Low power density V.S. high power density photocoagulation of ventricular myocardium for cardiac arrhythmias. Observations supporting non-linear tissue optics

Nd:YAG laser photocoagulation of canine myocardium using low power density (LPD) laser irradiation was compared with high power density (HPD) irradiation. LFD irradiation (102 W/cm²) produced lesions of mean maximum depth at 320 joules of 4.8 mm with width exceeding depth. HPD irradiation (16,000-24,000 W/cm²) achieved depth of 9.5 to 11.5 mm at 300 joules or less. LPD lesions are consistent with light dosimetry calculations from the known optical parameters of canine myocardium. However, these calculated dosimetries cannot account for lesions produced by HPD irradiation. This finding suggests the possibility of non-linear optical properties in biologic tissue.