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We investigated experimentally dependence of light fluence on treated depth with photosensitization reaction shortly
after photosensitizer injection in rabbit myocardial tissue in vivo. In this particular photosensitization reaction scheme,
the photosensitizer accumulation characteristics for target region are not available. Meanwhile, the photosensitizer dose
and hospitalization period under restricted light circumstance might be reduced. Since both photosensitizer and oxygen supply are governed by blood flow, this photosensitization reaction is influenced significantly by blood flow variation in
particular blood vessel occlusion. We employed the myocardial tissue to keep tissue blood flow during the
photosensitization reaction because vessel blood flow speed in myocardial tissue is fast to resist vascular occlusion.
Surgically exposed rabbits myocardial tissues were irradiated with the light fluence ranging 25-100 J/cm2 by a 663 nm
diode laser 30 min after the injection of 2 mg/kg water soluble chlorin photosensitizer, Talaporfin sodium. Two weeks
after the irradiation, the rabbits were sacrificed and the histological specimens of the irradiated area were made to
measure scar layer thickness. The scar layer tissue thickness of 0.2-3.0 mm was observed microscopically by the light
fluence ranging 25-100 J/cm2. The scarring threshold in the deposit light fluence was estimated to 15-25 J/cm3 based on the above mentioned relation assuming constant and uniform myocardial effective attenuation coefficient of 0.72 mm-1.
The estimated scarring threshold in the deposit light fluence was lower than the threshold of conventional PDT. Large variation of the estimated threshold value might be attributed to unconsidered PDT parameter such as flow rate inhomogeneity in the myocardial tissue. These results suggested that the photosensitization reaction investigated in this
study would be available to apply arrhythmia therapy such as atrial fibrillation.
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