The effect of ethnicity on the vascular responses to cold exposure of the extremities

Purpose

Cold injuries are more prevalent in individuals of African descent (AFD). Therefore, we investigated the effect of extremity cooling on skin blood flow (SkBF) and temperature (T _sk) between ethnic groups.

Methods

Thirty males [10 Caucasian (CAU), 10 Asian (ASN), 10 AFD] undertook three tests in 30 °C air whilst digit T _sk and SkBF were measured: (i) vasomotor threshold (VT) test—arm immersed in 35 °C water progressively cooled to 10 °C and rewarmed to 35 °C to identify vasoconstriction and vasodilatation; (ii) cold-induced vasodilatation (CIVD) test—hand immersed in 8 °C water for 30 min followed by spontaneous warming; (iii) cold sensitivity (CS) test—foot immersed in 15 °C water for 2 min followed by spontaneous warming. Cold sensory thresholds of the forearm and finger were also assessed.

Results

In the VT test, vasoconstriction and vasodilatation occurred at a warmer finger T _sk in AFD during cooling [21.2 (4.4) vs. 17.0 (3.1) °C, P = 0.034] and warming [22.0 (7.9) vs. 12.1 (4.1) °C, P = 0.002] compared with CAU. In the CIVD test, average SkBF during immersion was greater in CAU [42 (24) %] than ASN [25 (8) %, P = 0.036] and AFD [24 (13) %, P = 0.023]. Following immersion, SkBF was higher and rewarming faster in CAU [3.2 (0.4) °C min^?1] compared with AFD [2.5 (0.7) °C min^?1, P = 0.037], but neither group differed from ASN [3.0 (0.6) °C min^?1]. Responses to the CS test and cold sensory thresholds were similar between groups.

Conclusion

AFD experienced a more intense protracted finger vasoconstriction than CAU during hand immersion, whilst ASN experienced an intermediate response. This greater sensitivity to cold may explain why AFD are more susceptible to cold injuries.     

Prediction of shivering heat production from core and mean skin temperatures

Prediction formulae of shivering metabolism (M _shiv) are critical to the development of models of thermoregulation for cold exposure, especially when the extrapolation of survival times is required. Many such formulae, however, have been calibrated with data that are limited in their range of core temperatures (T _c), seldom involving values of less than 36°C. Certain recent studies of cold-water immersion have reported T _c as low as 33.25°C. These data comprise measurements of T _c (esophageal) and mean skin temperature (T¯ _s), and metabolism from 14 males [mean (SD); age?=?28 (5) years; height?=?1.78?(0.06)?m; body mass?=?77.7 (6.9)?kg; body fat (BF)?=?18.4 (4.5)%] during immersion in water as cold as 8°C for up to 1?h and subsequent self-rewarming via shivering under dry blanketed conditions. The data contain 3343 observations with mean (SD) T _c and T¯ _s of 35.92?(0.93)°C and 23.4?(8.9)°C, respectively, and have been used to re-examine the prediction of M _shiv. Rates of changes of these temperatures were not used in the analysis. The best fit of the formulae, which are essentially algebraic constructs with and without setpoints, are those with a quadratic expression involving T¯ _s. This is consistent with the findings of Benzinger (1969) who demonstrated that the thermosensitivity of skin is parabolic downwards with temperature peaking near a value of 20°C. Formulae that included a multiplicative interaction term between T _c and T¯ _s did not predict as well. The best prediction using 37°C and 33°C as the T _c and T _s setpoints, respectively, was found with BF as an attenuation factor: M _shiv (W?·?m^?2)?= [155.5?·?(37???T _c) + 47.0?·?(33???T¯ _s)???1.57?·?(33???T¯ _s)²] (%BF)^0.5.     

The effect of dynamic exercise on resting cold thermoregulatory responses measured during water immersion

The purpose of this study was to evaluate the effect of exercise on the subsequent post-exercise thresholds for vasoconstriction and shivering measured during water immersion. On 2 separate days, seven subjects (six males and one female) were immersed in water (37.5°C) that was subsequently cooled at a constant rate of ≈6.5°C?·?h^?1 until the thresholds for vasoconstriction and shivering were clearly established. Water temperature was then increased to 37.5°C. Subjects remained immersed for ≈20?min, after which they exited the water, were towel-dried and sat in room air (22°C) until both esophageal temperature and mean skin temperature (T¯ _sk) returned to near-baseline values. Subjects then either performed 15?min of cycle ergometry (at 65% maximal oxygen consumption) followed by 30?min of recovery (Exercise), or remained seated with no exercise for 45?min (Control). Subjects were then cooled again. The core temperature thresholds for both vasoconstriction and shivering increased significantly by 0.2°C Post-Exercise (P?T¯ _sk at the onset of vasoconstriction and shivering was different during Pre- and Post-Exercise Cooling, we compensated mathematically for changes in skin temperatures using the established linear cutaneous contribution of skin to the control of vasoconstriction and shivering (20%). The calculated core temperature threshold (at a designated skin temperature of 32.0°C) for vasoconstriction increased significantly from 37.1 (0.3)°C to 37.5 ( 0.3)°C post-exercise (P?P?相似文献   

The contribution of blood flow to the skin temperature responses during a cold sensitivity test

Purpose

The presumption in a cold sensitivity test (CST) used for cold injuries is that the skin temperature (T _sk) observed reflects the return of blood flow to the extremity following a local cold challenge. We questioned this assumption.

Methods

Six non-cold injured participants undertook two CSTs in 30 °C air. The control (CON) CST involved 12 min gentle exercise prior to immersing the foot into 15 °C water for 2 min followed by 15 min of spontaneous rewarming. The occlusion (OCC) CST was the same except that blood flow to the foot was occluded during the rewarming period. These results were compared to CSTs from six individuals with non-freezing cold injury and moderate–severe cold sensitivity (CS) and a non-perfused human digit model (NPDM).

Results

Before immersion, great toe skin blood flow (SkBF) was similar in CON and OCC conditions [255 (107) laser Doppler units (LDU)] and was higher than CS [59 (52) LDU]. During rewarming, SkBF in CON returned to 104 % of the pre-immersion value and was higher than both OCC and CS. Great toe T _sk before immersion was lower in CS [28.5 (2.1) °C] compared to CON [34.7 (0.4) °C], OCC [34.6 (0.9) °C] and NPDM [35.0 (0.4) °C]. During rewarming skin/surface temperature in OCC, CS and NPDM were similar and all lower than CON.

Conclusions

SkBF does contribute to the skin rewarming profile during a CST as a faster rate of rewarming was observed in CON compared to either OCC or NPDM. The lower T _sk in CS may be due to a reduced basal SkBF.     

Cold induced vasodilatation and cardiovascular responses in humans during cold water immersion of various upper limb areas

To study the physiological responses induced by immersing in cold water various areas of the upper limb, 20 subjects immersed either the index finger (T1), hand (T2) or forearm and hand (T3) for 30?min in 5°C water followed by a 15-min recovery period. Skin temperature of the index finger, skin blood flow (Q_sk) measured by laser Doppler flowmetry, as well as heart rate (HR) and mean arterial blood pressure (¯BP_a) were all monitored during the test. Cutaneous vascular conductance (CVC) was calculated as Q_sk?/?¯BP_a. Cold induced vasodilatation (CIVD) indices were calculated from index finger skin temperature and CVC time courses. The results showed that no differences in temperature, CVC or cardiovascular changes were observed between T2 and T3. During T1, CIVD appeared earlier compared to T2 and T3 [5.90 (SEM 0.32) min in T1 vs 7.95 (SEM 0.86) min in T2 and 9.26 (SEM 0.78) min in T3, P??1 in T2 and +15 (SEM 3) beats?·?min^?1 in T3, P??1 in T2, and ?15 (SEM 3) beats?·?min^?1 in T3, P?aincreased at the beginning of T1 but was lower than in T2 and T3 [+9.3 (SEM 2.5) mmHg in T1, P?P?相似文献   

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.     

The effect of ambient temperature on gross-efficiency in cycling

Time-trial performance deteriorates in the heat. This might potentially be the result of a temperature-induced decrease in gross-efficiency (GE). The effect of high ambient temperature on GE during cycling will be studied, with the intent of determining if a heat-induced change in GE could account for the performance decrements in time trial exercise found in literature. Ten well-trained male cyclists performed 20-min cycle ergometer exercise at 60% (power output at which VO_2max was attained) in a thermo-neutral climate (N) of 15.6 ± 0.3°C, 20.0 ± 10.3% RH and a hot climate (H) of 35.5 ± 0.5°C, 15.5 ± 3.2% RH. GE was calculated based on VO₂ and RER. Skin temperature (T _sk), rectal temperature (T _re) and muscle temperature (T _m) (only in H) were measured. GE was 0.9% lower in H compared to N (19.6 ± 1.1% vs. 20.5 ± 1.4%) (P < 0.05). T _sk (33.4 ± 0.6°C vs. 27.7 ± 0.7°C) and T _re (37.4 ± 0.6°C vs. 37.0 ± 0.6°C) were significantly higher in H. T _m was 38.7 ± 1.1°C in H. GE was lower in heat. T _m was not high enough to make mitochondrial leakage a likely explanation for the observed reduced GE. Neither was the increased T _re. Increased skin blood flow might have had a stealing effect on muscular blood flow, and thus impacted GE. Cycling model simulations showed, that the decrease in GE could account for half of the performance decrement. GE decreased in heat to a degree that could explain at least part of the well-established performance decrements in the heat.     

Using skin temperature gradients or skin heat flux measurements to determine thresholds of vasoconstriction and vasodilatation

Forearm–fingertip skin temperature differentials (T _sk-diff) are used to indicate vasomotor tone, vasoconstriction defined as having occurred when T _sk-diff≥4°C (Sessler et al. 1987, 1988a, b). This study was conducted to determine whether T _sk-diff or finger pad heat flux (HF) can be used to predict when vasoconstriction and vasodilatation occur. Seven subjects (one female) sat in water at [mean (SD)] 40.7 (0.8)°C until their core temperature (T _c) increased by 1°C, ensuring vasodilatation. The water was then cooled [at a rate of 0.6 (0.1)°C.min^–1] until T _c fell to 0.5°C below pretesting values, causing vasoconstriction. Subjects were then rewarmed in water [41.2 (1.0)°C]. Skin blood flow (SkBF) was measured using laser Doppler flowmetry (LDF) on the left second finger pad [immersed in water at 10.4 (1.4)°C as part of another experiment], and infrared plethysmography on the third finger pad of both hands. T _sk-diff and HF were measured on the right upper limb, which remained in air. When vasodilated, the subjects had a stable T _sk-diff and HF. During cooling, rapid-onset vasoconstriction occurred coincidental with large gradient changes in HF and T _sk-diff (inflection points). In two subjects the original vasoconstriction definition (T _sk-diff≥4°C) was not attained, in the other five this was achieved 31–51 min after vasoconstriction. During rewarming, the T _sk-diff and HF inflection points less accurately reflected the onset of vasodilatation, although with one exception they were within 5 min of the LDF changes. We conclude that T _sk-diff and HF inflection points predict vasoconstriction accurately, and better than T _sk-diff≥4°C. Electronic Publication     

Treatment of exertional heat stress developed during low or moderate physical work

Purpose

We examined whether treatment for exertional heat stress via ice water immersion (IWI) or natural recovery is affected by the intensity of physical work performed and, thus, the time taken to reach hyperthermia.

Methods

Nine adults (18–45 years; 17.9 ± 2.8 percent body fat; 57.0 ± 2.0 mL kg^?1 min^?1 peak oxygen uptake) completed four conditions incorporating either walking or jogging at 40 °C (20 % relative humidity) while wearing a non-permeable rain poncho. Upon reaching 39.5 °C rectal temperature (T _re), participants recovered either via IWI in 2 °C water or via natural recovery (seated in a ~29 °C environment) until T _re returned to 38 °C.

Results

Cooling rates were greater in the IWI [T _re: 0.24 °C min^?1; esophageal temperature (T _es): 0.24 °C min^?1] than the natural recovery (T _re and T _es: 0.03 °C min^?1) conditions (p < 0.001) with no differences between the two moderate and the two low intensity conditions (p > 0.05). Cooling rates for T _re and T _es were greater in the 39.0–38.5 °C (T _re: 0.19 °C min^?1; T _es: 0.31 °C min^?1) compared with the 39.5–39.0 °C (T _re: 0.11 °C min^?1; T _es: 0.13 °C min^?1) period across conditions (p < 0.05). Similar reductions in heart rate and mean arterial pressure were observed during recovery across conditions (p > 0.05), albeit occurred faster during IWI. Percent change in plasma volume at the end of natural recovery and IWI was 5.96 and 9.58 %, respectively (p < 0.001).

Conclusion

The intensity of physical work performed and, thus, the time taken to reach hyperthermia does not affect the effectiveness of either IWI treatment or natural recovery. Therefore, while the path to hyperthermia may be different for each patient, the path to recovery must always be immediate IWI treatment.     

Thermal status of wet-suited divers using closed circuit O2 apparatus in sea water of 17–18.5°C

A wet suit may not provide adequate thermal protection when diving in moderately cold water (17–18°C), and any resultant mild hypothermia may impair performance during prolonged diving. We studied heat exchange during a dive to a depth of 5?m in sea water (17–18.5°C) in divers wearing a full wet suit and using closed-circuit oxygen breathing apparatus. Eight fin swimmers dived for 3.1?h and six underwater scooter (UWS) divers propelled themselves through the water for 3.7?h. The measurements taken throughout the dive were the oxygen pressure in the cylinder and skin and rectal temperatures (T _re). Each subject also completed a cold score questionnaire. The T _re decreased continuously in all subjects. Oxygen consumption in the fin divers (1.40?l?·?min^?1) was higher than that of the UWS divers (1.05?l?·?min^?1). The mean total insulation was 0.087°C?·?m²?·?W^?1 in both groups. Mean body insulation was 37% of the total insulation (suit insulation was 63%). The reduction in T _re over the 1st hour was related to subcutaneous fat thickness. There was a correlation between cold score and T _re at the end of 1?h, but not after that. A full wet suit does not appear to provide adequate thermal protection when diving in moderately cold water.     

Hypohydration and acute thermal stress affect mood state but not cognition or dynamic postural balance

Equivocal findings have been reported in the few studies that examined the impact of ambient temperature (T _a) and hypohydration on cognition and dynamic balance. The purpose of this study was to determine the impact of acute exposure to a range of ambient temperatures (T _a 10–40 °C) in euhydration (EUH) and hypohydration (HYP) states on cognition, mood and dynamic balance. Thirty-two men (age 22 ± 4 years, height 1.80 ± 0.05 m, body mass 85.4 ± 10.8 kg) were grouped into four matched cohorts (n = 8), and tested in one of the four T _a (10, 20, 30, 40 °C) when EUH and HYP (?4 % body mass via exercise–heat exposure). Cognition was assessed using psychomotor vigilance, 4-choice reaction time, matching to sample, and grammatical reasoning. Mood was evaluated by profile of mood states and dynamic postural balance was tested using a Biodex Balance System. Thermal sensation (TS), core (T _core) and skin temperature (T _sk) were obtained throughout testing. Volunteers lost ?4.1 ± 0.4 % body mass during HYP. T _sk and TS increased with increasing T _a, with no effect of hydration. Cognitive performance was not altered by HYP or thermal stress. Total mood disturbance (TMD), fatigue, confusion, anger, and depression increased during HYP at all T _a. Dynamic balance was unaffected by HYP, but 10 °C exposure impaired balance compared to all other T _a. Despite an increase in TMD during HYP, cognitive function was maintained in all testing environments, demonstrating cognitive resiliency in response to body fluid deficits. Dynamic postural stability at 10 °C appeared to be hampered by low-grade shivering, but was otherwise maintained during HYP and thermal stress.     

Simulated and experimental temperature responses in man during exercise in varying environments

Stolwijk's mathematical model of thermoregulation is validated against reported human experience.Computed temperatures from the model were compared against experimental data obtained, using various matched environmental conditions. Three fit male subjects 168.5 ± 1.8 cm, 64.2 ± 1.9 kg, 24.5 ± 2.6 yr. (mean ± range) underwent pre-work and incremented work phases in the various conditions. Measurements of rectal temperature (T_R), tympanic temperature (T_T), and four skin sites ($\text{T}$_S) were taken. Air temperature (T_AIR), air velocity (V), relative humidity (RH) and work (W), were manipulated.Good simulations were achieved at 30°C air temperature, rectal temperature deviation $\text{d}$ < 0.10°C. As lower ambient temperatures were encountered initial transient drop of simulated core temperature was emphasized, and this was not reflected in experimental data. It is proposed that the process of thermoregulation during exercise in the cold requires further conceptual refinement in the model. By subdivision of active muscular layers in the passive system of the model superior simulations at low air temperatures, may be achieved.     

Effect of ambient temperature on endurance performance while wearing cross-country skiing clothing

This study assessed the effects of exposure to cold (?14 and ?9?°C), cool (?4 and 1?°C) and moderate warm (10 and 20?°C) environments on aerobic endurance performance-related variables: maximal oxygen consumption (VO_2max), running time to exhaustion (TTE), running economy and running speed at lactate threshold (LT). Nine male endurance athletes wearing cross-country ski racing suit performed a standard running test at six ambient temperatures in a climatic chamber with a wind speed of 5?m?s^?1. The exercise protocol consisted of a 10-min warm-up period followed by four submaximal periods of 5?min at increasing intensities between 67 and 91?% of VO_2max and finally a maximal test to exhaustion. During the time course mean skin temperature decreased significantly with reduced ambient temperatures whereas T _re increased during all conditions. T _re was lower at ?14?°C than at ?9 and 20?°C. Running economy was significantly reduced in warm compared to cool environments and was also reduced at 20?°C compared to ?9?°C. Running speed at LT was significantly higher at ?4?°C than at ?9, 10 and 20?°C. TTE was significantly longer at ?4 and 1?°C than at ?14, 10 and 20?°C. No significant differences in VO_2max were found between the various ambient conditions. The optimal aerobic endurance performance wearing a cross-country ski racing suit was found to be ?4 and 1?°C, while performance was reduced under moderate warm (10 and 20?°C) and cold (?14 and ?9?°C) ambient conditions.     

Do greater rates of body heat storage precede the accelerated reduction of self-paced exercise intensity in the heat?

Aim

To reevaluate the previous hypothesis that greater reductions in self-paced exercise intensity in the heat are mediated by early differences in the rate of body heat storage (S).

Methods

Eight trained volunteers cycled in 19 °C/1.8 kPa (COOL), 25 °C/1.2 kPa (NORM), and 34 °C/1.6 kPa (HOT), while maintaining an RPE of 16. Potential differences in S following the onset of exercise were assessed by comparing rates of esophageal temperature change (ΔT _es/Δt); and estimated S values using a traditional two-compartment thermometric model (S _therm) of changes in rectal (T _re) and skin (T _sk) temperature, and partitional calorimetry (S _cal).

Results

After 15 min of exercise, workload decreased more in HOT vs. COOL (P = 0.03), resulting in a shorter time (HOT: 40.7 ± 14.9 min; COOL: 53.5 ± 18.7 min; P = 0.04) to 70 % of initial workload. However, there were no preceding differences in ΔT _es/Δt between conditions (P = 0.18). S _therm values were different between HOT and COOL during the first 5 min of exercise (P < 0.05), primarily due to negative S _therm values (?32 ± 15 kJ min^?1) in COOL, which according to partitional calorimetric measurements, required improbably high (~56 kJ min^?1) rates of evaporation when no sweating on the back and thigh was observed until after 7.6 ± 1.5 min and 4.8 ± 1.7 min of exercise, respectively. S _cal values in the first 5 min of exercise confirmed S was actually positive in COOL (+21 ± 8 kJ min^?1) and not negative. Different S _therm values following the onset of exercise at different environmental temperatures are simply due to transient differences in the rate of change in T _sk.

Conclusion

Reductions in self-paced exercise intensity in the heat are not mediated by early differences in S following the onset of exercise.     

Temperature dependence of habituation of the initial responses to cold-water immersion

The initial responses to cold-water immersion, evoked by stimulation of peripheral cold receptors, include tachycardia, a reflex inspiratory gasp and uncontrollable hyperventilation. When immersed naked, the maximum responses are initiated in water at 10°C, with smaller responses being observed following immersion in water at 15°C. Habituation of the initial responses can be achieved following repeated immersions, but the specificity of this response with regard to water temperature is not known. Thirteen healthy male volunteers were divided into a control (C) group (n?=?5) and a habituation (H) group (n?=?8). Each subject undertook two 3-min head-out immersions in water at 10°C wearing swimming trunks. These immersions took place at a corresponding time of day with 4 days separating the two immersions. In the intervening period the C group were not exposed to cold water, while the H group undertook another six, 3-min, head-out immersions in water at 15°C. Respiratory rate (f _R), inspiratory minute volume (V˙ _I) and heart rate (f _H) were measured continuously throughout each immersion. Following repeated immersions in water at 15°C, the f _R, V˙ _I and f _H responses of the H group over the first 30 s of immersion were reduced (P??1, 50.5?l?·?min^?1 and 114 beats?·?min^?1 respectively, to 19.8 breaths?·?min^?1, 26.4?l?·?min^?1 and 98 beats?·?min^?1, respectively. In water at 10°C these responses were reduced (P??1, 67.6?l?· min^?1 and 128 beats?·?min^?1 to 24.0 breaths?·?min^?1, 29.5?l?·?min^?1 and 109 beats?·?min^?1, respectively over a corresponding period of immersion. Similar reductions were observed during the last 2.5?min of immersions. The initial responses of the C group were unchanged. It is concluded that habituation of the cold shock response can be achieved by immersion in warmer water than that for which protection is required. This suggests that repeated submaximal stimulation of the cutaneous cold receptors is sufficient to attenuate the responses to more maximal stimulation.     

Influence of hydration status and fluid replacement on heat tolerance while wearing NBC protective clothing

The purpose of the present study was to investigate the influence of hypohydration and fluid replacement on tolerance to an uncompensable heat stress. Eight healthy young males completed a matrix of six trials in an environmental chamber, set at 40°C and 30% relative humidity, while wearing nuclear, biological, and chemical protective clothing. Subjects performed either light (3.5 km · h⁻¹, 0% grade, no wind) or heavy (4.8 km · h⁻¹, 4% grade, no wind) treadmill exercise combined with three hydration states [euhydration with fluid replacement (EU/F), euhydration without fluid replacement (EU/NF), and hypohydration with fluid replacement (H/F)]. Hypohydration of 2.2% body mass was achieved by exercise and fluid restriction on the day preceding the trials. No differences in the endpoint mean skin temperature (Tˉ_sk), sweat rate, or rectal temperature (T _re) were observed among the hydration conditions for either work rate. During light exercise, the change in T _re (ΔT _re) was significantly higher with H/F than EU/F after 40 min, and heart rate was greater after 25 min. The heart rate was greater during EU/NF than during EU/F after 60 min. Tolerance times were significantly greater for EU/F than for either EU/NF or H/F. With heavy exercise, no differences in ΔT _re were observed across hydration conditions. Compared to EU/F, heart rates were higher after 10 and 30 min for H/F and EU/NF, respectively. Tolerance times were significantly less during H/F than with either of the EU conditions. Stroke volume was significantly decreased in H/F trials compared to EU/F trials for both light and heavy work rates, but no differences in cardiac output were observed. It was concluded that even minor levels of hypohydration significantly impaired exercise tolerance in a severely uncompensable heat stress environment at both light and heavy exercise intensities. Accepted: 17 June 1997     

Effects of repeated exercise/rest sessions at –10°C on skin and rectal temperatures in men wearing chemical protective clothing

The development of thermophysiological responses during four consecutive exercise/rest sessions in the cold was studied in men wearing chemical protective clothing and a face mask. Six men repeated four exercise/rest sessions during 8?h at –10°C. Each session consisted of step exercise (240 W?·?m^?2) for 60?min and rest for another 60?min. Rectal and skin temperatures were measured continuously and thermal sensations were obtained at 30-min intervals. Entering the cold from a warm environment and the onset of exercise resulted in a decrease in skin temperatures during the first session and the decrement in the temperatures of the extremities continued for 10–20?min during the following period of exercise. Torso skin temperature was at its lowest during the first rest period. After the first session of cold exposure the range and the level of variation in mean body temperature (T¯ _b) followed a pattern which was repeated until the end of the experiment. However, the torso skin temperatures increased gradually until the fourth session, while the temperatures of the extremities, in contrast, tended to decrease up to the third session. In conclusion, the present results indicated that although T¯ _b, reflecting the whole body heat balance, showed a typical pattern of change after the first session (2?h), the torso area was warming until the end of the cold exposure while the extremities continued to cool down up to the third session (6?h), obviously due to a prolonged redistribution of the circulation.     

Passive temperature lability in the elderly

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28°C water. The exercise was conducted to elevate tympanic temperature (T _ty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling ofT _ty, and changes inT _ty relative to resting levels (ΔT _ty) at which sweating abated and shivering commenced were defined as the ΔT _ty thresholds for the cessation of sweating (T _sw) and onset of shivering (T _sh), respectively. In addition toT _ty, oxygen uptake ( ; 1 · min⁻¹), sweating rate (g · m⁻² · min⁻¹), and forehead skin blood perfusion were also measured during the trials. The mean (SD)T _sw occurred at a significantly (P <0.005) higher ΔT _ty [0.48 (0.18)°C] in the elderly than in the young adults [0.21(0.06)°C], while the Tsh occurred at significantly (P < 0.005) lower ΔT _ty in the elderly [ −0.64 (0.34)°C] than in young adults [−0.22 (0.10)°C]. Decreases in ΔT _ty below the shivering threshold were met with a significantly (P <0.01) reduced . The range of temperature lability between Ts, andT _sh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39)°C] than in the young adults [0.43 (0.12)°C], and the slope of the vasoconstrictor response in the null-zone was significantly (P <0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels ofT _ty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.     

The effect of ambient temperature and exercise intensity on post-exercise thermal homeostasis

We have previously demonstrated a prolonged (65?min or longer) elevated plateau of esophageal temperature (T _es ) (0.5–0.6°C above pre-exercise values) in humans following heavy dynamic exercise (70% maximal oxygen consumption, V˙O_2max) at a thermoneutral temperature (T _a) of 29°C. The elevated T _es value was equal to the threshold T _es at which active skin vasodilation was initiated during exercise (Th_dil). A subsequent observation, i.e., that successive exercise/recovery cycles (performed at progressively increasing pre-exercise T _es levels) produced parallel increases of Th_dil and the post-exercise T _es, further supports a physiological relationship between these two variables. However, since all of these tests have been conducted at the same T _a (29°C) and exercise intensity (70% V˙O_2max) it is possible that the relationship is limited to a narrow range of T _a/exercise intensity conditions. Therefore, five male subjects completed 18?min of treadmill exercise followed by 20?min of recovery in the following T _a/exercise intensity conditions: (1) cool with light exercise, T _a?=?20°C, 45% V˙O_2max (CL); (2) temperature with heavy exercise, T _a?=?24°C, 75% O_{2 max} (TH); (3) warm with heavy exercise, T _a?=?29°C, 75% V˙O_2max (WH); and (4) hot with light exercise, T _a?=?40°C, 45% V˙O_2max (HL). An abrupt decrease in the forearm-to-finger temperature gradient (T _fa??T _fi) was used to identify the Th_dil during exercise. Mean pre-exercise T _es values were 36.80, 36.60, 36.72, and 37.20°C for CL, TH, WH, and HL conditions respectively. T _es increased during exercise, and end post-exercise fell to stable values of 37.13, 37.19, 37.29, and 37.55°C for CL, TH, WH, and HL trials respectively. Each plateau value was significantly higher than pre-exercise values (P?dil values (i.e., 37.20, 37.23, 37.37, and 37.48°C for CL, TH, WH, and HL) were comparable to the post-exercise T _es values for each condition. The relationship between Th_dil and post-exercise T _es remained intact in all T _a/exercise intensity conditions, providing further evidence that the relationship between these two variables is physiological and not coincidental.     

Circadian variation of sweating responses to passive heat stress

The aim of present study was to examine whether sweating responses to passive heat stress change with the circadian rhythm of internal temperature. Six men had their legs immersed in water at 42 °C for 60 min in an ambient temperature of 28 °C on four separate days. Experiments were conducted at four different times [ 06.00 h (morning), 12.00 h (daytime), 18.00 h (evening) and 24.00 h (night)]. We measured oesophageal temperature (T_oes), mean body temperature T¯_b, local sweating rate m˙_sw on the forehead, back, forearm and thigh, the densities of activated sweat gland (ASG) on the back, forearm and thigh, and the frequency of sweat expulsion per minute (F_sw) which has been suggested to represent central sudomotor activity. Sweat gland output (SGO) on each site was calculated by dividing m˙_sw by ASG. ASG was significantly higher on the forearm than on the back and thigh, and SGO was significantly lower on the forearm than on the back and thigh. However, ASG and SGO did not significantly change over the day. T¯_b and T_oes thresholds for the onset of sweating showed a significant change with both the temperature rhythms at rest prior to each procedure, while the slopes of the relationships F_sw– T¯_b and m˙_sw–F_sw showed no significant difference over the day. We suggest that the circadian variation of sweating response to passive heat stress is regulated by a central sudomotor mechanism rather than by sweat gland function.