Age predicts cardiovascular, but not thermoregulatory, responses to humid heat stress

Abstract

Cross-section comparisons of the effect of age on physiological responses to heat stress have yielded conflicting results, in part because of the inability to separate chronological age from factors which change in concert with the biological aging process. The present study was designed to examine the relative influence of age on cardiovascular and thermoregulatory responses to low intensity cycle exercise (60 W for 1 h) in a warm humid environment (35°C, 80% relative humidity). Specifically, the relative importance of age compared to other individual characteristics [maximal oxygen uptake ($\dot V{\text{O}}_{\text{2}}$ _max), physical activity level, anthropometry, and adiposity] was determined by multiple regression analysis in a heterogeneous sample of 56 subjects in which age (20–73 years) and$\dot V{\text{O}}_{\text{2}}$ _max (1.864–44 l · min⁻¹) were not interrelated. Dependent variables (with ranges) included final values of thermoregulatory responses [rectal temperature (T _re, 37.8–39.2°C), calculated heat storage (S, 3.4–8.1 J · g⁻¹), sweat loss (238–847 g · m⁻²)] and cardiovascular responses [heart rate (HR, 94–176 beats min⁻¹), forearm blood flow (FBF, 5.3–31.3 ml · 100 ml⁻¹ · min⁻¹), mean arterial blood pressure (MAP, 68–122 mmHg), and forearm vascular conductance (FVC = FBF · MAP⁻¹, 0.06–0.44 ml · 100 ml⁻¹ · min⁻¹ · mmHg⁻¹). Age had no significant influence onT _re,S, or sweat loss, all of which were closely related to$\dot V{\text{O}}_{\text{2}}$ _max. On the other hand, HR, MAP, FBF, and FVC were related to both age and$\dot V{\text{O}}_{\text{2}}$ _max. Anthropometric variables and adiposity had secondary, but statistically significant, effects on MAP, FBF, FVC, and sweat loss. With respect to exercise in a warm humid environment, it was concluded that the effect of age on body temperature and sweating was negligible compared to effects related to$\dot V{\text{O}}_{\text{2}}$ _max, but that chronological age had an independent effect on cardiovascular effector responses.