35.01 Measurement and Thermodynamic Modeling of Energy Flux During Intercostal Nerve Cryoablation

M. Carter1, M. Inge1, S. Zeineddin1, S. Linton1, J. B. Pitt1, P. Robson2, F. Abdullah1, S. Goldstein1  1Ann & Robert H. Lurie Children’s Hospital of Chicago, Division Of Pediatric Surgery, Chicago, IL, USA 2Icahn School of Medicine at Mount Sinai, Biomedical Engineering And Imaging Institute, New York, NY, USA

Introduction:
Intercostal spinal nerve cryoablation (cryo) is an increasingly adopted technique to decrease postoperative pain in patients undergoing thoracic or cardiac operations, particularly pectus excavatum correction. Concerns regarding cryo-induced systemic hypothermia have been raised in pediatric patients; however, assessment of the impact of a cooled cryoprobe on body temperature has not been performed to date. Therefore, we aimed to determine the energy flux from a maximally cooled cryoprobe and model the possible effects on a whole-body system.

Methods:
To directly measure energy flux (transfer over time), a maximally cooled cryoSPHERE probe (AtriCure, Inc., Mason, OH) was isolated in a well-mixed water bath at 37 oC. Real time temperatures were recorded. Subsequently, three models of increasing complexity were created to estimate intraoperative flux. Finally, perioperative temperatures of 50 patients who received cryoablation during modified Nuss procedure were compared by independent Student’s t-test to 50 patients who did not receive cryoablation.

Results:
Direct calorimetry measured average energy flux of the maximally cooled cryoprobe to be 28 J/s. Thermodynamic modeling demonstrated that: 1) The highest possible cryoprobe flux is less than estimated basal metabolic rate of a 50 kg human, and; 2) Flux in a best model of human tissue energy transfer using available literature is far less than the effects of basal metabolic rate and insensible losses. Clinically, there were no significant differences in the minimum intraoperative, end procedure and first postoperative body temperatures for the patients who received cryoablation when compared to those who did not receive cryoablation (Figure).

Conclusion:
By direct measurement and thermodynamic modeling, cryoprobe flux is significantly fewer joules per second than basal metabolic rate. Furthermore, in a high-volume clinical experience there were no differences in body temperature due to cryoablation employment. These findings contradict concerns regarding hypothermia secondary to cryoablation and add to the existing evidence supporting the safety of its use in children.