Thermoregulation and Individual Characteristics during Cold and Heat Stress

Open Access
- Author:
- DeGroot, David W.
- Graduate Program:
- Physiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 28, 2007
- Committee Members:
- William Lawrence Kenney Jr., Committee Chair/Co-Chair
James Anthony Pawelczyk, Committee Member
David Nathan Proctor, Committee Member
Mosuk Chow, Committee Member
George Havenith, Committee Member - Keywords:
- predicting modeling
core temperature
aging
skin blood flow - Abstract:
- Numerous individual characteristics influence both the physiological responses to cold stress and the ability to predict body core temperature (Tc) during exercise in the heat. This series of studies was designed to 1) determine age-related differences in the defense of Tc during resting, mild cold stress in young and older subjects matched for relevant body composition characteristics, 2) determine the relative influence of individual characteristics on the defense of Tc during resting, mild cold stress in young and older subjects and 3) determine the predictive accuracy of 3 models of human thermoregulation during a unique cross-desert expedition, with reference to differences in individualization allowed by each of the models. Aging and Cold Stress (Projects 1 and 2) The purpose of project 1 was to determine the influence of primary human aging on the defense of Tc during a mild cold transient. Thirty-six young (YS; 23±1 years, range 18-30) and 46 older (OS; 71±1 years, range 65-89) subjects underwent a slow transient cold air exposure from a thermoneutral baseline, during which esophageal (Tes) and mean skin temperatures (Tsk), O2 consumption, and skin blood flow (SkBF; laser-Doppler flowmetry) were measured. Cold exposure was terminated at the onset of visible sustained shivering. Net metabolic heat production (Mnet), heat debt, predicted change in mid-region temperature (Tmid), tissue insulation (It), and cutaneous vascular conductance (laser-Doppler flux/mean arterial pressure, expressed as percent change from baseline (ΔCVC%base)) were calculated. There were no baseline group differences for Tes, but Mnet was lower in older subjects (OS: 38.0±1.1; YS: 41.9±1.1 W•m-2, p<0.05). Tes was well maintained in YS but fell progressively in OS (p<0.01 for all timepoints after 35 min). The cutaneous vasoconstrictor response to mild cold stress was attenuated in OS (OS: 42±3 vs. YS: 53±4 ΔCVC%base, p<0.01). There were no group differences for Tsk or It, while Mnet remained lower in OS during cooling (p<0.05). The Tmid did not account for the drop in Tes in OS. Healthy aged humans, matched with a young subject population for relevant anthropometric characteristics, failed to maintain Tes; however, the mechanisms underlying this response are not clear. The purpose of the second project was to determine the relative influence of individual characteristics on Tc and It during mild cold stress. Forty-two young (23±1 years, range 18-30) and 46 older (71±1 years, range 65-89) subjects, varying widely in muscularity, adiposity, and body size, underwent a transient cooling protocol during which Tes was measured continuously and It was calculated. Multiple regression analyses were performed to determine predictors of Tes and It and standardized regression coefficients were analyzed to determine the relative influence of each predictor. Putative predictors included age, sex, weight, body surface area, body surface area-to-mass ratio, sum of skinfolds, %fat, appendicular skeletal muscle mass (ASMM), and thyroid hormone concentrations ([T3], [T4]). The sum of skinfolds explained 67% (p<0.01) of the Tes variance in young subjects vs. 2% (p=0.30) in older subjects. Conversely, ASMM explained a greater portion of the variance in older subjects for both Tes (older: 28%, p<0.01; young: 8%, p=0.05) and It (older: 46%, p<0.01; young: 17%, p<0.01). The residual variance for Tes was considerably larger in older subjects (59-72% vs. 14-42% in young), possibly due to varying rates of physiological aging. The individual characteristics that explain the variance in Tes and It, as well are the relative influence of these characteristics, differ between young and older subjects. Predicting Core Temperature during Exercise in the Heat (Project 3) Models of human thermoregulation have been developed and validated from short duration laboratory data utilizing relatively large sample sizes. However, data collected in field studies may introduce additional sources of variability that may reduce model accuracy. Additionally, the accuracy of existing models is uncertain when applied to individuals or small groups or longer duration exposures. Recently, 3 runners undertook a unique expedition to run across the Sahara Desert where we collected the environmental and physiological data necessary for the prediction of Tc by 3 models of thermoregulation over 2 days at the start of the expedition. The SCENARIO and Initial Capability Decision Aide (ICDA) models allow for input of individual characteristics while Fiala’s model relies solely on environment and exercise intensity. The runners intermittently ran 8.0 km/h over 6 h during NIGHT desert conditions and 7.0 km/h over 7 h 45 min on during DAY desert conditions. The Tc standard deviation (SD) was 0.34 and 0.51 for NIGHT and DAY, respectively. The root mean squared deviation (RMSD) of the Tc prediction was calculated and model accuracy was considered acceptable when RMSD<Tc SD. NIGHT RMSD= 0.45, 0.64, and 0.91 and DAY RMSD= 0.66, 0.70, and 2.09 for SCENARIO, Fiala and ICDA, respectively. Increased RMSD during day was likely due to overestimation of solar radiation effects. The RMSD was smaller when only the first 2 h of the exposure was analyzed. SCENARIO, which offers the user the greatest opportunity for individualization, resulted in the lowest RMSD. When SCENARIO was configured to match the individual characteristics assumed by Fiala, the RMSD increased from 0.66 to 0.82, illustrating the importance of individualizing the passive system. Summary of Findings Older humans fail to adequately defend Tc during mild cold stress compared to young subjects; different individual characteristics mediate the respective responses in each age group. Greater individualization of model inputs contributes to greater predictive accuracy in models of thermoregulation. Several variables, including solar radiation, fluctuations in exercise intensity and environmental conditions, and minute-by-minute errors in the estimation of the change in Tc, likely contributed to decreased model accuracy during prolonged exercise in desert conditions.