Cooperative phenomena in two-pulse, two-color laser photocoagulation of cutaneous blood vessels

A novel laser system has been developed to study the effects of multiple la ser pulses of differing wavelengths on cutaneous blood vessels in vivo, usi ng the hamster dorsal skin flap preparation and in vitro, using cuvettes of whole or diluted blood. The system permits sequenced irradiation with well -defined intrapulse spacing at 532 nm, using a long-pulse frequency-doubled Nd:YAG laser, and at 1064 nm, using a long-pulse Nd:YAG laser. Using this system, we have identified a parameter space where two pulses of different wavelengths act in a synergistic manner to effect permanent vessel damage a t radiant exposures where the two pulses individually have little or no eff ect. Using a two-color pump-probe technique in vitro, we have identified a phenomenon we call greenlight-induced infrared absorption, where a pulse of green light causes photochemical and photothermal modifications to the che mical constituents of blood and results in enhanced infrared absorption. We identify a new chemical species, met-hemoglobin, not normally present in h ealthy human blood but formed during laser photocoagulation which we believ e is implicated in the enhanced near-infrared absorption.