PubMed · 8703870
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A R Fielder. 1996. Evidently, evidence based.. https://doi.org/10.1136/bjo.80.4.273
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BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used extensively for epidermal protection during laser-induced photothermolysis of port wine stains and other vascular skin lesions. The efficacy of CSC depends critically on the heat transfer coefficient (H) at the skin surface for which, however, no reliable values exist. Reported values for H, based on tissue phantoms, vary from 1,600 to 60,000 W/m(2) K. STUDY DESIGN/MATERIALS AND METHODS: A simple experimental model was designed and constructed, consisting of a pure silver-measuring disk (diameter 10 mm, thickness approximately 1 mm), embedded in a thermal insulator. The disk was covered with a 10 microm thick stratum corneum layer, detached from in vivo human skin. The heat transfer coefficient of the stratum corneum/cryogen interface was measured during CSC with short spurts of atomized tetrafluoroethane. RESULTS: H was found to be dependent on the specific design of the cryogen valve and nozzle. With nozzles used in typical clinical settings, H was 11,500 W/m(2) K, when averaged over a 100 ms spurt, and 8,000 W/m(2) K when averaged over a 200 ms spurt. CONCLUSIONS: The presented model enables accurate prediction of H and thus improve control over temperature depth profile and cooling efficiency during laser therapy. Thereby, it may contribute to improvement of therapeutic outcome.
BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery. Since optimization of CSC permits the safe use of higher light doses, which improves therapeutic outcome in many patients with superficial skin lesions, studies have focused on understanding spray-surface interactions and cooling dynamics. The objective of this study is to measure accurately temperature variations at the sprayed surface and the effects of spurt duration (deltat) and nozzle-to-sprayed surface distance (L) on cooling dynamics during CSC. STUDY DESIGN/MATERIALS AND METHODS: A fast-response temperature measurement sensor is built using thin (20 microm) aluminum foil placed on top of a poly methyl-methacrylate resin (Plexiglass) with a 50 microm bead diameter thermocouple positioned in between. Liquid film residence time (t(r)) and minimum surface temperature (T(min)) are systematically measured as a function of deltat and L. RESULTS: Two distinct spray-surface interaction mechanisms are recognized. The transition between them occurs at a critical length L(c) approximately 25-30 mm. Noticeable characteristics include: (1) for spurts at L < L(c), shorter t(r), and lower T(min) are reached as compared to L > L(c), T(min) is dependent on deltat and L, while t(r) is a function of deltat only; (2) for spurts at L > L(c), T(min) still depends on L but not on deltat, while t(r) becomes a function of both deltat and L. Finally, for all deltat, t(r) reaches a maximum at L = 40 mm. CONCLUSIONS: Based on our results, a good choice to achieve low T(min) and t(r) for the treatment of superficial skin lesions may be met by using deltat of approximately 30-50 milliseconds and the shortest spray distance that is tolerable by the patient. Spurt durations (deltat) of more than 30-50 milliseconds at spray distances (L) greater than L(c) lead to higher T(min) and longer t(r). These parameters may be appropriate for laser therapy of deeper targets.