Does the rate of thermoregulatory sweating depend on the rate of change of core temperature?

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1 Does the rate of thermoregulatory sweating depend on the rate of change of core temperature? Brian Farnworth 1, Michel B. DuCharme 2,3, Ollie Jay 3 and Glen Kenny 3 1. BF Scientific Inc, 2020 Bennett Rd, Kelowna, BC, Canada, V1V 2C1 2. Defence R&D Canada, Quebec City, QC, Canada, G3J 1X5 3. Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, ON, Canada, K1N 6N5. INTRODUCTION Contact Person: michel.ducharme@drdc-rddc.gc.ca In a previously published set of experiments in a human calorimeter, Kenny et al (2008) measured the heat loss from 8 subjects (6M, 2F) during one hour of semi-recumbent cycling exercise at 40% of their maximum oxygen consumption followed by one hour of resting recovery. The measurement technique permitted the separate determination of heat loss by the combined dry mechanisms (i.e. conduction, convection and radiation) and evaporation. On the assumption that all sweat produced evaporated from the skin and allowing for respiratory mass loss according to Fanger ( 1970), the evaporative heat loss can be taken as a direct measure of thermoregulatory sweat production. It was observed that the sweat rate rises more quickly during the exercise period and falls more quickly during the subsequent rest period than does core temperature. This is shown in figure 1 with aural temperature (T au ) used as an index of core temperature (T core ).

2 Aural Temperature ( O C) Heat Loss from Sweat (W) Tau Sweat heat loss (W) Time (min) Figure 1. Evaporative heat loss from sweat evaporation and aural temperature during exercise (0-60 min) and rest ( min) from Kenny et al. (2008). The conventional view that sweat production is proportion to the rise in core temperature above a set point (Sawka and Wenger 1988) does not seem to be compatible with these results, since at a given core temperature the sweat rate is higher during exercise than during rest. For example, during exercise a T au of ~36.9ºC yields a whole-body sweat rate of ~240 W (3.2 g min -1 m -2 with a latent heat of vaporization of 2427 J/g) whereas during recovery the same approximate T au yields a whole-body sweat rate of ~90 W (1.2 g min -1 m -2 ) (Figure 1). In order to explain these data, it is postulated that the sweating response is driven by both the value of core temperature and its rate of change with time. METHODS A model is proposed where the evaporative heat loss is given by: 0 (1) H evap S p (T core T set ) S d dt dt core Where T set is the set point, and S p and S d are the sensitivities to the proportional and derivative terms respectively. Values of core temperature were taken from the experimental data set at 5 to 15 minute intervals. These were differentiated numerically and used in equation 1 to obtain a

3 Heat Loss from Sweat (W) calculated value of H evap. The parameters T set, S p and S d were adjusted to give the best fit of the calculation to the measured values of H evap. The conventional proportional model was tested by a least squares fit of a straight line to the experimental data RESULTS Using aural temperature as representative of core temperature the best visual fit gives the calculated curve in figure 2. Here H evap is plotted against T au to more clearly demonstrate the difference between the exercise and recovery periods. In a model where sweat rate is proportional to core temperature, figure 2 would be expected to be a straight line with positive slope with data from exercise and rest phases on the same line. Clearly the figure is open with the increase in sweat rate during exercise following a curve which is higher than that followed by the decrease during recovery. Also shown in figure 2 is the proportional model represented by the best linear fit (R 2 = 0.74) to the experimental data Experimental data Model calculation Linear fit to experimental data Exercise phase Rest phase Aural temperature ( O C) Figure 2. Heat loss from sweating (H evap ) plotted against aural temperature during exercise and subsequent rest. The data are represented as mean ± standard error from Kenny et al. (2008). DISCUSSION AND CONCLUSION

4 The calculated curve in figure 2 agrees well with the experiments except at the beginning of the recovery phase. The difference between calculation and experiment is an average of about 50 W for the first 15 minutes of the rest phase. This would correspond to the evaporation of about 18 g of liquid sweat. It is not clear whether this discrepancy is a defect in the model or a limitation of the experimental method. The calorimeter can measure heat losses with a precision of 2.3 W (Reardon 2006), but has a finite response time so there is inevitably some time lag between the production of sweat and the detection of the water vapour. However this latency has been found to not be greater than about 60 s. Another possibility is that some sweat produced during the exercise phase did not evaporate until the first part of the recovery phase. For example, some liquid sweat could have soaked into the subjects hair or clothing or remained on the skin and evaporated at a time later than when it was produced. Thus the measurement could be lower than actual sweat rate in the exercise phase and higher during the first part of the recovery phase. Compared to the total sweat evaporation of 465 g, this discrepancy is small, about 4%. Even though the discrepancy noted above is minor, it did preclude the use of a least squares technique to get the best fit of the calculation to the data. Optimizing the sensitivities to try to bring the two or three deviating points into the fit tended to make the overall picture worse. The reason for this is that the deviations at these points are probably systematic rather than random. Hence a visual fit was used. A similar analysis was performed using esophageal temperature rather than aural as an index of core temperature with results which were very similar except that the values of sensitivity for the best fit were different as shown in table 1. The value for S d is lower for the esophageal temperature since this index of core temperature showed a faster response at the onset of exercise. The slightly higher value of T set for esophageal temperature reflects the generally higher value of this index of core temperature throughout the experiment. Table 1. Model coefficients for the best fit to heat loss from sweating for two different indices of core temperatures. Index of core temperature S p (W/K) S d (W h / K) T set ( C) Aural Esophageal A model which takes the time rate of change of core temperature as an input to the drive for sweat production gives improved agreement with the experiments compared to the assumption that sweating depends on core temperature only directly. The mechanism which allows sweat rate to respond to the rate of change of core temperature is a matter of conjecture but since the thermoreceptors in the skin are known to respond to rate of change as well as actual temperature (Sawka, Wenger et al. 1996; Sawka and Wenger 1988) it does not seem unreasonable that those in the hypothalamus should behave similarly. Under steady state conditions, the derivative term vanishes and this model reduces to the conventional view of sweat rate being linear in core temperature. It has been postulated (Webb 1995) that the rate of heat storage in the body (metabolic rate external work heat loss) plays a role as input to thermoregulation. The derivative term

5 postulated here is similar in its effect. If all parts of the body have the same rate of change of temperature, heat storage is the product of the rate of change of temperature and the heat capacity of the body. Hence under those limited circumstances, the heat storage and the derivative term would be equivalent. In other circumstances, where different parts of the body (core, shell, muscles, etc) are changing temperature at different rates, the equivalence would not be exact but there may be qualitative similarity. ACKNOWLEGDEMENTS The experimental work was supported by the U.S. Army Medical Research and Material Command s Office of the Congressionally Directed Medical Research Programs (DAMD ). REFERENCES Fanger, PO. (1970) Thermal Comfort. McGraw-Hill, NY, pp28-30 Kenny GP, Webb P, DuCharme MB, Reardon FD and Jay O. (2008) Calorimetric measurement of postexercise net heat loss and residual body heat storage. Med Sci Sports Exerc Sep;40(9): Sawka MN and Wenger CB (1988). Physiological responses to acute exercise-heat stress. Human performance physiology and environmental medicine at terrestrial extremes. K. B. Pandolf, M. N. Sawka and R. R. Gonzalez. Indianapolis, Benchmark: Sawka MN, Wenger CB, et al. (1996). Thermoregulatory responses to acute exercise-heat stress and heat acclimation. Handbook of Physiology. Section 4: Webb, P. (1995). The physiology of heat regulation. Am J Physiol 268 (4 Pt 2): R Reardon FD, Leppik KE, Wegmann R, Webb P, DuCharme MB & Kenny GP. (2006). The Snellen human calorimeter revisited, re-engineered and upgraded: design and performance characteristics. Med Biol Eng Comput 44,

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