An application of photodynamic therapy for myocardial ablation, which would induce myocardial electrical conduction block, is proposed. For the proposed application, an extracellular photosensitization reaction (PR) is performed while photosensitizer is distributed in myocardial interstitial space by employing a short drug-light interval. Because the myocardial necrosis depth must be accurately controlled to prevent surrounding tissue injury during the myocardial ablation procedure, the necrosis depth during PR needs to be predicted. The purpose of this study is to investigate the availability of predicting PR-induced myocardial necrosis depth (dnec) using a defined fluorescence-fall amount (FA), which is the calculated result of fluorescence intensity decrease from the start of the PR multiplied by irradiation duration and corresponds to photosensitizer consumption amount under an assumption that the photosensitizer consumption rate is faster than the photosensitizer supply rate. The correlation between FA and dnec was experimentally investigated in vivo using an open-chested canine heart model with 2.5 and 5.0 mg/kg of talaporfin sodium at an irradiance of 5-20 W/cm 2 for 5-20 s. The fluorescence measurement was performed at a wavelength of 710 nm during the PR to derive FA. One week after the PR, a uniform necrosis depth was measured histopathologically as dmnec. A logarithmic correlation between dmnec and FA was confirmed with R2=0.69-0.80 and a dmnec range of 0.2-7.1 mm. The defined FA might be useful for predicting dnec for the extracellular PR in myocardium when using talaporfin sodium.
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