The importance of surface area for the cooling efficacy of mild therapeutic hypothermia

Aim of the studyMild hypothermia after cardiac arrest should be induced as soon as possible. There is a need for improved feasibility and efficacy of surface cooling in ambulances. We investigated which and how much area of the body surface should be covered to guarantee a sufficient cooling rate.MethodsEach of five adult, human-sized pigs (88-105 kg) was randomly cooled in three phases with pads that covered different areas of the body surface corresponding to humans (100% or 30% [thorax and abdomen] or 7% [neck]). The goal was to quickly lower brain temperature (Tbr) from 38 to 33 °C within a maximum of 120 min. Linear regression analysis was used to test the association between cooling efficacy and surface area. Data are presented as mean ± standard deviation.ResultsThe 100% and 30% cooling pads decreased the pigs' Tbr from 38 to 33 °C within 33 ± 7 min (8.2 ± 1.6 °C/h) and 92 ± 24 min (3.6 ± 1.1 °C/h). The 7% achieved a final Tbr of 35.8 ± 0.7 °C after 120 min (1.1 ± 0.4 °C/h). The 30% and 7% cooling surface areas achieved 37 ± 11% and 15 ± 7% of the cooling rate compared to the 100% cooling pads. For every additional percent of surface area cooled, the cooling rate increased linearly by 0.07 °C/h (95% CI 0.05-0.09, p = 0.001). No skin lesions were observed.ConclusionsThe cooling pads were effective and safe for rapid induction of mild hypothermia in adult, human-sized pigs, depending on the percentage of body surface area covered. Covering only the neck, chest, and abdomen might achieve satisfactory cooling rates.