Dennis A. Grahn

Hibernation in Black Bears: Independence of Metabolic Suppression from Body Temperature

Hibernating black bears suppress their metabolic rate to 25% of normal, but only slightly reduce their body temperature. Black bears hibernate for 5 to 7 months a year and, during this time, do not eat, drink, urinate, or defecate. We measured metabolic rate and body temperature in hibernating black bears and found that they suppress metabolism to 25% of basal rates while regulating body temperature from 30° to 36°C, in multiday cycles. Heart rates were reduced from 55 to as few as 9 beats per minute, with profound sinus arrhythmia. After returning to normal body temperature and emerging from dens, bears maintained a reduced metabolic rate for up to 3 weeks. The pronounced reduction and delayed recovery of metabolic rate in hibernating bears suggest that the majority of metabolic suppression during hibernation is independent of lowered body temperature.

8-OH-DPAT-sensitive neurons in the nucleus raphe magnus modulate thermoregulatory output in rats.

The nucleus raphe magnus (NRM) is purported to be a relay through which peripheral thermoafferent information is transmitted to thermointegrative centers located in the preoptic/anterior hypothalamus (POAH). Therefore, suppression of neural activity in the NRM should reduce thermoregulatory responses to peripheral thermal challenges, but not affect responses elicited by manipulation of POAH temperature. At low ambient temperatures lidocaine injections into the NRM of nonanesthetized rats resulted in decreases in POAH temperature, oxygen consumption, and electromyographic activity. At a warm ambient temperature, lidocaine injections into the NRM decreased the elevations in oxygen consumption and electromyographic activity elicited by cooling the POAH. The effects of lidocaine injections were duplicated by injection of a 5-HT(1A) agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) into the NRM. The effect of 8-OH-DPAT was eliminated by pre-treatment with a selective autoreceptor antagonist. These results suggest that NRM 5-HT neurons are modulating the relationship between output of thermointegrative centers and thermoregulatory effector responses rather than processing thermoafferent information.

Cooling via one hand improves physical performance in heat-sensitive individuals with Multiple Sclerosis: A preliminary study

BackgroundMany individuals afflicted with multiple sclerosis (MS) experience a transient worsening of symptoms when body temperature increases due to ambient conditions or physical activity. Resulting symptom exacerbations can limit performance. We hypothesized that extraction of heat from the body through the subcutaneous retia venosa that underlie the palmar surfaces of the hands would reduce exercise-related heat stress and thereby increase the physical performance capacity of heat-sensitive individuals with MS.MethodsTen ambulatory MS patients completed one or more randomized paired trials of walking on a treadmill in a temperate environment with and without cooling. Stop criteria were symptom exacerbation and subjective fatigue. The cooling treatment entailed inserting one hand into a rigid chamber through an elastic sleeve that formed an airtight seal around the wrist. A small vacuum pump created a -40 mm Hg subatmospheric pressure enviinside the chamber where the palmar surface of the hand rested on a metal surface maintained at 18–22°C. During the treatment trials, the device was suspended from above the treadmill on a bungee cord so the subjects could comfortably keep a hand in the device without having to bear its weight while walking on the treadmill.ResultsWhen the trials were grouped by treatment only, cooling treatment increased exercise durations by 33% (43.6 ± 17.1 min with treatment vs. 32.8 ± 10.9 min. without treatment, mean ± SD, p < 5.0·10-6, paired t-test, n = 26). When the average values were calculated for the subjects who performed multiple trials before the treatment group results were compared, cooling treatment increased exercise duration by 35% (42.8 ± 16.4 min with treatment vs. 31.7 ± 9.8 min. without treatment, mean ± SD, p < 0.003, paired t-test, n = 10).ConclusionThese preliminary results suggest that utilization of the heat transfer capacity of the non-hairy skin surfaces can enable temperature-sensitive individuals with MS to extend participation in day-to-day physical activities despite thermally stressful conditions. However, systematic longitudinal studies in larger cohorts of MS patients with specific deficits and levels of disability conducted under a variety of test conditions are needed to confirm these preliminary findings.

A NON-INVASIVE MEANS TO EFFECTIVELY RESTORE NORMOTHERMIA IN COLD STRESSED INDIVIDUALS: A PRELIMINARY REPORT

The combined application of subatmospheric pressure and heat to a forearm and hand has been previously reported to be an effective method for restoring normothermia in hypothermic postoperative patients. The objective of this study was to determine whether this technique also could be useful for the treatment of accidental hypothermia. Four otherwise healthy cold-stressed soldiers (core temperature <36.0 degrees C) were studied. In all four cases, application of the subatmospheric pressure induced an immediate local distention of the subcutaneous vasculature of the hand and fingers. Tympanic temperature reached a plateau in a normothermic range within 15 min in all subjects. The subjects also noted rapid elimination of shivering and improvement in subjective assessment of thermal comfort. These results are encouraging and warrant further investigation of the technique.

Sleep-Dependent Changes in the Thermoregulatory System

the thermoregulatory system is particularly convenient and interesting for studies of the influence of arousal state on homeostatic processes. State-related changes in thermoregulatory functions are substantial and are a general phenomenon in mammals and birds. The sleep phase of the daily rest/activity cycle is associated with decreases in metabolic rate and body temperature. In addition, the onset of sleep at any time of day tends to result in declines in metabolic rate and body temperature. Sleep is not a unitary phenomenon but consists of specific states definable by electroencephalographic (EEG), electromyographic (EMG), and electrooculographic (EOG) criteria. At a minimum, two sleep states, rapid-eye-movement (REM) and non-REM (NREM) sleep, are distinguished with these electrographic parameters. Changes in thermoregulatory functions in comparison to wakefulness are qualitatively different in these two sleep states. During periods of NREM sleep, adjustments are made in thermoregulatory responses that decrease heat production and/or increase heat loss in comparison to contiguous periods of wakefulness. During REM sleep there is a cessation of ongoing thermoregulatory responses (for reviews, see refs. 11 and 13).

Work volume and strength training responses to resistive exercise improve with periodic heat extraction from the palm.

Abstract Grahn, DA, Cao, VH, Nguyen, CM, Liu, MT, and Heller, HC. Work volume and strength training responses to resistive exercise improve with periodic heat extraction from the palm. J Strength Cond Res 26(9): 2558–2569, 2012—Body core cooling via the palm of a hand increases work volume during resistive exercise. We asked: (a) “Is there a correlation between elevated core temperatures and fatigue onset during resistive exercise?” and (b) “Does palm cooling between sets of resistive exercise affect strength and work volume training responses?” Core temperature was manipulated by 30–45 minutes of fixed load and duration treadmill exercise in the heat with or without palm cooling. Work volume was then assessed by 4 sets of fixed load bench press exercises. Core temperatures were reduced and work volumes increased after palm cooling (Control: Tes = 39.0 ± 0.1° C, 36 ± 7 reps vs. Cooling: Tes = 38.4 ± 0.2° C, 42 ± 7 reps, mean ± SD, n = 8, p < 0.001). In separate experiments, the impact of palm cooling on work volume and strength training responses were assessed. The participants completed biweekly bench press or pull-up exercises for multiple successive weeks. Palm cooling was applied for 3 minutes between sets of exercise. Over 3 weeks of bench press training, palm cooling increased work volume by 40% (vs. 13% with no treatment; n = 8, p < 0.05). Over 6 weeks of pull-up training, palm cooling increased work volume by 144% in pull-up experienced subjects (vs. 5% over 2 weeks with no treatment; n = 7, p < 0.001) and by 80% in pull-up naïve subjects (vs. 20% with no treatment; n = 11, p < 0.01). Strength (1 repetition maximum) increased 22% over 10 weeks of pyramid bench press training (4 weeks with no treatment followed by 6 weeks with palm cooling; n = 10, p < 0.001). These results verify previous observations about the effects of palm cooling on work volume, demonstrate a link between core temperature and fatigue onset during resistive exercise, and suggest a novel means for improving strength and work volume training responses.

A Chemical Heat Pack-Based Method For Consistent Heating of Intravenous Fluids

BACKGROUND Transfusion of cold intravenous fluids (IVF) can exacerbate hypothermia. Civilian and military guidelines recommend heated IVF for hypothermic patients; however, there is currently no ideal IVF heating system for use in resource-limited settings. OBJECTIVE Development of a system that uses flameless ration heaters (FRH) and an insulated sleeve for the consistent delivery of IVF at physiologically appropriate temperatures (40°-42°C) over the range of ambient conditions typical of the prehospital and wilderness environments. METHODS The temperatures of 0.9% normal saline (NS) 1-L bags were measured under 3 ambient conditions: 3°C, 10°C, and 20°C. The IVF was placed in an insulated pouch along with a predetermined number of activated FRH (5 FRH for 3°C, 4 FRH for 10°C, and 3 FRH for 20°C) for 10 minutes before removing the FRHs. The insulated IVF bag was drained through 280 cm of intravenous tubing at a flow rate of 77 mL/min. Raw temperature data for internal and delivery temperatures were collected and analyzed. RESULTS The temperature of the IVF throughout the delivery of 1 L of NS under the 3 ambient conditions was as follows (mean ± SD): at 3°C ambient, 47° ± 2.1°C internal and 42.6°C ± 1.4°C at delivery; at 10°C ambient, 52.3° ± 2.7°C and 45.2° ± 1.6°C; and at 20°C ambient, 45.5° ± 1°C and 39.7° ± 0.7°C. CONCLUSIONS The IVF heating system described here reliably delivered physiologically appropriate temperature intravenous fluids in 2 of the 3 ambient treatment conditions. With the appropriate number of FRH for the ambient conditions, this system enables the delivery of warmed IVF to provide active warming, which may be clinically beneficial in the prevention and treatment of hypothermia.

Vacuum Enhanced Heat Transfer Through Mammalian Radiator Structures

Most mammals, including humans, maintain relatively constant internal temperatures despite changes in ambient conditions and fluctuations in internal heat production. To achieve this end, an individual must be able to: 1) protect the internal milieu from external thermal challenges and 2) dissipate excess internally produced heat. Heat is produced as a byproduct of cellular metabolism and is lost to the environment across the body surface. The thermoregulatory challenge is to dissipate excess internally produced heat despite the insulation layers that protect the internal milieu from external thermal influences.Copyright

A method to reduce heat strain while clad in encapsulating outerwear

Abstract The use of personal protective equipment (PPE) increases the risk of heat related maladies. A means to enhance heat dissipation capacity of individuals clad in PPE would be of benefit. The glabrous skin regions of the hands, face, and feet are portals for direct heat transfer between the body core and the external environment. The effects of PPE outerwear and palmar glabrous skin cooling on heat storage were assessed. Subjects engaged in fixed load treadmill exercise in a thermoneutral environment (Ta = 20–24°C) or rested in a hot environment (45 ± 0.5°C). The use of PPE outerwear increased the rate of core temperature rise by five-fold during vigorous exercise. Palm cooling using a stationary water circulation system attenuated the rate of core temperature rise by 30–60% during rest and light, moderate, and vigorous exercise while wearing PPE outerwear. However, the subjects were tethered to the system. A wearable cooling system was devised that allowed free range of motion and unrestricted mobility. The wearable system provided thermal benefits equivalent to the use of the tethering cooling system. With optimization, this wearable cooling technique could neutralize the negative thermoregulatory effects of wearing PPE while engaged in light workload activities such as those encountered by healthcare professionals working in infectious disease treatment centers. For individuals working at higher workloads, such as firefighters, a wearable glabrous skin-based cooling system could extend work bout duration as well as enhance heat loss during episodic recovery periods.

Novel Application of Chemical Cold Packs for Treatment of Exercise-Induced Hyperthermia: A Randomized Controlled Trial

OBJECTIVE Heat-related illness is a common disease with significant morbidity and mortality. Despite no proven efficacy, application of chemical cold packs (CCP) to the skin overlying the large vessels of the neck, groin, and axillae is a traditional recommended cooling modality. The study objective was to compare the cooling rates of CCP applied to these traditional areas vs the glabrous skin surfaces of the cheeks, palms, and soles in exercise-induced hyperthermia. METHODS Ten healthy adult male volunteers walked on a treadmill in a heated room (40°±0.5°C) while wearing insulated military overgarments until their esophageal temperatures (Tes) reached 39.2°C. Each participant had three heat stress trials on separate days: no treatment followed by randomly ordered traditional (neck, groin, and axillae) cooling and glabrous skin cooling. RESULTS With no treatment, Tes remained stable after the first 5 minutes of the heat trial (ΔTes=0.12°±0.07°C/10 min). Traditional cooling followed a linear decline (ΔTes=0.17°±0.04°C/10 min; P<.001). Glabrous cooling enhanced the treatment effect by a steeper decline (ΔTes=0.30°±0.06°C/10 min; P<.001), significantly different from traditional cooling by 2-way analysis of variance (P<.001). CONCLUSIONS Application of CCP to glabrous skin surfaces was more effective for treating exercise-induced heat stress than the traditional CCP cooling intervention. This novel cooling technique may be beneficial as an adjunctive treatment for heat-related illness in the prehospital environment.