THERMAL RELAXATION OF PORT-WINE STAIN VESSELS PROBED IN-VIVO - THE NEED FOR 1-10 MILLISECOND LASER-PULSE TREATMENT

Although thermal relaxation times of cutaneous port-wine stain microve ssels have been calculated and used to formulate laser selective photo thermolysis, they have never been measured, A scheme to do so was devi sed by measuring the skin response to pairs of 585-nm dye laser pulses (250-360 microseconds each) as a function of the time interval betwee n the two pulses, in five volunteers with port-wine stains, After a pu mp pulse delivering 80% of the fluence necessary for causing purpura, the fluence of a second probe pulse necessary to cause purpura was det ermined and was found to increase with the interval between the two pu lses, in a manner consistent with thermal diffusion theory, Biopsy spe cimens were obtained from four of the five subjects to examine the nat ure and extent of vessel damage and to measure the port-wine stain ves sel diameters, Using diffusion theory, the thermal relaxation time was calculated based on the measured vessel diameters. These calculated v alues are consistent with the increase in radiant exposure (fluence) o f the probe pulse necessary to induce purpura for longer time delays, Two simple models for thermal relaxation of port-wine stain vessels ar e presented and compared with the data. The data and histologic assess ment of the vessel injury strongly suggest that pulse durations for id eal laser treatment are in the 1-10-millisecond region and depend on v essel diameter, No dermatologic lasers presently used for. port-wine s tain treatment operate in this pulse width domain.