Xiaojiang Xu

Measurement and prediction of peak shivering intensity in humans

Abstract Prediction equations of shivering metabolism are critical to the development of models of thermoregulation during cold exposure. Although the intensity of maximal shivering has not yet been predicted, a peak shivering metabolic rate (Shivpeak) of five times the resting metabolic rate has been reported. A group of 15 subjects (including 4 women) [mean age 24.7 (SD 6) years, mean body mass 72.1 (SD 12) kg, mean height 1.76 (SD 0.1) m, mean body fat 22.3 (SD 7)% and mean maximal oxygen uptake (V˙O2max) 53.2 (SD 9) ml O2 · kg−1 · min−1] participated in the present study to measure and predict Shivpeak. The subjects were initially immersed in water at 8°C for up to 70 min. Water temperature was then gradually increased at 0.8 °C · min−1 to a value of 20 °C, which it was expected would increase shivering heat production based on the knowledge that peripheral cold receptors fire maximally at approximately this temperature. This, in combination with the relatively low core temperature at the time this water temperature was reached, was hypothesized would stimulate Shivpeak. Prior to warming the water from 8 to 20 °C, the oxygen consumption was 15.1 (SD 5.5) ml · kg−1 · min−1 at core temperatures of approximately 35 °C. After the water temperature had risen to 20 °C, the observed Shivpeak was 22.1 (SD 4.2) ml O2 · kg−1 · min−1 at core and mean skin temperatures of 35.2 (SD 0.9) and 22.1 (SD 2.2) °C, respectively. The Shivpeak corresponded to 4.9 (SD 0.8) times the resting metabolism and 41.7 (SD 5.1)% of V˙O2max. The best fit equation predicting Shivpeak was Shivpeak (ml O2 · kg−1 · min−1)=30.5 + 0.348 ×V˙O2max (ml O2 · kg−1 · min−1) − 0.909 × body mass index (kg · m−2) − 0.233 × age (years); (P=0.0001; r2=0.872).

Methods of Evaluating Protective Clothing Relative to Heat and Cold Stress: Thermal Manikin, Biomedical Modeling, and Human Testing

Personal protective equipment (PPE) refers to clothing and equipment designed to protect individuals from chemical, biological, radiological, nuclear, and explosive hazards. The materials used to provide this protection may exacerbate thermal strain by limiting heat and water vapor transfer. Any new PPE must therefore be evaluated to ensure that it poses no greater thermal strain than the current standard for the same level of hazard protection. This review describes how such evaluations are typically conducted. Comprehensive evaluation of PPE begins with a biophysical assessment of materials using a guarded hot plate to determine the thermal characteristics (thermal resistance and water vapor permeability). These characteristics are then evaluated on a thermal manikin wearing the PPE, since thermal properties may change once the materials have been constructed into a garment. These data may be used in biomedical models to predict thermal strain under a variety of environmental and work conditions. When the biophysical data indicate that the evaporative resistance (ratio of permeability to insulation) is significantly better than the current standard, the PPE is evaluated through human testing in controlled laboratory conditions appropriate for the conditions under which the PPE would be used if fielded. Data from each phase of PPE evaluation are used in predictive models to determine user guidelines, such as maximal work time, work/rest cycles, and fluid intake requirements. By considering thermal stress early in the development process, health hazards related to temperature extremes can be mitigated while maintaining or improving the effectiveness of the PPE for protection from external hazards.

Thermoregulatory modeling for cold stress.

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

Comparison of Parallel and Serial Methods for Determining Clothing Insulation

This paper examines the fundamental differences between the parallel and serial methods for the calculation of clothing insulation using a thermal manikin and demonstrates the differences in the insulation values calculated using these two methods. The parallel method is based on the condition that manikin surface temperatures remain uniform (UST), while the serial method is based on the condition that manikin heat fluxes remain uniform (UHF). Eleven clothing ensembles were evaluated on manikins in UST mode. Three of them were also evaluated on manikins in UHF mode. Insulation values were then calculated using both the serial and parallel methods. Results from UST mode showed that the parallel insulation values ranged from 1.24 to 5.79 clo, while the serial insulation values ranged from 1.43 to 7.98 clo. Differences in the parallel and serial insulations increased as the insulation increased, and the serial insulations were approximately 14–38 % higher than the parallel insulations. Results from UHF mode showed that the parallel insulations were 1.30 clo to 5.89 clo and close to the serial insulations of 1.34 clo to 5.99 clo. In conclusion, the methods of insulation calculation should be determined by the operating mode of the manikin. Only the parallel method should be used when manikins are operated in UST mode and only the serial method should be used when manikins are operated in UHF mode. Insulation values calculated using the incorrect method will be misleading.

Prediction of hand manual performance during cold exposure

A model was developed to predict hand manual performance (HMP) impairment during cold exposure. HMP is defined as the performance (tasks completed per unit time) normalized relative to performance at a finger/hand skin temperature (FST) of approximately 33°C. An empirical algorithm describing the relationship between HMP and FST was developed from published data. This algorithm adequately predicts the critical FST thresholds for reduced HMP. The combination of this algorithm and FST predictive models is useful for risk assessment, evaluation of handwear and rescue equipment design when human testing is precluded for ethical or practical reasons.

A new look at survival times during cold water immersion

This paper presents an expanded dataset for survival times during cold water immersion. In 1946, the first set of human data for cold water survival was derived from the US Navy medical reports during WWII. Although this is the largest and most widely used data source, it has only 23 data points and immersion times are less than 5.5 h for water temperature below 20 °C. For the new dataset, data (i.e., immersion times, water temperatures, clothing worn, and in some cases, body masses, heights, and survival times for the deaths witnessed by survivors) was retrieved from 12 well-documented incidents of accidental immersions which involved 22 survivors and 21 deaths. These data were combined with the 1946 dataset to create the expanded dataset which included 122 data points. Analysis of the dataset revealed critical details pertinent to cold water survival: 1) immersion times, up to 75 h, at water temperatures below 20 °C, were longer than most immersion times documented in the 1946 dataset; 2) thermal protection (wetsuit or drysuit), high body mass, and partial immersion may significantly impact survival during immersion in cold water; 3) twenty-one actual survival times until witnessed death are added. A maximal survival time curve was derived to represent the survival limit which many victims are unlikely to approach and few can exceed except under unique circumstances.

The effect of localized microclimate heating on peripheral skin temperatures and manual dexterity during cold exposure

Reduced dexterity is a major problem in cold weather, with a need for a countermeasure that increases hand (Thand) and finger (Tfing) temperatures and improves dexterity. The purpose of this study was to determine whether electric heat (set point, 42°C) applied to the forearm (ARM, 82 W), face (FACE, 9.2 W), or combination of both (COMB, 91.2 W), either at the beginning of cold exposure (COLD; 0.5°C, 120 min; 2 clo insulation, seated, bare-handed) or after Tfing fell to 10.5°C [delayed trials (D)], improves Thand, Tfing, dexterity, and finger key pinch strength (Sfing). Volunteers ( n = 8; 26 ± 9 yr) completed 7 experimental trials in COLD: ARM, ARM-D, FACE, FACE-D, COMB, COMB-D, and no heating (CON). Temperatures were measured before (BASE) and throughout COLD. Tests of dexterity [Purdue Pegboard assembly (PP) and magazine loading (MAGLOAD)] and Sfing were measured at BASE and after 45 and 90 min of COLD. Data presented are at minute 90. Thand was warmer ( P < 0.001) during ARM (18.0 ± 2.6°C) and COMB (18.9 ± 2.0°C) versus CON (15.3 ± 1.5°C) and FACE (15.8 ± 1.5°C) for heating that was initiated at the beginning of COLD. Tfing was higher ( P < 0.04) during COMB (12.7 ± 5.1°C) versus CON (9.7 ± 2.1°C) and FACE (8.9 ± 2.2°C). The change from BASE for PP (no. of pieces) was less ( P < 0.005) in COMB (-4.5 ± 3.3) and ARM (-5.0 ± 6.0) versus CON (-13.0 ± 7.3) and FACE (-10.0 ± 8.3), and for MAGLOAD, it tended ( P = 0.06) to be less in COMB (-8.9 ± 6.2 cartridges) versus CON (-14.8 ± 3.7 cartridges). There was no change in Sfing from BASE (10.5 kg) to minute 90 in ARM or COMB (0.7 ± 1.4 and -0.2 ± 1.7 kg, respectively) but a decrease ( P < 0.01) in CON and FACE (-2.1 ± 2.0 and -1.6 ± 1.9 kg, respectively). There were no differences in Thand, Tfing, dexterity, and Sfing at minute 90 when comparing heating that was initiated at the beginning of COLD versus delayed heating. In conclusion, heating using either COMB or ARM, compared with CON and FACE, improved Thand and Tfing and reduced the decline in dexterity by 20%-50% and Sfing by 90%. Furthermore, delayed heating had no deleterious effect on Thand, Tfing, dexterity, and Sfing compared with heating that started at the beginning of cold exposure. NEW & NOTEWORTHY The present study demonstrated that, during sedentary cold air exposure, localized heating that was applied from the beginning of cold exposure on the forearm increases hand and finger temperatures and finger strength, leading to subsequent improvements in manual dexterity. In addition, localized heating that was delayed until finger temperatures cooled significantly also caused higher peripheral temperatures, leading to better strength and manual dexterity, compared with no heating.

Estimation of core body temperature from skin temperature, heat flux, and heart rate using a Kalman filter

Core body temperature (TC) is a key physiological metric of thermal heat-strain yet it remains difficult to measure non-invasively in the field. This work used combinations of observations of skin temperature (TS), heat flux (HF), and heart rate (HR) to accurately estimate TC using a Kalman Filter (KF). Data were collected from eight volunteers (age 22 ± 4 yr, height 1.75 ± 0.10 m, body mass 76.4 ± 10.7 kg, and body fat 23.4 ± 5.8%, mean ± standard deviation) while walking at two different metabolic rates (∼350 and ∼550 W) under three conditions (warm: 25 °C, 50% relative humidity (RH); hot-humid: 35 °C, 70% RH; and hot-dry: 40 °C, 20% RH). Skin temperature and HF data were collected from six locations: pectoralis, inner thigh, scapula, sternum, rib cage, and forehead. Kalman filter variables were learned via linear regression and covariance calculations between TC and TS, HF, and HR. Root mean square error (RMSE) and bias were calculated to identify the best performing models. The pectoralis (RMSE 0.18 ± 0.04 °C; bias -0.01 ± 0.09 °C), rib (RMSE 0.18 ± 0.09 °C; bias -0.03 ± 0.09 °C), and sternum (RMSE 0.20 ± 0.10 °C; bias -0.04 ± 0.13 °C) were found to have the lowest error values when using TS, HF, and HR but, using only two of these measures provided similar accuracy.

Quantitative evaluation of personal protective ensembles relative to heat strain

Personal protective equipment (PPE) exacerbates heat strain experienced by users through: (a) increases in thermal (Rt) and evaporative (Ret) resistances; and (b) increases in metabolic rate (M·) during physical activity driven in large part by ensemble weight. This study aimed to quantify the effects of PPE Rt & Ret and ensemble weight on heat strain during walking.