Effect of pressure on thermal insulation in humans wearing wet suits

The present work was undertaken to determine the critical water temperature (Tcw), defined as the lowest water temperature a subject can tolerate at rest for 3 h without shivering, of wet-suited subjects during water immersion at different ambient pressures. Nine healthy males wearing neoprene wet suits (5 mm thick) were subjected to immersion to the neck in water at 1, 2, and 2.5 ATA while resting for 3 h. Continuous measurements of esophageal (T(es)) and skin (Tsk) temperatures and heat loss from the skin (Htissue) and wet suits (Hsuit) were recorded. Insulation of the tissue (Itissue), wet suits (Isuit), and overall total (Itotal) were calculated from the temperature gradient and the heat loss. The Tcw increased curvilinearly as the pressure increased, whereas the metabolic heat production during rest and immersion was identical over the range of pressure tested. During the 3rd h of immersion, Tes was identical under all atmospheric pressures; however, Tsk was significantly higher (P less than 0.05) at 2 and 2.5 ATA compared with 1 ATA. A 42 (P less than 0.001) and 50% (P less than 0.001), reduction in Isuit from the 1 ATA value was detected at 2 and 2.5 ATA, respectively. However, overall mean Itissue was maximal and independent of the pressure during immersion at Tcw. The Itotal was also significantly smaller in 2 and 2.5 ATA compared with 1 ATA. The Itissue provided most insulation in the extremities, such as the hand and foot, and the contribution of Isuit in these body parts was relatively small. On the other hand, Itissue of the trunk areas, such as the chest, back, and thigh, was not high compared with the extremities, and Isuit played a major role in the protection of heat drain from these body parts.     

Water temperature and intensity of exercise in maintenance of thermal equilibrium

This study examined the thermal and metabolic responses of six men during exercise in water at critical temperature (Tcw, 31.2 +/- 0.5 degrees C), below Tcw (BTcw, 28.8 +/- 0.6 degrees C), at thermoneutrality (Ttn, 34 degrees C), and above Ttn (ATtn, 36 degrees C). At each water temperature (Tw) male volunteers wearing only swimming trunks completed four 1-h experiments while immersed up to the neck. During one experiment, subjects remained at rest (R), and the other three performed leg exercise (LE) at three different intensities (LE-1, 2 MET; LE-2, 3 MET; LE-3, 4 MET). In water warmer than Tcw, there was no difference in metabolic rate (M) during R. The M for each work load was independent of Tw. Esophageal temperature (Tes) remained unchanged during R in water of ATtn (36 degrees C). However, Tes significantly (P less than 0.05) declined over 1 h during R at Ttn (delta Tes = -0.39 degrees C), Tcw (delta Tes = -0.54 degrees C), and BTcw (delta Tes = -0.61 degrees C). All levels of underwater exercise elevated Tes and M compared with R at all Tw. In water colder than Tcw, the ratio of heat loss from limbs compared with the trunk became greater as LE intensity increased, indicating a preferential increase in heat loss from the limbs in cool water. Tissue insulation (Itissue) was lower during LE than at R and was inversely proportional to the increase in LE intensity. A linearly inverse relationship was established between Tw and M in maintaining thermal equilibrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Critical water temperature during water immersion at various atmospheric pressures

The present work was undertaken to determine the effect of atmospheric pressure [ranging from a high altitude of 4,300 m above sea level or 0.6 atmospheres absolute (ATA) to depths of 10 m deep or 2 ATA] on the critical water temperature (Tcw), defined as the lowest water temperature a subject can tolerate at rest for 2 h without shivering, of the unprotected subject during water immersion. Nine healthy males wearing only shorts were subjected to immersion to the neck in water at 0.6, 1, and 2 ATA while resting for 2 h. Continuous measurements included esophageal (Tes) and skin (Tsk) temperatures, direct heat loss from the skin (Htissue), and insulation of the tissue (Itissue). The Tcw was significantly higher at 0.6 ATA than 1 and 2 ATA: however, Tcw at 1 ATA was identical to that at 2 ATA. The metabolic heat production remained unchanged among the pressures. During the 2-h immersion in Tcw, Tes was identical among all atmospheric pressures: however, Tsk was significantly higher (P less than 0.05) at 0.6 ATA and was identical between 1 and 2 ATA. The overall mean Itissue was near maximal during immersion in Tcw in each pressure, and no difference was detected among the pressures. However, Itissue at the acral extremities (arm, hand, and foot) decreased significantly at 0.6 ATA, and subsequently heat loss from these parts was increased, which elevated an extremity-to-trunk heat loss ratio to 1.4 at 0.6 ATA from 1.1 at 1 and 2 ATA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in thermal insulation during underwater exercise in Korean female wet-suit divers

The present work was undertaken to examine the effect of wet suits on the pattern of heat exchange during immersion in cold water. Four Korean women divers wearing wet suits were immersed to the neck in water of critical temperature (Tcw) while resting for 3 h or exercising (2-3 met on a bicycle ergometer) for 2 h. During immersion both rectal (Tre) and skin temperatures and O2 consumption (VO2) were measured, from which heat production (M = 4.83 VO2), skin heat loss (Hsk = 0.92 M +/- heat store change based on delta Tre), and thermal insulation were calculated. The average Tcw of the subjects with wet suits was 16.5 +/- 1.2 degrees C (SE), which was 12.3 degrees C lower than that of the same subjects with swim suits (28.8 +/- 0.4 degrees C). During the 3rd h of immersion, Tre and mean skin temperatures (Tsk) averaged 37.3 +/- 0.1 and 28.0 +/- 0.5 degrees C, and skin heat loss per unit surface area 42.3 +/- 2.66 kcal X m-2 X h. The calculated body insulation [Ibody = Tre - Tsk/Hsk] and the total shell insulation [Itotal = (Tre - TW)/Hsk] were 0.23 +/- 0.02 and 0.5 +/- 0.04 degrees C X kcal-1 X m2 X h, respectively. During immersion exercise, both Itotal and Ibody declined exponentially as the exercise intensity increased. Surprisingly, the insulation due to wet suit (Isuit = Itotal - Ibody) also decreased with exercise intensity, from 0.28 degrees C X kcal-1 X m2 X h at rest to 0.12 degrees C X kcal-1 X m2 X h at exercise levels of 2-3 met.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of body temperature on the development of fatigue during prolonged exercise in the heat.

We investigated whether fatigue during prolonged exercise in uncompensable hot environments occurred at the same critical level of hyperthermia when the initial value and the rate of increase in body temperature are altered. To examine the effect of initial body temperature [esophageal temperature (Tes) = 35.9 +/- 0.2, 37.4 +/- 0. 1, or 38.2 +/- 0.1 (SE) degrees C induced by 30 min of water immersion], seven cyclists (maximal O2 uptake = 5.1 +/- 0.1 l/min) performed three randomly assigned bouts of cycle ergometer exercise (60% maximal O2 uptake) in the heat (40 degrees C) until volitional exhaustion. To determine the influence of rate of heat storage (0.10 vs. 0.05 degrees C/min induced by a water-perfused jacket), four cyclists performed two additional exercise bouts, starting with Tes of 37.0 degrees C. Despite different initial temperatures, all subjects fatigued at an identical level of hyperthermia (Tes = 40. 1-40.2 degrees C, muscle temperature = 40.7-40.9 degrees C, skin temperature = 37.0-37.2 degrees C) and cardiovascular strain (heart rate = 196-198 beats/min, cardiac output = 19.9-20.8 l/min). Time to exhaustion was inversely related to the initial body temperature: 63 +/- 3, 46 +/- 3, and 28 +/- 2 min with initial Tes of approximately 36, 37, and 38 degrees C, respectively (all P < 0.05). Similarly, with different rates of heat storage, all subjects reached exhaustion at similar Tes and muscle temperature (40.1-40.3 and 40. 7-40.9 degrees C, respectively), but with significantly different skin temperature (38.4 +/- 0.4 vs. 35.6 +/- 0.2 degrees C during high vs. low rate of heat storage, respectively, P < 0.05). Time to exhaustion was significantly shorter at the high than at the lower rate of heat storage (31 +/- 4 vs. 56 +/- 11 min, respectively, P < 0.05). Increases in heart rate and reductions in stroke volume paralleled the rise in core temperature (36-40 degrees C), with skin blood flow plateauing at Tes of approximately 38 degrees C. These results demonstrate that high internal body temperature per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Furthermore, time to exhaustion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.     

Effect of work load on cutaneous vascular response to exercise.

The purpose of the present study was to examine whether intensity of exercise affects skin blood flow response to exercise. For this purpose, six healthy men cycled, in a random order on different days, for 15 min at 50, 60, 70, 80, and 90% of their maximum oxygen consumption (VO2max) at a room temperature of 25 degrees C. At the end of exercise, esophageal temperature (Tes) averaged 37.4 +/- 0.2, 37.7 +/- 0.2, 37.9 +/- 0.2, 38.6 +/- 0.3, and 38.9 +/- 0.4 degrees C (SE) at the 50, 60, 70, 80, and 90% work loads, respectively. At the two highest work loads, no steady state was observed in Tes. Skin blood flow was estimated by measuring forearm blood flow (FBF) with strain-gauge plethysmography and by laser-Doppler flowmetry on the upper back. Both techniques showed that skin blood flow response to rising Tes was markedly reduced at the 90% work load compared with other work loads. At the end of exercise, FBF averaged 7.5 +/- 1.7, 10.7 +/- 3.1, 9.6 +/- 2.1, 11.3 +/- 2.6, and 5.4 +/- 1.3 (SE) ml.min-1.100 ml-1 (P less than 0.01) at the 50, 60, 70, 80, and 90% VO2max work loads, respectively. The corresponding values for Tes threshold for cutaneous vasodilation (FBF) were 37.42 +/- 0.16, 37.48 +/- 0.13, 37.59 +/- 0.13, 37.79 +/- 0.19, and 38.20 +/- 0.22 degrees C (P less than 0.05) at 50, 60, 70, 80, and 90% VO2max, respectively. In two subjects, no cutaneous vasodilation was observed at the 90% work load.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estrogen replacement in middle-aged women: thermoregulatory responses to exercise in the heat.

Thermoregulatory, cardiovascular, and body fluid responses during exercise in the heat were tested in five middle-aged (48 +/- 2 yr) women before and after 14-23 days of estrogen replacement therapy (ERT). The heat and exercise challenge consisted of a 40-min rest period followed by semirecumbent cycle exercise (approximately 40% maximal O2 uptake) for 60 min. At rest, the ambient temperature was elevated from a thermoneutral (dry bulb temperature 25 degrees C; wet bulb temperature 17.5 degrees C) to a warm humid (dry bulb temperature 36 degrees C; wet bulb temperature 27.5 degrees C) environment. Esophageal (Tes) and rectal (Tre) temperatures were measured to estimate body core temperature while arm blood flow and sweating rate were measured to assess the heat loss response. Mean arterial pressure and heart rate were measured to evaluate the cardiovascular response. Blood samples were analyzed for hematocrit (Hct), hemoglobin ([Hb]), plasma 17 beta-estradiol (E2), progesterone (P4), protein, and electrolyte concentrations. Plasma [E2] was significantly (P < 0.05) elevated by ERT without affecting the plasma [P4] levels. After ERT, Tes and Tre were significantly (P < 0.05) depressed by approximately 0.5 degrees C, and the Tes threshold for the onset of arm blood flow and sweating rate was significantly (P < 0.05) lower during exercise. After ERT, heart rate during exercise was significantly lower (P < 0.05) without notable variation in mean arterial pressure. Isotonic hemodilution occurred with ERT evident by significant (P < 0.05) reductions in Hct and [Hb], whereas plasma tonicity remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rectal and rectal vs. esophageal temperatures in paraplegic men during prolonged exercise

This study investigated the rectal (Tre), esophageal (Tes), and skin (Tsk) temperature changes in a group of trained traumatic paraplegic men pushing their own wheelchairs on a motor-driven treadmill for a prolonged period in a neutral environment. There were two experiments. The first experiment (Tre and Tsk) involved a homogeneous group (T10-T12/L3) of highly trained paraplegic men [maximum O2 uptake (VO2max) 47.5 +/- 1.8 ml.kg-1.min-1] exercising for 80 min at 60-65% VO2max.Tre and Tsk (head, arm, thigh, and calf) and heart rate (HR) were recorded throughout. O2 uptake (VO2), minute ventilation (VE), CO2 production (VCO2), and heart rate (HR) were recorded at four intervals. During experiment 1 significant changes in HR and insignificant changes in VCO2, VE, and VO2 occurred throughout prolonged exercise. Tre increased significantly from 37.1 +/- 0.1 degrees C (rest) to 37.8 +/- 0.1 degrees C after 80 min of exercise. There were only significant changes in arm Tsk. Experiment 2 involved a nonhomogeneous group (T5-T10/T11) of active paraplegics (VO2max 39.9 +/- 4.3 ml.kg-1.min-1) exercising at 60-65% VO2max for up to 45 min on the treadmill while Tre and Tes were simultaneously recorded. Tes rose significantly faster than Tre during exercise (dT/dt 20 min: Tes 0.050 +/- 0.003 degrees C/min and Tre 0.019 +/- 0.005 degrees C/min), and Tes declined significantly faster than Tre at the end of exercise. Tes was significantly higher than Tre at the end of exercise. Our results suggest that during wheelchair propulsion by paraplegics, Tes may be a better estimate of core temperature than Tre.     

Effect of positive-pressure breathing on cardiovascular and thermoregulatory responses to exercise

Five healthy male volunteers performed 20 min of both seated and supine cycle-ergometer exercise (intensity, 50% maximal O2 uptake) in a warm environment (Tdb = 30 degrees C, relative humidity = 40-50%) with and without breathing 10 cmH2O of continuous positive airway pressure (CPAP). The final esophageal temperature (Tes) at the end of 20 min of seated exercise was significantly higher during CPAP (mean difference = 0.18 +/- 0.04 degree C, P less than 0.05) compared with control breathing (C). The Tes threshold for forearm vasodilation was significantly higher (P less than 0.05) during seated CPAP exercise than C (C = 37.16 +/- 0.13 degrees C, CPAP = 37.38 + 0.12 degree C). The highest forearm blood flow (FBF) at the end of exercise was significantly lower (P less than 0.05) during seated exercise with CPAP (mean +/- SE % difference from C = -30.8 +/- 5.8%). During supine exercise, there were no significant differences in the Tes threshold, highest FBF, or final Tes with CPAP compared with C. The added strain on the cardiovascular system produced by CPAP during seated exercise in the heat interacts with body thermoregulation as evidenced by elevated vasodilation thresholds, reduced peak FBF, and slightly higher final esophageal temperatures.     

Core temperature "null zone".

An experimental protocol was designed to investigate whether human core temperature is regulated at a "set point" or whether there is a neutral zone between the core thresholds for shivering thermogenesis and sweating. Nine male subjects exercised on an underwater cycle ergometer at a work rate equivalent to 50% of their maximum work rate. Throughout an initial 2-min rest period, the 20-min exercise protocol, and the 100-min recovery period, subjects remained immersed to the chin in water maintained at 28 degrees C. On completion of the exercise, the rate of forehead sweating (Esw) decayed from a mean peak value of 7.7 +/- 4.2 (SD) to 0.6 +/- 0.3 g.m-2.min-1, which corresponds to the rate of passive transpiration, at core temperatures of 37.42 +/- 0.29 and 37.39 +/- 0.48 degrees C, as measured in the esophagus (Tes) and rectum (Tre), respectively. Oxygen uptake (VO2) decreased rapidly from an exercising level of 2.11 +/- 0.25 to 0.46 +/- 0.09 l/min within 4 min of the recovery period. Thereafter, VO2 remained stable for approximately 20 min, eventually increased with progressive cooling of the core region, and was elevated above the median resting values determined between 15 and 20 min at Tes = 36.84 +/- 0.38 degrees C and Tre = 36.80 +/- 0.39 degrees C. These results indicate that the core temperatures at which sweating ceases and shivering commences are significantly different (P less than 0.001) regardless of whether core temperature is measured within the esophagus or rectum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of body mass and morphology on thermal responses in water

Ten male volunteers were divided into two groups based on body morphology and mass. The large-body mass (LM) group (n = 5) was 16.3 kg heavier and 0.22 cm2 X kg-1 X 10(-2) smaller in surface area-to-mass ratio (AD X wt-1) (P less than 0.05) than the small-body mass (SM) group (n = 5). Both groups were similar in total body fat and skinfold thicknesses (P greater than 0.05). All individuals were immersed for 1 h in stirred water at 26 degrees C during both rest and one intensity of exercise (metabolic rate approximately 550 W). During resting exposures metabolic rate (M) and rectal temperature (Tre) were not different (P greater than 0.05) between the LM and SM groups at min 60. Esophageal temperature (Tes) was higher (P less than 0.05) for the SM group at min 60, although the change in Tes during the 60 min between groups was similar (LM, -0.4 degrees C; SM, -0.2 degrees C). Tissue insulation (I) was lower (P less than 0.05) for SM (0.061 degrees C X m-2 X W-1) compared with the LM group (0.098 degrees C X m-2 X W-1). During exercise M, Tre, Tes, and I were not different (P greater than 0.05) between groups at min 60. These data illustrate that a greater body mass between individuals increases the overall tissue insulation during rest, most likely as a result of a greater volume of muscle tissue to provide insulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma volume during heat stress and exercise in women

Five women were studied during exercise and passive heating to determine whether PV dynamics were affected by the menstrual cycle. The exercise bout (80% VO2 peak) on a modified cycle ergometer and the passive heat stress were done in a hot environment (Ta = 50 degrees C, Pw = 1.61 kPa) during the follicular and luteal phase. Esophageal temperature (Tes) was measured continuously. Blood samples were drawn after each 0.2 degree C increase in Tes and VO2 was measured at that time. Initial PV was estimated at rest during the follicular phase. PV changes from rest were calculated at each Tes from Hb and Hct. During passive heating, PV decreased by a mean volume of 156 (+/- 80) ml to 2.83 (+/- 0.09) l in the follicular phase. During the luteal phase, there was a larger volume reduction (300 +/- 100 ml) during passive heating, and the final PV was lower than in the follicular phase and averaged 2.47 +/- 0.18 l. During exercise, PV decreased 463 (+/- 90) ml to 2.50 (+/- 0.11) l in the follicular and 381 (+/- 70) ml to 2.50 (+/- 0.23) l in the luteal phase. These data indicate that there is a menstrual cycle effect on PV dynamics during passive heating such that more fluid is shifted out of the vasculature during the luteal phase. During severe exercise there is a greater fluid loss during the follicular phase, yet the final PV is not different between phases.     

Effects of age and exercise on physiological dead space during simulated dives at 2.8 ATA.

Physiological dead space (Vds), end-tidal CO(2) (Pet(CO(2))), and arterial CO(2) (Pa(CO(2))) were measured at 1 and 2.8 ATA in a dry hyperbaric chamber in 10 older (58-74 yr) and 10 younger (19-39 yr) air-breathing subjects during rest and two levels of upright exercise on a cycle ergometer. At pressure, Vd (liters btps) increased from 0.34 +/- 0.09 (mean +/- SD of all subjects for normally distributed data, median +/- interquartile range otherwise) to 0.40 +/- 0.09 (P = 0.0060) at rest, 0.35 +/- 0.13 to 0.45 +/- 0.11 (P = 0.0003) during light exercise, and 0.38 +/- 0.17 to 0.45 +/- 0.13 (P = 0.0497) during heavier exercise. During these conditions, Pa(CO(2)) (Torr) increased from 33.8 +/- 4.2 to 35.7 +/- 4.4 (P = 0.0059), 35.3 +/- 3.2 to 39.4 +/- 3.1 (P < 0.0001), and 29.6 +/- 5.6 to 37.4 +/- 6.5 (P < 0.0001), respectively. During exercise, Pet(CO(2)) overestimated Pa(CO(2)), although the absolute difference was less at pressure. Capnography poorly estimated Pa(CO(2)) during exercise at 1 and 2.8 ATA because of wide variability. Older subjects had higher Vd at 1 ATA but similar changes in Vd, Pa(CO(2)), and Pet(CO(2)) at pressure. These results are consistent with an effect of increased gas density.     

No evidence for brain stem cooling during face fanning in humans.

The interpeak latencies (IPLs) of the acoustically evoked brain stem potentials depend on brain stem temperature. This was used to see whether face fanning during hyperthermia lowers brain stem temperature. In 15 subjects, three thermally stable conditions were maintained by a water bath. In each condition the IPLs were determined in 10 separate trials. In condition A esophageal temperature (Tes) was 36.9 +/- 0.3 degrees C and increased to 38.6 +/- 0.2 degrees C in condition B. In conditions A and B the head was enclosed in a ventilated hood (air temperature 38 degrees C, relative humidity 100%) to suppress any direct heat loss from the head. From conditions A to B the IPL at peaks I-V decreased by 0.146 ms/degrees C change in Tes, reflecting a change in brain stem temperature. In condition C the hood was removed and the face was fanned by a cold air-stream (8-15 degrees C, 4-10 m/s) to maximize direct heat loss from the head. Skin temperature at the sweating forehead decreased from 38 to 23 degrees C, whereas Tes in condition C was maintained at the same level as in condition B (38.5 +/- 0.2 degrees C). The IPL at peaks I-V showed no difference between conditions B and C. It is concluded that face fanning in hyperthermic subjects does not dissociate brain stem temperature from Tes.     

Effect of beta-adrenergic blockade on thermoregulation during exercise

To resolve conflicting reports concerning the effects of beta-blockade (BB) on thermoregulatory reflexes during exercise, we studied six fit men during 40 min of cycle ergometer exercise at 60% maximum O2 consumption at ambient temperatures of 22 and 32 degrees C. Two hours before exercise, each subject ingested a capsule containing either 80 mg of propranolol or placebo in single-blind fashion. Heart rate at 40 min of exercise was reduced (P less than 0.01) from 125 to 103 beats min at 22 degrees C and 137 to 104 beats min at 32 degrees C, demonstrating effective BB. After 40 min of exercise, esophageal temperature (Tes) was elevated with BB (P less than 0.05) from 37.66 +/- 0.04 to 38.14 +/- 0.03 and 38.13 +/- 0.04 to 38.41 +/- 0.04 degrees C at 22 and 32 degrees C, respectively. The elevated Tes resulted from a reduced core-to-skin heat flux at both temperatures, indicated by a reduction in the slope of the forearm blood flow (FBF)-Tes relationship, and a decrease in maximal FBF. Systolic blood pressure was decreased 20 mmHg with BB (P less than 0.01), whereas diastolic blood pressure was unchanged, reducing arterial pulse pressure (PP). Because PP was decreased and cardiac filling pressure was presumably not reduced (since cardiac stroke volume was elevated), we suggest that at least a part of the relative increase in peripheral vasomotor tone during BB was the consequence of reduced sinoaortic baroreceptor stimulation.     

Effects of human menstrual cycle on thermoregulatory vasodilation during exercise

To investigate the effects of the menstrual cycle and of exercise intensity on the relationship between finger blood flow (FBF) and esophageal temperature (Tes), we studied four women, aged 20-32 years. Subjects exercised at 40% and 70% VO2max in the semi-supine posture at an ambient temperature of 20 degrees C. Resting Tes was higher during the luteal phase than the follicular phase (P less than 0.01). There were no significant differences between the two phases in FBF, oxygen consumption, carbon dioxide production, heart rate or minute ventilation at rest and during exercise, respectively. Each regression line of the FBF-Tes relationship consists of two distinct segments of FBF change to Tes (slope 1 and 2). FBF increased at a threshold Tes for vasodilation ([Tes 0]) and the rate of FBF rise became greater at ([Tes 0]) and the rate of FBF rise became greater at another Tes above this threshold ([Tes 0']). For both levels of exercise, [Tes 0] and [Tes 0'] were shifted upward during the luteal phase, but the slopes of the FBF-Tes relationship were almost the same in the two phases of the menstrual cycle. Increasing exercise intensity induced a significant decrease in slope 1 of the FBF-Tes relationship during the follicular (P less than 0.01) and the luteal phases (P less than 0.02), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heat acclimation increases skin vasodilation and sweating but not cardiac baroreflex responses in heat-stressed humans.

In the present study, to test the hypothesis that exercise-heat acclimation increases orthostatic tolerance via the improvement of cardiac baroreflex control in heated humans, we examined cardiac baroreflex and thermoregulatory responses, including cutaneous vasomotor and sudomotor responses, during whole body heating before and after a 6-day exercise-heat acclimation program [4 bouts of 20-min exercise at 50% peak rate of oxygen uptake separated by 10-min rest in the heat (36 degrees C; 50% relative humidity)]. Ten healthy young volunteers participated in the study. On the test days before and after the heat acclimation program, subjects underwent whole body heat stress produced by a hot water-perfused suit during supine rest for 45 min and 75 degrees head-up tilt (HUT) for 6 min. The sensitivity of the arterial baroreflex control of heart rate (HR) was calculated from the spontaneous changes in beat-to-beat arterial pressure and HR. The HUT induced a presyncopal sign in seven subjects in the preacclimation test and in six subjects in the postacclimation test, and the tilting time did not differ significantly between the pre- (241 +/- 33 s) and postacclimation (283 +/- 24 s) tests. Heat acclimation did not change the slope in the HR-esophageal temperature (Tes) relation and the cardiac baroreflex sensitivity during heating. Heat acclimation decreased (P < 0.05) the Tes thresholds for cutaneous vasodilation in the forearm and dorsal hand and for sweating in the forearm and chest. These findings suggest that short-term heat acclimation does not alter the spontaneous baroreflex control of HR during heat stress, although it induces adaptive change of the heat dissipation response in nonglabrous skin.     

Insulation and body temperature of prepubescent children wearing a thermal swimsuit during moderate-intensity water exercise

This study investigated thermal swimsuits (TSS) effects on body temperature and thermal insulation of prepubescent children during moderate-intensity water exercise. Nine prepubescent children (11.0+/-0.7 yrs) were immersed in water (23 degrees C) and pedalled on an underwater cycle-ergometer for 30 min with TSS or normal swimsuits (NSS). The rectal temperature (Tre) was maintained slightly higher with TSS than with NSS. The total insulation (Itotal) was significantly higher with TSS. The DeltaTre, Deltamean body temperature (Tb), and tissue insulation (Itissue) in the NSS condition were correlated with % body fat, which indicated that the insulation layer of subjects with low body fat was thinner than that of obese subjects, and tended to decrease body temperature. Wearing TSS increased Itotal, thereby reducing heat loss from subjects' skin to the water. Consequently, subjects with TSS were able to maintain higher body temperatures. In addition, TSS is especially advantageous for subjects with low body fat to compensate for the smaller Itissue.     

Control of internal temperature threshold for active cutaneous vasodilation by dynamic exercise.

Exercise induces shifts in the internal temperature threshold at which cutaneous vasodilation begins. To find whether this shift is accomplished through the vasoconstrictor system or the cutaneous active vasodilator system, two forearm sites (0.64 cm2) in each of 11 subjects were iontophoretically treated with bretylium tosylate to locally block adrenergic vasoconstrictor control. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at those sites and at two adjacent untreated sites. Mean arterial pressure (MAP) was measured noninvasively. Cutaneous vascular conductance was calculated as LDF/MAP. Forearm sweat rate was also measured in seven of the subjects by dew point hygrometry. Whole body skin temperature was raised to 38 degrees C, and supine bicycle ergometer exercise was then performed for 7-10 min. The internal temperature at which cutaneous vasodilation began was recorded for all sites, as was the temperature at which sweating began. The same subjects also participated in studies of heat stress without exercise to obtain vasodilator and sudomotor thresholds from rest. The internal temperature thresholds for cutaneous vasodilation were higher during exercise at both bretylium-treated (36.95 +/- 0.07 degrees C rest, 37.20 +/- 0.04 degrees C exercise, P less than 0.05) and untreated sites (36.95 +/- 0.06 degrees C rest, 37.23 +/- 0.05 degrees C exercise, P less than 0.05). The thresholds for cutaneous vasodilation during rest or during exercise were not statistically different between untreated and bretylium-treated sites (P greater than 0.05). The threshold for the onset of sweating was not affected by exercise (P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of thermal responses between rest and leg exercise in water

This study examined both the thermal and metabolic responses of individuals in cool (30 degrees C, n = 9) and cold (18 degrees C, n = 7; 20 degrees C, n = 2) water. Male volunteers were immersed up to the neck for 1 h during both seated rest (R) and leg exercise (LE). In 30 degrees C water, metabolic rate (M) remained unchanged over time during both R (115 W, 60 min) and LE (528 W, 60 min). Mean skin temperature (Tsk) declined (P less than 0.05) over 1 h during R, while Tsk was unchanged during LE. Rectal (Tre) and esophageal (Tes) temperatures decreased (P less than 0.05) during R (delta Tre, -0.5 degrees C; delta Tes, -0.3 degrees C) and increased (P less than 0.05) during LE (delta Tre, 0.4 degrees C; Tsk, 0.4 degrees C). M, Tsk, Tre, and Tes were higher (P less than 0.05) during LE compared with R. In cool water, all regional heat flows (leg, chest, and arm) were generally greater (P less than 0.05) during LE than R. In cold water, M increased (P less than 0.05) over 1 h during R but remained unchanged during LE. Tre decreased (P less than 0.05) during R (delta Tre, -0.8 degrees C) but was unchanged during LE. Tes declined (P less than 0.05) during R (delta Tes, -0.4 degrees C) but increased (P less than 0.05) during LE (delta Tes, 0.2 degrees C). M, Tre, and Tes were higher (P less than 0.05), whereas Tsk was not different during LE compared with R at 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)