Cryogen spray cooling in laser dermatology: Effects of ambient humidity and frost formation

Background and Objective: Dynamics of cryogen spray deposition, water conde nsation and frost formation is studied in relationship to cooling rate and efficiency of cryogen spray cooling (CSC) in combination with laser dermato logic surgery. Study Design/Materials and Methods: A high-speed video camera was used to i mage the surface of human skin during and after CSC using a commercial devi ce. The influence of ambient humidity on heat extraction dynamics was measu red in an atmosphere-controlled chamber using an epoxy block with embedded thermocouples. Results: A layer of liquid cryogen may remain on the skin after the spurt t ermination and prolong the cooling time well beyond that selected by the us er. A layer of frost starts forming only after the liquid cryogen retracts. Condensation of ambient water vapor and subsequent frost formation deposit latent heat to the target site and may significantly impair the CSC coolin g rate. Conclusion: Frost formation following CSC does not usually affect laser dos age delivered for therapy of subsurface targets. Moreover, frost formation may reduce the risk of cryo-injury associated with prolonged cooling. The e pidermal protection during CSC assisted laser dermatologic surgery can be f urther improved by eliminating the adverse influence of ambient humidity. ( C) 2001 Wiley-Liss, Inc.