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

Abstract

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. 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. 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. 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.