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

AIM OF THE STUDY Mild 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. METHODS Each of five adult, human-sized pigs (88-105kg) 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 120min. Linear regression analysis was used to test the association between cooling efficacy and surface area. Data are presented as mean±standard deviation. RESULTS The 100% and 30% cooling pads decreased the pigs' Tbr from 38 to 33°C within 33±7min (8.2±1.6°C/h) and 92±24min (3.6±1.1°C/h). The 7% achieved a final Tbr of 35.8±0.7°C after 120min (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. CONCLUSIONS The 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.

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