Influence of wavelength on response to laser photothermolysis of blood vessels: implications for port wine stain laser therapy.
OBJECTIVE
Treatment of port wine stains (PWS) by photothermolysis can be improved by optimizing laser parameters. We have studied the all-important role of wavelength (lambda) by performing pulsed laser photothermolysis in the vasculature of the chick chorioallantoic membrane (CAM) assay.


METHODS
The CAM contains an extensive microvascular network ranging from capillaries with diameter D < 20 microm to blood vessels of D approximately 120 microm. The CAM assay enables observation and real-time video documentation of blood flow in pre-capillary arterioles (A) and post-capillary venules (V). The ScleroPlus trade mark laser (Candela Corp., Wayland, MA) has a smooth output over its fixed 1.5 ms pulse duration and allows the operator to vary several treatment parameters such as wavelength, spot size, and energy. Blood vessels in the CAM were irradiated at two clinically relevant wavelengths, lambda = 587 or 597 nm, constant spot size (7 mm), and at different exposures (E = 5-12 J/cm(2)). Threshold exposure (E(th)) (at which non-reversible damage occurred) were 5 J/cm(2) at 587 nm and 8 J/cm(2) at 597 nm. Mathematical modeling was developed to interpret initial (within 30 seconds) injury of arterioles and venules at both wavelengths as a function of D when varying E.


RESULTS
Vessel injury was graded off-line from videotapes. For all combinations of lambda and E, measurable injury was evident within 30 seconds of irradiation and maximal damage was reached in less than 10 minutes. Vessel damage was found to depend strongly on lambda. Damage decreased with increasing vessel size. For all D, damage of arterioles was greater than for corresponding venules.


CONCLUSIONS
Selection of the correct wavelength is crucial for successful laser therapy of hypervascular lesions and, ideally, should be based on knowledge of vessel diameters for a specific PWS lesion and of optical penetration depths in blood. As a general statement, smaller blood vessels (D = 10-60 microm) should be treated using shorter wavelengths such as 585 nm. To ensure deposition of sufficient laser energy, irradiation at 585 nm, characterized by lower optical penetration depth in blood, may be preferable for PWS therapy.
Kimel
Sol
S
Beckman Laser Institute, University of California, Irvine, CA 92612, USA.
Svaasand
Lars O
LO
Hammer-Wilson
Marie J
MJ
Nelson
J Stuart
JS
D000818:Animals; D001808:Blood Vessels / Q000528:radiation effects*; D002642:Chick Embryo; D006801:Humans; D053685:Laser Therapy / Q000379:methods*; D023421:Models, Animal; D008955:Models, Cardiovascular; D010782:Photolysis / Q000528:radiation effects; D010789:Phototherapy / Q000379:methods; D019339:Port-Wine Stain / Q000628:therapy; D011827:Radiation; D014656:Vascular Surgical Procedures / Q000379:methods*