Abstract
Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28 degrees C water. The exercise was conducted to elevate tympanic temperature (Tty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling of Tty, and changes in Tty relative to resting levels (delta Tty) at which sweating abated and shivering commenced were defined as the delta Tty thresholds for the cessation of sweating (Tsw) and onset of shivering (Tsh), respectively. In addition to Tty, oxygen uptake (VO2; l.min-1), sweating rate (g.m-2.min-1), and forehead skin blood perfusion were also measured during the trials. The mean (SD) Tsw occurred at a significantly (P < 0.005) higher delta Tty [0.48 (0.18) degrees C] in the elderly than in the young adults [0.21 (0.06) degrees C], while the Tsh occurred at significantly (P < 0.005) lower delta Tty in the elderly [-0.64 (0.34) degrees C] than in young adults [-0.22 (0.10) degrees C]. Decreases in delta Tty below the shivering threshold were met with a significantly (P < 0.01) reduced VO2. The range of temperature lability between Tsw and Tsh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39) degrees C] than in the young adults [0.43 (0.12) degrees C], and the slope of the vasoconstrictor response in the null-zone was significantly (P < 0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels of Tty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.
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