New vbeam study: Vascular response to laser photothermolysis as a function of ..

[Guest guest]

New v-beam study related to PWS, but very applicable to treatment of

facial vessels in rosacea. One of the more technical papers I've

seen on this subject.

URL and abstract below.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?

cmd=Retrieve&db=PubMed&list_uids=11870797&dopt=Abstract

Lasers Surg Med 2002;30(2):160-9

Vascular response to laser photothermolysis as a function of pulse

duration, vessel type, and diameter: Implications for port wine stain

laser therapy.

Kimel S, Svaasand LO, Cao D, Hammer- MJ, JS.

Beckman Laser Institute and Medical Clinic, University of California,

Irvine, California 92612.

Background and Objective Treatment of port wine stains (PWS) by

photothermolysis can be improved by optimizing laser parameters on an

individual patient basis. We have studied the critical role of pulse

duration (t(p)) on the treatment efficacy. Study Design/Materials and

Methods The V-beam laser (Candela) allowed changing t(p) over user-

specified discrete values between 1.5 and 40 milliseconds by

delivering a series of 100 microsecond spikes. For the 1.5 and 3

millisecond pulses, three spikes were observed at intervals t(p)/2

and for t(p)[?]greater-than-or-equal[?]6 milliseconds, four spikes

separated by t(p)/3. The ScleroPlus laser (Candela) has a smooth

output over its fixed 1.5 milliseconds duration. Blood vessels in the

chick chorioallantoic membrane (CAM) were irradiated at fixed

wavelength (595 nm), spot size (7 mm), radiant exposure (15 Jcm(minus

sign2)), and at variable t(p). The CAM contains an extensive

microvascular network ranging from capillaries with diameter D[?]<[?]

30 mum to blood vessels of D[?]approximate[?]120 mum. The CAM assay

allows real-time video documentation, and observation of blood flow

in pre-capillary arterioles (A) and post-capillary venules (V).

Vessel injury was graded from recorded videotapes. Mathematical

modeling was developed to interpret results of vessel injury when

varying t(p) and D. A modified thermal relaxation time was introduced

to calculate vessel wall temperature following laser exposure.

RESULTS: Arterioles. For increasing t(p), overall damage was found to

decrease. For fixed t(p), damage decreased with vessel size. Venules.

For all D, damage was smaller than for corresponding arterioles.

There was no dependence of damage on t(p). For given t(p), no

variation of damage with D was observed. Photothermolysis due to

spiked (V-beam) vs. smooth (Scleroplus) delivery of laser energy at

fixed t(p) (1.5 milliseconds), showed similar vessel injuries for al

values of D (P>0.05). CONCLUSIONS: The difference between initial

arteriole and venule damage could be explained by the threefold

higher absorption coefficient at 595 nm in (oxygen-poor!) arterioles.

In human patients, PWS consist of ectatic venules (characterized by

higher absorption), so that these considerations favor the use of 595-

nm irradiation for laser photothermolysis. For optimal treatment of

PWS it is proposed that t(p) be between 0.1 and 1.5 milliseconds.

This is based on a modified relaxation time tau(d)prime prime or

minute, defined as the time required for heat conduction into the

full thickness of the vessel wall, which is assumed to have a

thickness DeltaD approximate[?]0.1D. The corresponding tau(d)prime

prime or minute will be a factor of about six smaller than given in

the literature. For vessels with D between 30 and 300 mum, tau(d)

prime prime or minute ranges from 0.1 to 1.5 milliseconds. Copyright

2002 Wiley-Liss, Inc.