Impaired Metabolic And Tissue Insulation Responses To Cold In Aged Humans

Abstract

The cutaneous vasoconstrictor (VC) response to whole body cold exposure is reduced with aging. Additionally, resting metabolic rate (M) and basal core temperature (esophageal, Tes) are often reduced in older subjects. The interaction of these variables on calculated tissue insulation [It = (Tes-Tsk)/(Mnet -S), where S is body heat storage (negative in the case of heat loss) and Tsk is mean skin temperature], when older adults are exposed to cold has not been determined. PURPOSE To determine the effects of attenuated VC and reduced metabolic rate on S and It in older and young subjects. METHODS 15 young (Y; 24±3 yrs) and 7 older (O;68±4 yrs) subjects underwent a cooling trial in which dry bulb temperature (Tdb) was progressively decreased. Data were analyzed at baseline and during the last time point where maximal steady state VC occurred, but before shivering began. Tes, Tsk, and red blood cell flux (laser Doppler flowmetry) were measured continuously, and O2 consumption and CO2 production were measured every 10 min. Laser Doppler flux units were normalized to mean arterial pressure (cutaneous vascular conductance, CVC) and expressed as % baseline. It, S, and M were calculated using standard equations. RESULTS There was no difference between groups in time to onset of shivering (O: 77±11 min, Y: 76±18 min). Tes was lower in O at baseline (36.86±0.05 vs 37.03±0.03°C, p=0.04) and at maximum VC (36.80±0.05 vs 37.03±0.04°C, p=0.002) and Tes was well maintained in both groups. Both groups decreased CVC during cooling, with a trend toward impaired VC in older subjects (O: −69±3 % baseline, Y: −62±2% baseline, p=0.09). Both groups increased It to a similar extent (Y:0.084±0.004 to 0.096±0.004°C·m2·W−1, p=0.04; O:0.089±0.006 to 0.105±0.006 °C·m2·W−1, p=0.06). Y did not increase M, but O increased M by ∼14% (35.4±1.9 to 40.3±1.9W·m−2, p=0.08). There were no differences between groups throughout the trial for S or Tsk. When a subgroup of the study population was matched for body mass index, body surface area, and % body fat, these differences between groups remained. CONCLUSIONS Older subjects increase M to a greater extent than Y to compensate for an attenuated VC in order to maintain Tes. Supported by NIH Grant R01 AG07004