Spinal and corticospinal excitability in response to reductions in skin and core temperatures via whole-body cooling.

Cold stress impairs fine and gross motor movements. Although peripheral effects of muscle cooling on performance are well understood, less is known about central mechanisms. This study characterized corticospinal and spinal excitability during surface cooling, reducing skin (Tsk) and esophageal (Tes) temperatures. Ten subjects (3 females) wore a liquid-perfused suit and were cooled (9 °C perfusate, 90 min) and rewarmed (41 °C perfusate, 30 min). Transcranial magnetic stimulation (eliciting motor evoked potentials [MEPs]), as well as transmastoid (eliciting cervicomedullary evoked potentials [CMEPs]) and brachial plexus (eliciting maximal compound motor action potentials [Mmax]) electrical stimulation, were applied at baseline, every 20 min during cooling, and following rewarming. Sixty minutes of cooling reduced Tsk by 9.6 °C (P < 0.001), but Tes remained unchanged (P = 0.92). Tes then decreased by ∼0.6 °C in the next 30 min of cooling (P < 0.001). Eight subjects shivered. During rewarming, shivering was abolished, and Tsk returned to baseline, while Tes did not increase. During cooling and rewarming, Mmax, MEP, and MEP/Mmax remained unchanged from baseline. However, CMEP and CMEP/Mmax increased during cooling by ∼85% and 79% (P < 0.001), respectively, and remained elevated post-rewarming. The results suggest that spinal excitability is facilitated by reduced Tsk during cooling and reduced Tes during warming, while corticospinal excitability remains unchanged. ClinicalTrials.gov ID: NCT04253730. Novelty: This is the first study to characterize corticospinal and spinal excitability during whole-body cooling and rewarming in humans. Whole body cooling did not affect corticospinal excitability. Spinal excitability was facilitated during reductions in both skin and core temperatures.

Comparison of Electric Resistive Heating Pads and Forced-Air Warming for Pre-hospital Warming of Non-shivering Hypothermic Subjects.

INTRODUCTION: Victims of severe hypothermia require external rewarming, as self-rewarming through shivering heat production is either minimal or absent. The US Military commonly uses forced-air warming in field hospitals, but these systems require significant power (600-800 W) and are not portable. This study compared the rewarming effectiveness of an electric resistive heating pad system (requiring 80 W) to forced-air rewarming on cold subjects in whom shivering was pharmacologically inhibited. MATERIALS AND METHODS: Shivering was inhibited by intravenous meperidine (1.5 mg/kg), administered during the last 10 min of cold-water immersion. Subjects then exited from the cold water, were dried and lay on a rescue bag for 120 min in one of the following conditions: spontaneous rewarming only (rescue bag closed); electric resistive heating pads (EHP) wrapped from the anterior to posterior torso (rescue bag closed); or, forced-air warming (FAW) over the anterior surface of the body (rescue bag left open and cotton blanket draped over warming blanket). Supplemental meperidine (to a maximum cumulative dose of 3.3 mg/kg) was administered as required during rewarming to suppress shivering. RESULTS: Six healthy subjects (3 m, 3 f) were cooled on three different occasions, each in 8°C water to an average nadir core temperature of 34.4 ± 0.6°C (including afterdrop). There were no significant differences between core rewarming rates (spontaneous; 0.6 ± 0.3, FAW; 0.7 ± 0.2, RHP; 0.6 ± 0.2°C/h) or post-cooling afterdrop (spontaneous; 1.9 ± 0.4, FAW; 1.9 ± 0.3, RHP; 1.6 ± 0.2°C) in any of the 3 conditions. There were also no significant differences between metabolic heat production (S; 74 ± 20, FAW; 66 ± 12, RHP; 63 ± 9 W). Total heat gain was greater with FAW (36 W gain) than EHP (13 W gain) and spontaneous (13 W loss) warming (p < 0.005). CONCLUSIONS: Total heat gain was greater in FAW than both EHP, and spontaneous rewarming conditions, however, there were no observed differences found in rewarming rates, post-cooling afterdrop or metabolic heat production. The electric heat pad system provided similar rewarming performance to a forced-air warming system commonly used in US military field hospitals for hypothermic patients. A battery-powered version of this system would not only relieve pressure on the field hospital power supply but could also potentially allow extending use to locations closer to the field of operations and during transport. Such a system could be studied in larger groups in prospective trials on colder patients.

Is active recovery during cold water immersion better than active or passive recovery in thermoneutral water for postrecovery high-intensity sprint interval performance?

High-intensity exercise is impaired by increased esophageal temperature (Tes) above 38 °C and/or decreased muscle temperature. We compared the effects of three 30-min recovery strategies following a first set of three 30-s Wingate tests (set 1), on a similar postrecovery set of Wingate tests (set 2). Recovery conditions were passive recovery in thermoneutral (34 °C) water (Passive-TN) and active recovery (underwater cycling; ∼33% maximum power) in thermoneutral (Active-TN) or cold (15 °C) water (Active-C). Tes rose for all conditions by the end of set 1 (∼1.0 °C). After recovery, Tes returned to baseline in both Active-C and Passive-TN but remained elevated in Active-TN (p < 0.05). At the end of set 2, Tes was lower in Active-C (37.2 °C) than both Passive-TN (38.1 °C) and Active-TN (38.8 °C) (p < 0.05). From set 1 to 2 mean power did not change with Passive-TN (+0.2%), increased with Active-TN (+2.4%; p < 0.05), and decreased with Active-C (-3.2%; p < 0.05). Heart rate was similar between conditions throughout, except at end-recovery; it was lower in Passive-TN (92 beats·min-1) than both exercise conditions (Active-TN, 126 beats·min-1; Active-C, 116 beats·min-1) (p < 0.05). Although Active-C significantly reduced Tes, the best postrecovery performance occurred with Active-TN. Novelty An initial set of 3 Wingates increased Tes to ∼38 °C. Thirty minutes of Active-C was well tolerated, and decreased Tes and blood lactate to baseline values, but decreased subsequent Wingate performance.