Hand and finger skin temperatures in convective and contact cold exposure

The present study aimed at investigating the spatial variability of skin temperature (T _sk) measured at various points on the hand during convective and cold contact exposure. A group of 8 subjects participated in a study of convective cooling of the hand (60 min) and 20 subjects to contact cooling of the finger pad (5 min). Experiments were carried out in a small climatic chamber into which the hand was inserted. For convective cold exposure,T _sk was measured at seven points on the palmar surface of the fingers of the left hand, one on the palmar surface and one on the dorsal surface of the hand. The air temperature inside the mini-chamber was 0, 4, 10 and 16°C. With the contact cold exposure, the subjects touched at constant pressures an aluminium cube cooled to temperatures of –7, 0 and 7°C in the same mini-chamber. ContactT _sk was measured on the finger pad of the index finger of the left hand. TheT _sk of the proximal phalanx of the index finger (on both palm and back sides), and of the middle phalanx of the little finger was also measured. The variation ofT _sk between the proximal and the distal phalanx of the index finger was between 1.5 to 10°C during the convective cold exposure to an air temperature of 0°C. Considerable gradients persisted between the hand and fingers (from 2 to 17°C at 0°C air temperature) and between the phalanges of the finger (from 0.5 to 11.4°C at 0°C air temperature). The onset of cold induced vasodilatation (CIVD) on different fingers varied from about 5 to 15 min and it did not always appear in every finger. For contact cold exposure, whenT _sk on the contact skin cooled down to nearly 0°C, the temperature at the area close to the contact skin could still be 30°C. Some cases of CIVD were observed in the contact skin area, but not on other measuring points of the same finger. These results indicated that local thermal stimuli were the main determinents of CIVD. Representative hand skin temperature may require five or more measuring points. Our results strongly emphasised a need to consider the large spatial and individual variations in the prediction and modelling of extremity cooling.     

Nasal mucosal vasodilatation in response to passive hyperthermia in humans

The present study was conducted to measure nasal mucosal blood flow (NMBF) during body warming. Five subjects [mean (SD) 24 (2) years], wearing only shorts and a thick felt hat with ear flaps, were immersed to the neck in a bath at 40 (0.5)°C. Tympanic (T _ty), esophageal (T _es), mean unweighted skin (T _sk), nose skin and ear pinna skin were recorded at 1-min intervals. NMBF on the lower septal wall was estimated using a laser Doppler flow meter. At rest T _ty and T _es were both 36.5°C. T _ty dropped significantly below T _es during body warming, despite impeded heat loss from the head due to the felt hat. T _ty increased to 37.3°C and T _es increased to 37.5°C during the immersion. During the immersion all skin temperatures were steady or increasing, ruling out the possibility of a contamination of T _ty from (T _sk), Body warming significantly (P = 0.001) increased NMBF by approximately three times from resting values at the end of immersion. During the period of increasing core temperatures NMBF was significantly correlated to T _ty (r = 0.93, P = 0.0001) and T _es (r = 0.97, P = 0.0001), suggesting the blood flow change in this tissue was a thermo-regulatory response. The increased NMBF during hyperthermia supports the hypothesis of respiratory cooling involvement in selective brain cooling of humans.     

Disturbance of thermal homeostasis during post-exercise hyperthermia

The response of core temperature to exercise was investigated during recovery in order to avoid the antagonistic competition between exercise and thermal reflexes for the same effector systems which control skin blood flow. Five healthy, non-training males [mean (SD) age, 23.8 (2.04) years] were habituated to 29° C at relative 50% humidity for more than 2 h and then exercised by treadmill running at about 75% maximum oxygen uptake for 18 min. They then remained at 29° C for up to 65 min of recovery. Oesophageal (T _es), rectal (T _re) and skin temperatures (T _sk) were recorded at 5-s intervals throughout. The abrupt fall of temperature gradient from the forearm to finger was used to identify the T _es for skin vessel dilatation (T _dil) during exercise. Mean (SE) Ts rose from a resting value of 36.67 (0.15)° C to 38.22 (0.24)° C, mean T _re rose from 37.09 (0.25)° C to 38.23 (0.15)° C, and T _dil occurred at 37.39 (0.32)° C. Within 10 min of recovery mean T _es fell to 37.31 (0.24)° C, where it remained a significant 0.64° C above its pre-exercise (PrEx) level (P0.018) but insignificantly different from T _dil for the remaining 55 min of recovery. Meanwhile, T _re fell gradually throughout recovery to 37.64 (0.18)° C. The T _sk at all non-acral sites except the thigh had recovered to PrEx levels by 20–30 min post-exercise (PoEx). The rapid PoEx fall of T _es to the level of T _dil and the subsequent plateau above PrEx values suggests that heat dissipation during recovery was primarily passive once T _es had fallen to T _dil, even though T _es and T _re were significantly elevated. The relationship of these results to the set-point and load error concepts of thermal control is discussed.These data have been presented at the Canadian Physiological Society Winter meeting, January 1993, but have not been previously published     

Thermoregulatory responses of young and older men to cold exposure

Summary Nine young (20–25 years) and ten older (60–71 years) men, matched for body fatness and surface area :mass ratio, underwent cold tests in summer and winter. The cold tests consisted of a 60-min exposure, wearing only swimming trunks, to an air temperature of 17°C (both seasons) and 12°C (winter only). Rectal (T _re) and mean skin ( _sk) temperatures, metabolic heat production (M), systolic (BP_S) and diastolic (BP_d) blood pressures and heart rate (f _c) were measured. During the equilibrium period (28°C air temperature) there were no age-related differences inT _re, _sk, BP_S, BP_d, orf _c regardless of season, although M of the older men was significantly lower (P<0.003). The decrease inT _re and _sk (due to the marked decrease in six of the older men) and the increase in BPS and BPd were significantly greater (P<0.004) for the older men during all the cold exposures. The rate of increase inM was significantly greater (P<0.01) for the older group when exposed to 12°C in winter and 17°C in summer (due to the marked increase in four of the older men). This trend was not apparent during the 17°C exposure in winter. There was no age-related difference in f_c during the exposures. Significant decreases inT _re and _sk and increases inM, BP_S and BP_d during the 12°C exposure were observed for the older group (P< 0.003) compared to their responses during the 17°C exposure in winter. In contrast,T _re,M, BP_S in the young group were not affected as much by the colder environment. It was concluded that older men have more variable responses and some appear more or less responsive to mild and moderate cold air than young men.     

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T_sk) and body core temperature (T_c) on these factors. Twenty-two males [O_2max=64.2 (8.9) ml kg^–1 min^–1, body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4°C rectal temperature (T_re) and then cooled back to 37.4^oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T_c of 39.4^oC reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T_c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T_c reduces VA during isometric MVC, and neither T_sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.     

Responses of young and older men during prolonged exercise in dry and humid heat

Summary Eight young, sedentary men (aged 34 years, SD 3) and six older moderately active, unacclimated men (aged 57 years, SD 2) walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [dry bulb temperature (T _db) 21°C, relative humidity (r.h.) 43%)], a warm humid (T _db 30°C, r.h. 80%) and a hot dry (T _db 40°C, r.h. 20%) environment while wearing ordinary working clothes (0.7 c/o). Their oxgen consumption, heart rate (f _c), rectal (T _re) and mean skin temperature (T_sk), sweat rate (SR), and evaporative rate (ER) were measured during the tests. The ratings of thermal sensation (TS) and perceived exertion (RPE) were assessed using standard scales. In the heat stress tests, the number of experiments discontinued did not significantly differ between the two groups. The mean levels and end-exercise values of T _re, T_sk, f _c, TS and RPE were not significantly different between the young and older subjects in any of the environments. In the warm humid environment, however, the T _re and RPE of the older subjects increased continuously (P<0.05) during the test compared to the young subjects. No significant difference between the groups was observed in SR or in ER. In the hot dry environment, however, the ER of older men increased more slowly compared to the young men. In spite of some time-related differences observed in T _re, RPE, and ER, the older subjects did not exhibit higher f _c during exercise in the heat, they were not more hyperthermic and their performance times were similar to the young subjects. Therefore, it was concluded that older calendar age is not necessarily associated with a reduced ability to exercise in a hot environment and other factors, such as physical activity habits and aerobic capacity, may be equally important in determining heat tolerance in the elderly.     

Thermosensitivity of preoptic neurones in a hibernator at high and low ambient temperatures

Summary The effect of ambient temperature on the thermosensitivity of preoptic neurones was studied in euthermic golden hamsters. At skin temperatures (T_sk) of 20°C, preoptic units were still responsive to hypothalamic temperatures (T_hy) below 10°C, while at T_sk=36°C these neurones became inactive at T_hy=15°C on the average. These studies suggest that thermoreceptive preoptic neurones, influenced by a high activity of cutaneous cold-receptors, are capable of sensing core temperatures even in deep hibernation.Supported by the Deutsche Forschungsgemeinschaft, Wu 63/2     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

Summary In six male subjects the sweating thresholds, heart rate (f _c, as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO_2max)], were compared at ambient temperatures (T _a) of 5° C (LT) and 24° C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO₂) at the end of each rest period was higher in LT than MT (P< 0.05). During 20-min exercise at 40%VO_2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (T _re) but at lower mean skin (T _sk) and mean body temperatures (T _b) in LT than MT experiments (P<0.001). The exercise inducedVO₂ increase was greater only at the end of the light (40%VO_2max) exercise in the cold in comparison with MT (P<0.001). Bothf _c and blood lactate concentration [la]_b were lower at the end of LT than MT for moderate (60%VO_2max) and heavy (80%VO_2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower (T _b) andT _sk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24° C since the increases inT _re,f _c and [la]_b were lower at the lowerT _a.     

Blood flow through the ophthalmic veins during exercise in humans

Summary The blood from the face flows into the intracranium through the ophthalmic veins when human subjects become hyperthermic. To investigate a possible mechanism underlying this change in direction of flow, five young men were subjected to either passive body warming or exercise on a cycle ergometer, in a climatic chamber whose air temperature and relative humidity were 28°C and 40%. Tympanic (T _ty) and oesophageal temperatures, forehead sweat rate (m _sw), skin blood flow. (Q _sk) and blood flow through the ophthalmic vein (Q _ov) were measured, and the mean skin (T _sk) and mean body (T _b) temperatures were computed. Passive body warming was induced by a box-shaped body warming unit enclosing all but the subject's head. Exercise was performed either at an intensity of 60% maximal oxygen consumption or with the intensity increasing in increments. During both tests, m _sw and Q _sk started to increase shortly after the imposition of the heat load. The Q _ov began to change with the venous blood flowing from the face into the intracranium and a complete reversal in the direction of Q _ov (from the face to the intracranium). came significantly later than the increases in Q _sw and Q _sk. The T _ty at the time of flow reversal was the same in both tests. The T _sk (and hence T _b) at flow reversal was, however, significantly higher during passive body warming than during exercise. The mechanism for switching the direction of Q _ov appeared to have been triggered by a high temperature in the brain, and not by thermal input from the periphery of the body. In a febrile subject who volunteered for this study, the direction of Q _ov was consistently inwards even when sitting quietly. From these results, we suggest that there are elements within the brain that control the mechanisms for switching the direction of venous flow through the emissary veins to keep the brain cool during hyperthermia.     

Regulation of local sweating in sleep-deprived exercising humans

Summary Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

Vasomotor response of the human face: laser-Doppler measurements during mild hypo- and hyperthermia

The skin of the face is reputed not to vasoconstrict in response to cold stress because the face skin temperature remains steady during hypothermia. The purpose of the present work was to measure the vasomotor response of the human face to whole-body hypothermia, and to compare it with hyperthermia. Six male subjects were immersed in cold and in warm water to obtain the two conditions. Skin blood flow, evaporation, and skin temperature (T_sk) were recorded in three loci of the face, the forehead, the infra orbital area, and the cheek. Tympanic (T_ty) and oesophageal (T_oes) temperatures were also recorded during the different thermal states. Normothermic measurements served as control. Blood flow was recorded with a laser-Doppler flowmeter, evaporation measured with an evaporimeter. Face T_sk remained stable between normo-, hypo-, and hyperthermia. Facial blood flow, however, did not follow the same pattern. The facial blood flow remained at minimal vasoconstricted level when the subjects' condition was changed from normo- to hypothermia. When the condition changed from hypo- to hyperthermia a 3 to 9–fold increase in the blood flow was recorded. From these results it was concluded that a vasoconstriction seems to be the general vasomotor state in the face during normothermia.     

Effects of selective cutaneous denervation on hypothalamic thermosensitivity in rats

The effect of altering input from cutaneous thermoreceptors of the face and trunk on the relationship between hypothalamic temperature (T _hy) and heat production (HP) was studied in three rats. The signal from cutaneous receptors was altered in two ways: by altering skin temperature (T _sk) and by sectioning nerves supplying cutaneous receptors. It was found that whenT _sk was lowered in normal ratsT _hy threshold for thermoregulatory HP was elevated, but the slope of the relationship betweenT _hy and HP was not significantly altered. After the spinal nerves serving the trunk skin were sectioned, the slope was reduced and the threshold was elevated markedly at both test ambient temperatures (T _a), butT _a had essentially the same effect on theT _hy vs. HP relationship after cutaneous denervation as before. Clearly, eliminating input from trunk cutaneous thermoreceptors has a different effect than does lowering or raisingT _sk, but thermoregulation is being achieved by the same basic mechanism before and after cutaneous denervation. After the cranial nerves supplying the skin of the face were also sectioned, there was a further elevation in theT _hy threshold for HP atT _a=25° C but no change atT _a=15°C. It is concluded that cutaneous denervation does not substantially interfere with the rat's ability to regulate its body temperature, and that the reduced Thy sensitivity and increased Thy threshold exhibited after cutaneous denervation is the result of input from intact warm- and cold-thermoreceptors located in the core and in tissues intermediate to core and skin.     

Post-exercise thermal homeostasis as a function of changes in pre-exercise core temperature

We have previously reported that, following continuous exercise, a prolonged elevated plateau of esophageal temperature (T _es) was directly related to the T _es at the time of cutaneous vasodilation (Th_dil) during exercise. In order to investigate the hypothesis that the factors which result in an increase of the post-exercise Thdil and define the post-exercise T _es elevation are related to pre-exercise T _es, nine healthy, young [24.0 (1.9) years], non-training males rested at 29°C, 50% humidity for > 1 h (control). They then completed three successive cycles of 15 min treadmill running at 70% maximal oxygen consumption ( ) followed by 30 min rest. Esophageal, rectal (T _re) and skin (T _sk) temperatures and forearm cutaneous blood flow were recorded at 5-s intervals throughout. Laser-Doppler flowmetry of forearm skin blood flow was used to identify the Thdil during exercise. Pre-exercise T _es was 36.74 (0.25)°C and post-exercise Tes fell to stable and significant (P < 0.05) elevations above pre-exercise values at 37.22(0.27)°C, 37.37(0.27)°C and 37.48(0.26)°C following each successive work bout respectively. Correspondingly, Th_dil during each work bout rose in proportion to, and was not different than, the post-exercise T _es in the following recovery [37.20(0.23)°C, 37.41(0.24)°C and 37.58(0.24)°C]. Although the increases were less with each successive exercise bout, the differences between each exercise bout, in terms of post-exercise Tes and Th_dil values, were significant (P < 0.05). These results reinforce our previous observations of elevations in Th_dil and post-exercise Tes after a single exercise bout and lead to the tentative conclusions that (1) pre-exercise Test has a direct influence on Thdil and post-exercise Test and (2) the exercise-induced increase of Th_dil persists into recovery, influencing post-exercise thermal recovery.     

Thermoregulatory and subjective responses of clothed men in the cold during continuous and intermittent exercise

Summary Thermoregulatory and thermal subjective responses were studied in ten male, clothed subjects during continuous (C) and intermittent (I) exercise at the same average level of oxygen consumption. The subjects performed both I and C twice, dressed in two different three-layer cold-protective clothing ensembles of two thermal insulation levels [total clothing insulation = 2.59 clo (L) and 3.20 clo (H)]. Experiments were carried out at an ambient temperature of –10°C. Rectal temperatures increased similarly in both types of exercise. Mean skin temperature (T _sk) was lower in 1 compared to C with both levels of clothing insulation. Over the last 0.5 h of the experimentT _sk was approximately 1.3°C lower in 1 than in C for clothing L. The skin evaporation rate was higher in clothing H than L but did not differ between I and C. Subjective ratings for thermal sensations of the whole body (BTS) and hands were close to neutral in I and around slightly warm in C. The BTS was lower in I than in C and was lower in L compared to H. It was concluded that, at equal average energy expenditure, thermal responses to intermittent and continuous exercise in the Gold differ in clothed subjects, principally as a result of different patterns of heat exchange.     

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing

This study examined the effectiveness of endurance training and heat acclimation in reducing the physiological strain imposed by exercising in the heat while wearing protective clothing. Seven young men underwent 8 weeks of physical training [60–80% maximal aerobic power (VO_2max) for 30–45 min · day^–1, 3–4 days · week^–1 at < 25° C] followed by 6 days of heat acclimation (45–55% VO_2max for 60 min · day^–1 at 40° C, 30% relative humidity). Nine other young men underwent corresponding periods of control observation and heat acclimation. Before and after each treatment, subjects completed a treadmill walk (4.8 km · h^–1, 2% grade) in a climatic chamber (40° C, 30% relative humidity), wearing in turn normal combat clothing or clothing protecting against nuclear, biological, and chemical (NBC) agents. Criteria for halting this test were: (1) a rectal temperature (T _re) of 39.3° C; (2) a heart rate (f _c) 95% of the subject's observed maximum, maintained for 3 min; (3) unwillingness of the subject to continue; (4) the elapse of 120 min. The training regimen increased mean VO_2max by 16% and mean plasma volume by 8%. When tested in normal combat clothing, the rates of increase in T _re and f _c were slower after training. However, when wearing NBC protective clothing, the only significant change induced by training was a higher mean skin temperature (T _sk) in the early part of the test. Heat acclimation increased the mean plasma volume of untrained subjects by 8%, but their VO_2max remained unchanged. When tested in normal combat clothing, acclimation decreased their mean values of T _re, T _sk, f _c, and metabolic rate. When wearing NBC protective clothing, the only significant decrease after acclimation was in overall T _re. In trained subjects, heat acclimation induced no further improvement in any physiological variable when wearing normal combat clothing, but reduced overall T _re and T _sk when wearing NBC protective clothing. Training- or acclimation-induced increases of sweat secretion (an average increment of 0.14–0.23 kg · h^–1) were not accompanied by any statistically significant increase in sweat evaporation when wearing NBC protective clothing. Moreover, tolerance times were unchanged in either normal combat (116–120 min) or NBC protective clothing (47–52 min). We conclude that neither endurance training nor heat acclimation do much to improve exercise tolerance when wearing NBC protective clothing in hot environments, because any added sweat secretion decreases blood volume and increases discomfort without augmenting body cooling.     

Effects of leg activity and ambient barometric pressure on foot swelling and lower-limb skin temperature during 8 h of sitting

Summary Prolonged immobilization in an upright position often leads to discomfort and oedema in the feet of otherwise healthy subjects. To determine the significance of leg activity and ambient pressure on oedema formation, skin temperature (T_sk) and discomfort, 6 volunteers sat for 8 h with one leg immobilized and the other spontaneously active; one day at sea level (750 mmHg) and one day at reduced barometric pressure (540 mmHg). Foot swelling was measured by water plethysmography. Leg movements were continuously monitored by a Vitalog computer, and foot discomfort was estimated by analog-visual scales. The 8 hour swelling averaged 5.7% in the inactive foot, and 2.7% in the active foot (p<0.001). T_sk of the inactive foot levelled off towards ambient temperature (21 ° C) within 4 h. For the active foot this fall was reduced by 2–3 ° C (p<0.025). The increase in foot discomfort during the day was lowest in the active foot (p<0.005). High foot T_sk was associated with a high foot swelling rate. Reduced ambient barometric pressure had no effects on foot swelling or T_sk. It is concluded that modest leg activity during 8 h of sitting has several effects on the circulation in the feet: some effects promote and some prevent oedema formation. However, the net result is a reduction in foot swelling.     

Skin blood flow during incremental exercise in a thermoneutral and a hot dry environment

Summary Eight physically fit men performed two incremental bicycle ergometer tests, one in an ambient temperature of 25° C and the other at 40° C. Oesophageal temperature (T_es) increased continuously throughout the tests up to 38.0 and 38.3° C, respectively. In both enviroments, forearm blood flow (plethysmography) was linearly related to T_es above the T_es threshold for vasodilation, but at the heaviest work loads this relationship was clearly attenuated and therefore indicated skin vasoconstriction, which tended to be more pronounced at 25° C. During recovery at 25° C, in some subjects the forearm blood flow increased above the levels observed at the end of the graded exercise in spite of a decreasing T_es. Skin blood flow, measured by laser Doppler flow meter at the shoulder, was quantitatively different but, on average, seemed to reveal the same response pattern as the forearm blood flow. In spite of the higher level of skin blood flow in the heat, blood lactate accumulation did not differ between the two environments. The present results suggest that there is competition between skin vasoconstriction and vasodilation at heavy work rates, the former having precedence in a thermoneutral environment to increase muscle perfusion. During short-term graded exercise in a hot environment, skin vasoconstriction with other circulatory adjustments seems to be able to maintain adequate muscle perfusion at heavy work levels, but probably not during maximum exercise.     

Brain and core temperatures and peripheral vasomotion during sleep and wakefulness at various ambient temperatures in the rat

Changes in brain, core and tail skin temperatures (T _br, T _c and T _t) associated with transitions in the arousal states were recorded in rats throughout the 24-h diurnal cycle at 10 °C, 21 °C and 29 °C. Falling asleep was accompanied by decreases in both T _br and T _c and vasodilation at 10 C and 21 °C. At 29 °C, tail vessels were permanently dilated, and further dilation was not found on sleep onset. T _br and T _c, however, continued to decrease during non-rapid-eye-movement sleep (NREMS); these changes are likely to result from reductions in heat production and increased conductive heat loss. The changes in T _br, T _c and T _t on awakening mirrored those on falling asleep. It is suggested that the suppression of sleep in the cold and the enhancement of NREMS in the heat promote thermoregulation. Rapid-eye-movement sleep (REMS) was associated with sharp rises in T _br. The rise in T _br was the largest in the cold and was attenuated at 29 °C. T _c decreased and T _t increased in the cold, whereas T _c tended to increase and T _t to decrease in the heat. The paradoxical peripheral vasomotion during REMS supports previous suggestions on severe thermoregulatory impairment during REMS in other species.     

Local sweating responses during recovery sleep after sleep deprivation in humans

Changes in the central control of sweating were investigated in five sleep-deprived subjects (kept awake for 40 h) during their recovery sleep under warm ambient conditions [operative temperature (T _o) was either 35 or 38° C]. Oesophageal (T _oes) and mean skin (T _sk) temperatures, chest sweat rate (m _sw,ch), and concomitant electro-encephalographic data were recorded. Throughout the night at 35 or 38° C T _o, m _sw,ch changes were measured at a constant local chest skin temperature (T _ch) of 35.5° C. The results showed that body temperatures (T _oes and T _sk) of sleep-deprived subjects were influenced by thermal and hypnogogic conditions. The m _sw,ch levels correlated positively with T _oes in the subjects studied during sleep stage 1–2 (light sleep: LS), sleep stage 3–4 (slow wave sleep: SWS) and rapid eye movement (REM) sleep. Contrary to what has been reported in normal sleep, firstly, the T _oes threshold for sweating onset differed between REM sleep and both LS and SWS, and, secondly, the slopes of the m _sw,ch versus T _oes relationships were unchanged between REM and non-REM (i.e. LS or SWS) sleep. The changes observed after sleep deprivation were hypothesized to be due to alterations in the functioning of the central nervous system controller.     

Comparison of two cool vests on heat-strain reduction while wearing a firefighting ensemble

This study evaluated the effectiveness of a six-pack versus a four-pack cool vest in reducing heat strain in men dressed in firefighting ensemble, while resting and exercising in a warm/humid environment [34.4°C (day bulb), 28.9°C (wet bulb)]. Male volunteers (n = 12) were monitored for rectal temperature (T _re), mean skin temperature (T _sk), heart rate, and energy expenditure during three test trials: control (no cool vest), four-pack vest, and six-pack vest. The cool vests were worn under the firefighting ensemble and over Navy dungarees. The protocol consisted of two cycles of 30 min seated rest and 30 min walking on a motorized treadmill (1.12 m · s^–1, 0% grade). Tolerance time for the control trial (93 min) was significantly less than both vest trials (120 min). Throughout heat exposure, energy expenditure varied during rest and exercise, but no differences existed among all trials (P > 0.05). During the first 60 min of heat exposure, physiological responses were similar for the four-pack and six-pack vests. However, during the second 60 min of heat exposure the six-pack vest had a greater impact on reducing heat strain than the four-pack vest. PeakT _e andT _sk at the end of heat exposure for 6-pack vest [mean (SD) 38.0(0.3)°C and 36.8(0.7)°C] were significantly lower compared to four-pack [38.6 (0.4)°C and 38.1(0.5)°C] and controls [38.9(0.5)°C and 38.4(0.5)°C]. Our findings suggest that the six-pack vest is more effective than the four-pack vest at reducing heat strain and improves performance of personnel wearing a firefighting ensemble.