Much strain but no gain (in sweat output, that is)

Abstract

Drs. Hsieh, McNeeley, and Chelimsky, in "The Clinical Thermoregulatory Sweat Test Induces Maximal Sweating," have reported the results of a study designed to test the common assumption that maximal sweat rates over the whole body surface are achieved by the standard clinical thermoregulatory testing conditions [1].The intriguing and surprising conclusion of their work suggests that the addition of moderate exercise to the thermal environmental stimulus produces no further gain in sweat rate.For their study, Hsieh et al. used a thermal environment of 50~ air temperature, 50% relatively humidity, and skin temperature maintained at 40~ These parameters are at, or slightly in excess of, the range of published laboratory standards. For example, the criteria for the thermoregulatory sweat test as published in modern textbooks of clinical autonomic testing specify an air temperature of between 45 ~ and 50~ a relative humidity of 35% to 40%, and a skin temperature of 39.0 ~ to 39.5~ [2]. Criteria published in 1989 were an air temperature of 44 ~ to 50~ a relative humidity of 40% to 50%, and a skin temperature of 38.5 ~ to 39.5~ [3]. These latter parameters were originally selected for the Mayo Clinic Thermoregulatory Laboratory by myself two decades ago, with this overriding principle: to produce a tolerable heat stimulus that would cause a maximal sweat rate over all of the body surface in supine, nonexercising individuals. These conditions were derived from a study whose results are summarized in Figure 1.Sweating was measured from the forearm and foot and in all 35 subjects (ages 20-75 y). The sweat rate reached a plateau while oral temperature continued to rise, suggesting that maximal sweat output was achieved. We then selected 38.0~ (oral/core) as a test end-point temperature limit because all control subjects had maximized, stable sweat rates for at least 5 minutes at that temperature. What we did not test or know was whether exercise would increase sweat output further. That was the question addressed and answered by Hsieh et al. in their study.As mentioned, the subjects in Hsieh et al. were studied at 50~ 50% relatively humidity, and 40.0 ~ _+ 0.5~ skin temperature, slightly exceeding the standards currently used in most laboratories that I am aware of. (The authors need to acknowledge these as their laboratory standards.) This appears to explain the higher average core temperature rise of 2.0~ in their subjects (n = 8, male, ages 16-22 y) compared with a 1.2~ rise in our control subjects, during the thermal heat (sans exercise) portion of their study.Many authors have demonstrated an increase in sweat rate when exercise is coupled with a warm environment [4,5], hence the initial surprise about the lack of such evidence in the study of Hsieh et al. If one looks at normalized mean sweat rates across these studies (in nl/min/cm2), however, it becomes apparent that Hsieh et al. is one of the few studies in which the thermal environmental stimulus 390 38.0-]