Skin model surface temperatures during single and multiple cryogen spurts used in laser dermatologic surgery

Background: Although cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery, concern has been expressed that CSC may induce cryo-injury. In order to address this concern, it is necessary to evaluate the effects of prolonged exposure of human skin phantoms (HSP) to CSC. Objective: To measure the minimum surface temperature (T-min) and the time at which it occurs (t(Tmin)) as well as determine the time the sprayed HSP surface remains below 0degreesC (sub-zero time, Deltat(s)) and -26degreesC (residence time, Deltat(r)) during the application of single (SCS) and multiple (MCS) cryogen spurts. Two initial HSP substrate temperatures were studied, T-i: 23 and 70degreesC. Study Design/Materials and Methods: An epoxy-based HSP was constructed to measure T-min, t(Tmin), Deltat(s), and Deltat(r), for 17 spray patterns: I SCS with a total cryo-delivery time (Deltat(c)) of 40 milliseconds; 8 MCS patterns with identical Deltat(c), but with a total cooling time (Deltat(total)) varying from 50 to 280 milliseconds; and 8 SCS patterns that matched the Deltat(total) of the MCS patterns. Results: For both T-i, our results show that it is possible to distinguish between two different cooling regimes. For Deltat(total) less than or equal to 110 milliseconds, the differences between SCS and MCS patterns with the same Deltat(total) for all variables (T-min, t(Tmin), Deltat(s), Deltat(r)) are negligible. Most importantly, all these variables show a remarkable linear dependence with Deltat(total). In the interval 110 milliseconds < Delta t(total) < 280 milliseconds, T-min and t(Tmin) are similar for SCS and MCS, while Delta(s) and Deltat(r) show more pronounced differences between the two spray patterns. In this interval, the values of Tmin and Deltat(s) for MCS remain invariant and similar to the corresponding values for Deltat(total) = 110 milliseconds. Conclusions: These results suggest that: (1) similar epidermal protection may be attained with SCS and MCS for Deltat(total) less than or equal to 110 milliseconds; and (2) for 110 milliseconds < Delta t(total) <= 280 milliseconds, MCS help to maintain Delta t(s) similar to that of SCS at Delta t(total) = 100 milliseconds, which may be beneficial to prevent cryo-injury.