Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise

This investigation determined the effect of different rates of dehydration, induced by ingesting different volumes of fluid during prolonged exercise, on hyperthermia, heart rate (HR), and stroke volume (SV). On four different occasions, eight endurance-trained cyclists [age 23 +/- 3 (SD) yr, body wt 71.9 +/- 11.6 kg, maximal O2 consumption 4.72 +/- 0.33 l/min] cycled at a power output equal to 62-67% maximal O2 consumption for 2 h in a warm environment (33 degrees C dry bulb, 50% relative humidity, wind speed 2.5 m/s). During exercise, they randomly received no fluid (NF) or ingested a small (SF), moderate (MF), or large (LF) volume of fluid that replaced 20 +/- 1, 48 +/- 1, and 81 +/- 2%, respectively, of the fluid lost in sweat during exercise. The protocol resulted in graded magnitudes of dehydration as body weight declined 4.2 +/- 0.1, 3.4 +/- 0.1, 2.3 +/- 0.1, and 1.1 +/- 0.1%, respectively, during NF, SF, MF, and LF. After 2 h of exercise, esophageal temperature (Tes), HR, and SV were significantly different among the four trials (P < 0.05), with the exception of NF and SF. The magnitude of dehydration accrued after 2 h of exercise in the four trials was linearly related with the increase in Tes (r = 0.98, P < 0.02), the increase in HR (r = 0.99, P < 0.01), and the decline in SV (r = 0.99, P < 0.01). LF attenuated hyperthermia, apparently because of higher skin blood flow, inasmuch as forearm blood flow was 20–22% higher than during SF and NF at 105 min (P < 0.05). There were no differences in sweat rate among the four trials. In each subject, the increase in Tes from 20 to 120 min of exercise was highly correlated to the increase in serum osmolality (r = 0.81-0.98, P < 0.02-0.19) and the increase in serum sodium concentration (r = 0.87-0.99, P < 0.01-0.13) from 5 to 120 min of exercise. In summary, the magnitude of increase in core temperature and HR and the decline in SV are graded in proportion to the amount of dehydration accrued during exercise.

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Circadian rhythm in sweating and cutaneous blood flow

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1984.246.3.r321 ◽

1984 ◽

Vol 246 (3) ◽

pp. R321-R324 ◽

Cited By ~ 20

Author(s):

L. A. Stephenson ◽

C. B. Wenger ◽

B. H. O'Donovan ◽

E. R. Nadel

Keyword(s):

Blood Flow ◽

Circadian Rhythm ◽

Body Temperature ◽

Forearm Blood Flow ◽

Sweat Rate ◽

Circadian Variation ◽

Esophageal Temperature ◽

Cutaneous Blood Flow ◽

Cutaneous Vasodilation ◽

Cutaneous Blood

To characterize the changes in the control of the heat loss responses associated with the circadian variation in body temperature, we studied five men during 20 min of exercise in 25 degrees C on 6 separate days. Experiments were conducted at six times, equally spaced over the 24-h day. Esophageal temperature (Tes) and chest sweat rate (msw) were measured continuously, and forearm blood flow (FBF) was measured one to two times per minute. The thresholds for sweating and forearm vasodilation were significantly higher at 1600 and 2000 than at 2400 and 0400, averaging 0.57 and 0.65 degrees C higher, respectively, at 1600 than at 0400. Resting Tes and the Tes thresholds for cutaneous vasodilation and sweating during exercise all showed a similar circadian rhythm. The level at which core temperature is regulated therefore varies over the 24-h day with the zenith occurring around 1600 and the nadir at 0400. However, whereas the slope of the msw-to-Tes relation did not change over the 24-h day, the slope of the FBF-to-Tes relation tended to increase between 0400 and 2400, implying that the circadian rhythm may be more complex than just a shift in the central reference temperature.

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Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume

Journal of Applied Physiology ◽

10.1152/jappl.1992.73.3.903 ◽

1992 ◽

Vol 73 (3) ◽

pp. 903-910 ◽

Cited By ~ 126

Author(s):

S. J. Montain ◽

E. F. Coyle

Keyword(s):

Blood Flow ◽

Oxygen Uptake ◽

Blood Volume ◽

Skin Blood Flow ◽

Forearm Blood Flow ◽

Sodium Concentration ◽

Maximum Oxygen Uptake ◽

Fluid Replacement ◽

Fluid Ingestion ◽

Cardiovascular Drift

The purpose of this experiment was to determine whether fluid ingestion attenuates the hyperthermia and cardiovascular drift that occurs during exercise dehydration due to increases in blood volume. In addition, forearm blood flow, which is indicative of skin blood flow, was measured to determine whether the attenuation of hyperthermia and cardiovascular drift during exercise with fluid ingestion is due to higher skin blood flow. On three different occasions, seven trained cyclists [mean age, body weight, and maximum oxygen uptake: 23 +/- 3 yr, 73.9 +/- 10.5 kg, and 4.75 +/- 0.34 (SD) l/min, respectively] cycled at a power output equal to 62–67% maximum oxygen uptake for 2 h in a warm environment (33 degrees C, 50% relative humidity, wind speed 2.5 m/s). During exercise, they randomly received no fluid (NF) or a volume of a carbohydrate-electrolyte fluid replacement solution (FR) sufficient to replace 80 +/- 2% of sweat loss or were intravenously infused with 5.3 ml/kg of a blood volume expander (BVX; 6% dextran in saline). The infusion of 398 +/- 23 ml of BVX maintained blood volume at levels similar to that when 2,404 +/- 103 ml of fluid were ingested during FR and greater than that when no fluid was ingested during the 2nd h of exercise (P less than 0.05). However, BVX and NF resulted in similar esophageal and rectal temperatures, forearm blood flow, and elevations in serum osmolality and sodium concentration during 2 h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

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Reflex regulation of sweat rate by skin temperature in exercising humans

Journal of Applied Physiology ◽

10.1152/jappl.1984.56.5.1283 ◽

1984 ◽

Vol 56 (5) ◽

pp. 1283-1288 ◽

Cited By ~ 8

Author(s):

J. M. Johnson ◽

D. S. O'Leary ◽

W. F. Taylor ◽

M. K. Park

Keyword(s):

Blood Flow ◽

Regression Analysis ◽

Linear Regression ◽

Skin Temperature ◽

Sweat Rate ◽

Linear Regression Analysis ◽

Multiple Linear Regression Analysis ◽

Esophageal Temperature ◽

Reflex Regulation ◽

Forearm Skin

To find whether sweat rate (SR) and forearm skin blood flow ( SkBF ) were reflexly affected by skin temperature (Tsk) we used water-perfused suits to rapidly elevate Tsk during exercise. With this elevation in Tsk, there was a period of little net change in esophageal temperature (Tes) but marked responses in SR and SkBF . During this period a rise in Tsk of 4.2 +/- 0.3 degrees C was associated with an increase in SR of 0.44 +/- 0.09 mg X cm-2 X min-1 and an increase in SkBF of 3.27 +/- 0.42 ml X 100 ml-1 X min-1. Multiple linear regression analysis as well as comparison with control studies in which Tsk was kept cool also reveal a consistent role for Tsk in the reflex regulation of SR and SkBF . Responses in SR and FBF were much more marked at levels of Tsk below 33 degrees C. Below a Tsk of 33 degrees C, SR rose 0.30 +/- 0.06 mg X cm-2 X min-1 per degrees C rise in Tsk, whereas above 33 degrees SR rose only 0.05 +/- 0.01 mg X cm2 X min per degrees C. FBF rose 2.81 +/- 0.60 and 0.77 +/- 0.18 ml X 100 ml-1 X min-1 per degrees C rise in Tsk at the lower and upper ranges of Tsk, respectively.

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Effect of exercise hemoconcentration and hyperosmolality on exercise responses

Journal of Applied Physiology ◽

10.1152/jappl.1988.65.2.519 ◽

1988 ◽

Vol 65 (2) ◽

pp. 519-524 ◽

Cited By ~ 25

Author(s):

S. M. Fortney ◽

N. B. Vroman ◽

W. S. Beckett ◽

S. Permutt ◽

N. D. LaFrance

Keyword(s):

Heart Rate ◽

Blood Flow ◽

Relative Humidity ◽

Plasma Osmolality ◽

Esophageal Temperature ◽

O2 Uptake ◽

Sweating Rate ◽

Cycle Exercise ◽

Short Term ◽

Warm Environment

We investigated the effects of a decrease in plasma volume (PV) and an increase in plasma osmolality during exercise on circulatory and thermoregulatory responses. Six subjects cycled at approximately 65% of their maximum O2 uptake in a warm environment (30 degrees C, 40% relative humidity). After 30 min of control (C) exercise (no infusion), PV decreased 13.0%, or 419 +/- 106 (SD) ml, heart rate (HR) increased to 167 +/- 3 beats/min, and esophageal temperature (Tes) rose to 38.19 +/- 0.09 degrees C (SE). During infusion studies (INF), infusates were started after 10 min of exercise. The infusates contained 5% albumin suspended in 0.45, 0.9, or 3.0% saline. The volume of each infusate was adjusted so that during the last 10 min of exercise PV was maintained at the preexercise level and osmolality was allowed to differ. HR was significantly lower (10-16 beats/min) during INF than during C. Tes was reduced significantly during INF, with trends for increased skin blood flow and decreased sweating rates. No significant differences in HR, Tes, or sweating rate occurred between the three infusion conditions. We conclude that the decrease in PV, which normally accompanies moderate cycle exercise, compromises circulatory and thermal regulations. Increases in osmolality appear to have small if any effects during such short-term exercise.

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Cutaneous blood flow during exercise is higher in endurance-trained humans

Journal of Applied Physiology ◽

10.1152/jappl.2000.88.2.738 ◽

2000 ◽

Vol 88 (2) ◽

pp. 738-744 ◽

Cited By ~ 56

Author(s):

Ricardo G. Fritzsche ◽

Edward F. Coyle

Keyword(s):

Blood Flow ◽

Oxygen Uptake ◽

Forearm Blood Flow ◽

Peak Oxygen Uptake ◽

Metabolic Rates ◽

Esophageal Temperature ◽

Cutaneous Blood Flow ◽

Cycling Exercise ◽

C 50 ◽

Cutaneous Blood

This study determined whether cutaneous blood flow during exercise is different in endurance-trained (Tr) compared with untrained (Untr) subjects. Ten Tr and ten Untr men (62.4 ± 1.7 and 44.2 ± 1.8 ml ⋅ kg− 1 ⋅ min− 1, respectively; P < 0.05) underwent three 20-min cycling-exercise bouts at 50, 70, and 90% peak oxygen uptake in this order, with 30 min rest in between. The environmental conditions were neutral (∼23–24°C, 50% relative humidity, front and back fans at 2.5 m/s). Because of technical difficulties, only seven Tr and seven Untr subjects completed all forearm blood flow and laser-Doppler cutaneous blood flow (CBF) measurements. Albeit similar at rest, at the end of all three exercise bouts, forearm blood flow was ∼40% higher in Tr compared with Untr subjects (50%: 4.64 ± 0.50 vs. 3.17 ± 0.20, 70%: 6.17 ± 0.61 vs. 4.41 ± 0.37, 90%: 6.77 ± 0.62 vs. 5.01 ± 0.37 ml ⋅ 100 ml− 1 ⋅ min− 1, respectively; n = 7; all P < 0.05). CBF was also higher in Tr compared with Untr subjects at all relative intensities ( n = 7; all P < 0.05). However, esophageal temperature was not different in Tr compared with Untr subjects at the end of any of the aforementioned exercise bouts (50%: 37.8 ± 0.1 vs. 37.9 ± 0.1°C, 70%: 38.1 ± 0.1 vs. 38.1 ± 0.1°C, and 90%: 38.8 ± 0.1 vs. 38.6 ± 0.1°C, respectively). We conclude that a higher CBF may allow Tr subjects to achieve an esophageal temperature similar to that of Untr, despite their higher metabolic rates and thus higher heat production rates, during exercise at 50–90% peak oxygen uptake.

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Hypohydration affects forearm vascular conductance independent of heart rate during exercise

Journal of Applied Physiology ◽

10.1152/jappl.1992.73.4.1232 ◽

1992 ◽

Vol 73 (4) ◽

pp. 1232-1237 ◽

Cited By ~ 9

Author(s):

C. G. Tankersley ◽

D. H. Zappe ◽

T. G. Meister ◽

W. L. Kenney

Keyword(s):

Heart Rate ◽

Blood Flow ◽

Mean Arterial Pressure ◽

Forearm Blood Flow ◽

Body Core Temperature ◽

Esophageal Temperature ◽

Vascular Conductance ◽

Cutaneous Vasodilation ◽

Forearm Vascular Conductance ◽

Body Core

Elevated body core temperature stimulates cutaneous vasodilation, which can be modified by nonthermal factors. To test whether hypohydration affects forearm vascular conductance discretely from relative alterations in heart rate (HR), eight trained cyclists exercised progressively for 20 min each at 60, 120, and 180 W [approximately 22, 37, and 55% of maximal cycling O2 consumption (VO2peak), respectively] in a warm humid environment (dry bulb temperature 30 degrees C; wet bulb temperature 24 degrees C). Esophageal temperature and forearm blood flow were measured every 30 s, and mean arterial pressure and HR were measured at rest and during each exercise intensity (minutes 15, 35, and 55). In the hypovolemic (HP) compared with the euvolemic (EU) state, blood volume was contracted by 24-h fluid restriction an average of 510 ml, and this difference was sustained throughout exercise. The esophageal temperature and HR responses were similar between EU and HP states at 60 and 120 W but were significantly (P < 0.05) higher in HP by the end of 180 W. In contrast, the forearm blood flow response was significantly (P < 0.05) depressed during exercise at 120 and 180 W in HP, whereas mean arterial pressure remained similar between conditions. When body core temperature is elevated in a hypohydrated state, forearm vascular conductance is reduced at exercise intensities of approximately 37% VO2peak, which is independent of relative changes in HR. These findings are consistent with the notion that during exercise an attenuated cutaneous vasodilation is elicited by alterations in regionalized sympathetic outflow, which is unaccompanied by activation of cardiac pacemaker cells.

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Sweating and skin blood flow during exercise: effects of age and maximal oxygen uptake

Journal of Applied Physiology ◽

10.1152/jappl.1991.71.1.236 ◽

1991 ◽

Vol 71 (1) ◽

pp. 236-242 ◽

Cited By ~ 77

Author(s):

C. G. Tankersley ◽

J. Smolander ◽

W. L. Kenney ◽

S. M. Fortney

Keyword(s):

Blood Flow ◽

Heat Loss ◽

Sweat Rate ◽

Weighted Average ◽

Exercise Bout ◽

Physically Active ◽

Warm Environment ◽

Age Related ◽

Repeated Exercise ◽

O2 Pulse

Individuals greater than or equal to 60 yr of age are more susceptible to hyperthermia than younger people. However, the mechanisms involved remain unclear. To gain further insight, we examined the heat loss responses of 7 young (24–30 yr) and 13 older (58–74 yr) men during 20 min of cycle exercise [67.5% maximal O2 uptake (VO2max)] in a warm environment (30 degrees C, 55% relative humidity). Forearm blood flow (FBF) and chest sweat rate (SR) were plotted as a function of the weighted average of mean skin and esophageal temperatures [Tes(w)] during exercise. The sensitivity and threshold for each response were defined as the slope and Tes(w) at the onset of the response, respectively. When the young sedentary men were compared with a subgroup (n = 7) of the older physically active men with similar VO2max, the SR and FBF responses of the two groups did not differ significantly. However, when the young men were compared with a subgroup of older sedentary men with a similar maximal O2 pulse, the SR and FBF sensitivities were significantly reduced by 62 and 40%, respectively. These findings suggest that during a short exercise bout either 1) there is no primary effect of aging on heat loss responses but, rather, changes are associated with the age-related decrease in VO2max or 2) the decline in heat loss responses due to aging may be masked by repeated exercise training.

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Effects of airflow and work load on cardiovascular drift and skin blood flow

Journal of Applied Physiology ◽

10.1152/jappl.1984.56.5.1411 ◽

1984 ◽

Vol 56 (5) ◽

pp. 1411-1417 ◽

Cited By ~ 36

Author(s):

J. D. Shaffrath ◽

W. C. Adams

Keyword(s):

Heart Rate ◽

Blood Flow ◽

Skin Blood Flow ◽

Prolonged Exercise ◽

Work Load ◽

Progressive Increase ◽

Cycle Ergometry ◽

O2 Uptake ◽

Mean Values ◽

Cardiovascular Drift

Cardiovascular drift (CVD) can be defined as a progressive increase in heart rate (HR), decreases in stroke volume (SV) and mean arterial pressure (MAP), and a maintained cardiac output (Q) during prolonged exercise. To test the hypothesis that the magnitude of CVD would be related to changes in skin blood flow ( SkBF ), eight healthy, moderately trained males performed 70-min bouts of cycle ergometry in a 2 X 2 assortment of airflows (less than 0.2 and 4.3 m X s-1) and relative work loads (43.4% and 62.2% maximal O2 uptake). Ambient temperature and relative humidity were controlled to mean values of 24.2 +/- 0.8 degrees C and 39.5 +/- 2.4%, respectively. Q, HR, MAP, SkBF , skin and rectal temperatures, and pulmonary gas exchange were measured at 10-min intervals during exercise. Between the 10th and 70th min during exercise at the higher work load with negligible airflow, HR and SkBF increased by 21.6 beats X min-1 and 14.0 ml X 100 ml-1 X min-1, respectively, while SV and MAP decreased by 16.4 ml and 11.3 mmHg. The same work load in the presence of 4.3 m X s-1 airflow resulted in nonsignificant changes of 7.6 beats X min-1, 4.0 ml X (100 ml-1 X min)-1, -2.7 ml, and -1.7 mmHg for HR, SkBF , SV, and MAP. Since nonsignificant changes in HR, SkBF , SV, and MAP were observed at the lower work load in both airflow conditions, the results emphasize that CVD occurs only in conditions which combine high metabolic and thermal circulatory demands.(ABSTRACT TRUNCATED AT 250 WORDS)

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Effect of hyperosmolality on control of blood flow and sweating

Journal of Applied Physiology ◽

10.1152/jappl.1984.57.6.1688 ◽

1984 ◽

Vol 57 (6) ◽

pp. 1688-1695 ◽

Cited By ~ 161

Author(s):

S. M. Fortney ◽

C. B. Wenger ◽

J. R. Bove ◽

E. R. Nadel

Keyword(s):

Blood Flow ◽

Maximal Exercise ◽

Forearm Blood Flow ◽

Fluid Restriction ◽

O2 Consumption ◽

Rest Interval ◽

Ambient Conditions ◽

Exercise Tests ◽

Sweating Threshold ◽

Humidity Environment

To study the effect of hyperosmolality on thermoregulatory responses, five men [average maximal O2 consumption (VO2 max) = 48 ml X kg-1 X min-1] cycled at 65-75% VO2max for up to 30 min in a 30 degrees C, 40% relative humidity environment under three conditions. First, control tests (C) were performed where preexercise plasma volume (PV) and osmolality (Osm) averaged 3,800 ml and 282 mosmol X kg-1, respectively. Second, exercise tests (D) were performed following dehydration induced by fluid restriction and mild exercise (30% VO2max) in hot (40 degrees C) ambient conditions. Each subject then rested in cool surroundings 1 h before performing the exercise test. Preexercise PV and Osm averaged 3,606 ml and 293 mosmol X kg-1, respectively. Third, exercise tests (I) were performed following dehydration, but during the 1-h rest interval, 3% saline was infused so that PV was restored to 3,826 ml and Osm averaged 294 mosmol X kg-1 prior to exercise. During D, esophageal temperatures (Tes) were significantly higher than C, an avg 0.56 degrees C after 20 min exercise due to a 0.22 degrees C increase in Tes threshold for vasodilation, a 39% reduction in slope of the forearm blood flow (BF)-Tes relationship, a 32% average reduction in maximal exercise BF, and a 0.22 degrees C increase in Tes sweating threshold. During I, responses were similar to D, except the BF-Tes slope and the maximum BF were not significantly different from C. Thus hyperosmolality modifies thermoregulation by elevating thresholds for both vasodilation and sweating even without decreases in PV.

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