Passive temperature lability in the elderly

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°C water. The exercise was conducted to elevate tympanic temperature (T _ty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling ofT _ty, and changes inT _ty relative to resting levels (ΔT _ty) at which sweating abated and shivering commenced were defined as the ΔT _ty thresholds for the cessation of sweating (T _sw) and onset of shivering (T _sh), respectively. In addition toT _ty, oxygen uptake ($\dot VO_2$; 1 · min⁻¹), sweating rate (g · m⁻² · min⁻¹), and forehead skin blood perfusion were also measured during the trials. The mean (SD)T _sw occurred at a significantly (P <0.005) higher ΔT _ty [0.48 (0.18)°C] in the elderly than in the young adults [0.21(0.06)°C], while the Tsh occurred at significantly (P < 0.005) lower ΔT _ty in the elderly [ −0.64 (0.34)°C] than in young adults [−0.22 (0.10)°C]. Decreases in ΔT _ty below the shivering threshold were met with a significantly (P <0.01) reduced$\dot VO_2$. The range of temperature lability between Ts, andT _sh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39)°C] than in the young adults [0.43 (0.12)°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 ofT _ty, 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.