Influence of ventilation of the face on thermoregulation in Man during hyper- and hypothermia

Summary It has been suggested that a thermal counter-current exchange may occur in the cerebral vascular bed of humans, thereby creating for the brain a state of relative thermal independance with regard to the rest of the body. However, worrying questions have arisen concerning this suggestion. Experiments were carried out on seven young male volunteers. Hyper- and hypothermic conditions were produced by immersion in water at 38.5°C and 25°C, respectively. During the last few minutes of immersion, the face was cooled or warmed by ventilation with a 200 l·min^–1 air flow at 5°C or 40°C, respectively. Internal and peripheral temperatures were recorded. Blood flow in the anastomotic vessels between face and brain was measured by Doppler techniques associated with computerized frequency analysis. The general responses were as classically described, i.e. an increase in peripheral and central temperatures during immersion in the warm bath and a decrease in these variables in the cold bath. The reactions produced by cooling or warming the face were small and easily explained by the direct changes of the heat load they induced. Whatever the thermal conditions, the blood flow in the anastomotic vessels between the vascular bed of the face and that of the brain was never reversed. It was concluded that there was no experimental evidence for an efficient thermal counter-current exchange in the vascular bed of the human head.     

Functional morphology of the vascular plexuses associated with the cetacean uterus

The cetacean reproductive system is surrounded by thermogenic locomotory muscle and insulating blubber. This arrangement suggests elevated temperatures at the uterus that could induce detrimental effects on fetal development. We present anatomical evidence for a complex countercurrent heat exchange system that could function to regulate the thermal environment of the uterus and a developing fetus. Cooled venous blood from the surfaces of the dorsal fin and flukes enters the abdominal cavity via the lumbo-caudal venous plexus. This plexus is juxtaposed to the arterial and venous plexuses associated with the uterus. The morphology of the lumbo-caudal venous plexus suggests that it acts as a “heat sink” for the adjacent tissues. Heat may be transferred to the cool, lumbo-caudal venous plexus from the warm blood in the arterial and venous plexuses supplying the uterus. Heat may also be transferred from adjacent locomotory muscles to the cool lumbo-caudal venous plexus. The countercurrent heat exchanger created by the juxtaposition of the lumbo-caudal venous plexus with the uterovarian arterial plexus is similar in design to that of the countercurrent heat exchanger described for male cetaceans. The functional implications of introducing cool superficial blood into the abdominal cavity of a diving, and locomoting female cetacean are discussed. © 1993 Wiley-Liss, Inc.     

Does the temperature of water ingested during exertional-heat stress influence gastrointestinal injury,symptoms, and systemic inflammatory profile?

Objectives

The study aimed to determine the effects of temperature of ingested water during exertional-heat stress on gastrointestinal injury, symptoms and systemic inflammatory responses.

Cross-sectional area of the murine aorta linearly increases with increasing core body temperature

Purpose: The cardiovascular (CV) system plays a vital role in thermoregulation. To date, the response of core vasculature to increasing core temperature has not been adequately studied in vivo. Our objective was to non-invasively quantify the arterial response in murine models due to increases in body temperature, with a focus on core vessels of the torso and investigate whether responses were dependent on sex or age.

Methods: Male and female, adult and aged mice were anaesthetised and underwent magnetic resonance imaging (MRI). Data were acquired from the circle of Willis (CoW), heart, infrarenal aorta and peripheral arteries at core temperatures of 35, 36, 37 and 38?°C (±0.2?°C).

Results: Vessels in the CoW did not change. Ejection fraction decreased and cardiac output (CO) increased with increasing temperature in adult female mice. Cross-sectional area of the aorta increased significantly and linearly with temperature for all groups, but at a diminished rate for aged animals (p?2/°C; aged, 0.017 and 0.011?mm²/°C). Aged male mice had a diminished response in the periphery (% increase in femoral artery area from 35 to 38?°C, male and female: adult, 67 and 65%; aged, 0.1 and 57%).

Conclusion: Previously unidentified increases in aortic area due to increasing core temperature are biologically important because they may affect conductive and convective heat transfer. Leveraging non-invasive methodology to quantify sex and age dependent vascular responses due to increasing core temperature could be combined with bioheat modelling in order to improve understanding of thermoregulation.     

Validity and reliability of the myTemp ingestible temperature capsule

Objectives

An accurate and non-invasive measurement of core body temperature (Tc) is of great importance to quantify exercise-induced increases in Tc in athletes or to assess changes in Tc in patient populations. The use of ingestible gastrointestinal telemetric temperature capsules is widely accepted as a surrogate marker for Tc, but widespread implementation is lacking due to the high costs of these disposable capsules. A new and cheaper temperature capsule system (i.e. myTemp) was recently introduced. The aim of present study is to determine the validity and test–retest reliability of the myTemp system.

Exacerbated heat strain during consecutive days of repeated exercise sessions in heat

ObjectivesAn exercise session in a hot environment may increase thermal strain during subsequent exercise sessions on the same and consecutive days. Therefore, this study was conducted to determine lasting physiological strain from moderate-high intensity, intermittent exercise in heat on subsequent exercise.DesignRepeated measures laboratory study.MethodsSeventeen healthy, recreationally active men (age: 22 ± 3 y, maximal oxygen consumption: 54.6 ± 5.3 mL kg⁻¹ min⁻¹) underwent two intermittent moderate-high intensity aerobic exercise sessions separated by 2 h of rest one day, followed by one session 24 h later in a 40 °C, 40% relative humidity environment. Heart rate, rectal temperature, heat stress perception, and environmental symptoms were assessed.Results100%, 35%, and 71% of participants completed the full exercise protocol during the first exercise session, second exercise session, and the following day, respectively. Exercising heart rate and rectal temperature were greater during the second exercise session (189 ± 11 bpm, 38.80 ± 0.47 °C) than the first identical exercise session (180 ± 17 bpm, p = 0.004; 38.41 ± 0.52 °C, p = 0.001), respectively. Immediate post-exercise heart rate, rectal temperature, thirst, thermal sensation, fatigue, and perceived exertion were similar among exercise sessions despite a shorter exercise duration during the second exercise session (93 ± 27 min, p = 0.001) and the following day (113 ± 12 min, p = 0.032) than the first exercise session (120 ± 0 min).ConclusionsModerate-high-intensity intermittent exercise in the heat resulted in greater heat strain during a second exercise session the same day, and exercise the subsequent day.     

Mutual connections of the raphe system and hypothalamus in relation to fever

Previous studies in our laboratory have shown that lesions in certain brain stem regions prevent elevations of body temperature after administration of bacterial pyrogen. In the present experiments wer examined the morphology of the connections of these "brain stem thermoregulatory centers" which are represented in all raphe and paraphe nuclei of the brain stem reticular formation. The methods of anterograde degeneration and tracing of retrograde transport of horseradish peroxidase were used to identify afferent and efferent connections within this "thermoregulatory field." Abundant mutual connections between the raphe nuclei and hypothalamus were found. All nuclei of the raphe system receive afferents from the medial and lateral hypothalamus. All raphe nuclei have efferent projections to the medial reticular formation, and the raphe nuclei of the pons and mesencephalon provide ascending fibers to the hypothalamus. A lesion of any part (origin, course, termination field) of this mutual raphe-hypothalamic pathway system will prevent development of fever in response to bacterial pyrogen.     

Association between the increase in brain temperature and physical performance at different exercise intensities and protocols in a temperate environment

There is evidence that brain temperature (T_brain) provides a more sensitive index than other core body temperatures in determining physical performance. However, no study has addressed whether the association between performance and increases in T_brain in a temperate environment is dependent upon exercise intensity, and this was the primary aim of the present study. Adult male Wistar rats were subjected to constant exercise at three different speeds (18, 21, and 24 m/min) until the onset of volitional fatigue. T_brain was continuously measured by a thermistor inserted through a brain guide cannula. Exercise induced a speed-dependent increase in T_brain, with the fastest speed associated with a higher rate of T_brain increase. Rats subjected to constant exercise had similar T_brain values at the time of fatigue, although a pronounced individual variability was observed (38.7-41.7°C). There were negative correlations between the rate of T_brain increase and performance for all speeds that were studied. These results indicate that performance during constant exercise is negatively associated with the increase in T_brain, particularly with its rate of increase. We then investigated how an incremental-speed protocol affected the association between the increase in T_brain and performance. At volitional fatigue, T_brain was lower during incremental exercise compared with the T_brain resulting from constant exercise (39.3±0.3 vs 40.3±0.1°C; P<0.05), and no association between the rate of T_brain increase and performance was observed. These findings suggest that the influence of T_brain on performance under temperate conditions is dependent on exercise protocol.     

Beneficial effects of a ketamine/atropine combination in soman-poisoned rats under a neutral thermal environment

Exposure to organophosphorus (OP) compounds, such as pesticides and the chemical warfare agents (soman and sarin), respectively represents a major health problem and a threat for civilian and military communities. OP poisoning may induce seizures, status epilepticus and even brain lesions if untreated. We recently proved that a combination of atropine sulfate and ketamine, a glutamatergic antagonist, was effective as an anticonvulsant and neuroprotectant in mice and guinea-pigs exposed to soman. Since OP exposure may also occur in conditions of heat strain due to climate, wearing of protective gears or physical exercise, we previously demonstrated that ketamine/atropine association may be used in a hot environment without detrimental effects. In the present study, we assess soman toxicity and evaluate the effects of the ketamine/atropine combination on soman toxicity in a warm thermoneutral environment. Male Wistar rats, exposed to 31 °C (easily reached under protective equipments), were intoxicated by soman and treated with an anesthetic dose of ketamine combined with atropine sulfate. Body core temperature and spontaneous locomotor activity were continuously monitored using telemetry. At the end of the warm exposure, blood chemistry and brain mRNA expression of some specific genes were measured. In soman-intoxicated animals, metabolic and genic modifications were related to convulsions rather than to soman intoxication by itself. In the warm environment, ketamine/atropine combination did not produce any side-effect on the assessed variables. Furthermore, the ketamine/atropine combination exhibited beneficial therapeutic effects on soman-intoxicated rats such as a limitation of convulsion-induced hyperthermia and of the increase in some blood chemistry markers.     

Effects of blockade of central dopamine D1 and D2 receptors on thermoregulation,metabolic rate and running performance

To assess the effects of a blockade of central D₁- and D₂-dopaminergic receptors on metabolic rate, heat balance and running performance, 10 nmol (2 μl) of a solution of the D₁ antagonist SCH-23390 hydrochloride (SCH,n= 6), D₂ antagonist eticlopride hydrochloride (Eti, n = 6), or 2 μl of 0.15 MNaCl (SAL, n = 6) was injected intracerebroventricularly into Wistar rats before the animals began graded running until fatigue (starting at i0 m/min, increasing by 1 m/min increment every 3 min until fatigue, 5% inclination). Oxygen consumption and body temperature were recorded at rest, during exercise and following 30 min of recovery. Control experiments with injection of two doses (10 and 20 nmol/rat) of either SCH or Eti solution were carried out in resting rats as well. Body heating rate, heat storage, workload and mechanical efficiency were calculated. Although SCH and Eti treatments did not induce thermal effects in resting animals, they markedly reduced running performance (–83%, SCH; -59% Eti, p < 0.05) and decreased maximal oxygen uptake (–79%, SCH; -45%, Eti, p < 0.05) in running rats. In addition, these treatments induced a higher body heating rate and persistent hyperthermia during the recovery period. Our data demonstrate that the alteration in dopamine transmission induced by the central blockade of dopamine- D₁ and D₂ receptors impairs running performance by decreasing the tolerance to heat storage. This blockade also impairs the dissipation of exercise-induced heat and metabolic rate recovery during the post-exercise period. Our results provide evidence that central activation of either dopamine- Di or D₂ receptors is essential for heat balance and exercise performance.