide. The lesion size was thus positively correlated to the laser power, irradiation time, and photosensitizer concentration, and was independent of the applied contact force and oxygen concentration. In addition, the concentration of the photosensitizer strongly correlated with the changes in the pulse oximetry data and fluorescence of the backscattering laser, suggesting that a clinically appropriate condition could be estimated in real time. Conclusion Photodynamic therapy-mediated cardiac lesions might be controllable by regulating the photosensitizer concentration, laser output, and irradiation time.
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