The effect of lower body cooling on the changes in three core temperature indices

Rectal (T(re)), ear canal (T(ear)) and esophageal (T(es)) temperatures have been used in the literature as core temperature indices in humans. The aim of the study was to investigate if localized lower body cooling would have a different effect on each of these measurements. We hypothesized that prolonged lower body surface cooling will result in a localized cooling effect for the rectal temperature not reflected in the other core measurement sites. Twelve participants (mean ± SD; 26.8 ± 6.0 years; 82.6 ± 13.9 kg; 179 ± 10 cm, BSA = 2.00 ± 0.21 m(2)) attended one experimental session consisting of sitting on a rubberized raft floor surface suspended in 5 °C water in a thermoneutral air environment (approximately 21.5 ± 0.5 °C). Experimental conditions were (a) a baseline phase during which participants were seated for 15 min in an upright position on an insulated pad (1.408 K ⋅ m(2) ⋅ W(-1)); (b) a cooling phase during which participants were exposed to the cooling surface for 2 h, and (c) an insulation phase during which the baseline condition was repeated for 1 h. Temperature data were collected at 1 Hz, reduced to 1 min averages, and transformed from absolute values to a change in temperature from baseline (15 min average). Metabolic data were collected breath-by-breath and integrated over the same temperature epoch. Within the baseline phase no significant change was found between the three indices of core temperature. By the end of the cooling phase, T(re) was significantly lower (Δ = -1.0 ± 0.4 °C) from baseline values than from T(ear) (Δ = -0.3 ± 0.3 °C) and T(es) (Δ = -0.1 ± 0.3 °C). T(re) continued to decrease during the insulation phase from Δ -1.0 ± 0.4 °C to as low as Δ -1.4 ± 0.5 °C. By the end of the insulation phase T(re) had slightly risen back to Δ -1.3 ± 0.4 °C but remained significantly different from baseline values and from the other two core measures. Metabolic data showed no variation throughout the experiment. In conclusion, the local cooling of the buttock area results in a drop in rectal temperature compromising the validity of the rectal temperature as a core temperature index under these conditions.

Calorimetric measurement of postexercise net heat loss and residual body heat storage

PURPOSE

Previous studies have shown a rapid reduction in postexercise local sweating and blood flow despite elevated core temperatures. However, local heat loss responses do not illustrate how much whole-body heat dissipation is reduced, and core temperature measurements do not accurately represent the magnitude of residual body heat storage. Whole-body evaporative (H(E)) and dry (H(D)) heat loss as well as changes in body heat content (DeltaH(b)) were measured using simultaneous direct whole-body and indirect calorimetry.

METHODS

Eight participants cycled for 60 min at an external work rate of 70 W followed by 60 min of recovery in a calorimeter at 30 degrees C and 30% relative humidity. Core temperature was measured in the esophagus (T(es)), rectum (T(re)), and aural canal (T(au)). Regional muscle temperature was measured in the vastus lateralis (T(vl)), triceps brachii (T(tb)), and upper trapezius (T(ut)).

RESULTS

After 60 min of exercise, average DeltaH(b) was +273 +/- 57 kJ, paralleled by increases in T(es), T(re), and T(au) of 0.84 +/- 0.49, 0.67 +/- 0.36, and 0.83 +/- 0.53 degrees C, respectively, and increases in T(vl), T(tb), and T(ut) of 2.43 +/- 0.60, 2.20 +/- 0.64, and 0.80 +/- 0.20 degrees C, respectively. After a 10-min recovery, metabolic heat production returned to pre-exercise levels, and H(E) was only 22.9 +/- 6.9% of the end-exercise value despite elevations in all core temperatures. After a 60-min recovery, DeltaH(b) was +129 +/- 58 kJ paralleled by elevations of T(es) = 0.19 +/- 0.13 degrees C, T(re) = 0.20 +/- 0.03 degrees C, T(au) = 0.18 +/- 0.04 degrees C, Tvl = 1.00 +/- 0.43 degrees C, T(tb) = 0.92 +/- 0.46 degrees C, and T(ut) = 0.31 +/- 0.27 degrees C. Despite this, H(E) returned to preexercise levels. Only minimal changes in H(D) occurred throughout.

CONCLUSION

We confirm a rapid reduction in postexercise whole-body heat dissipation by evaporation despite elevated core temperatures. Consequently, only 53% of the heat stored during 60 min of exercise was dissipated after 60 min of recovery, with the majority of residual heat stored in muscle tissue.

Self-rescue swimming in cold water: the latest advice

According to the 2006 Canadian Red Cross Drowning Report, 2007 persons died of cold-water immersion in Canada between 1991 and 2000. These statistics indicate that prevention of cold-water immersion fatalities is a significant public health issue for Canadians. What should a person do after accidental immersion in cold water? For a long time, aquatic safety organizations and government agencies stated that swimming should not be attempted, even when a personal flotation device (PFD) is worn. The objective of the present paper is to present the recent scientific evidence making swimming a viable option for self-rescue during accidental cold-water immersion. Early studies in the 1960s and 1970s led to a general conclusion that "people are better off if they float still in lifejackets or hang on to wreckage and do not swim about to try to keep warm". Recent evidence from the literature shows that the initial factors identified as being responsible for swimming failure can be either easily overcome or are not likely the primary contributors to swimming failure. Studies over the last decade reported that swimming failure might primarily be related not to general hypothermia, but rather to muscle fatigue of the arms as a consequence of arm cooling. This is based on the general observation that swimming failure developed earlier than did systemic hypothermia, and can be related to low temperature of the arm muscles following swimming in cold water. All of the above studies conducted in water between 10 and 14 degrees C indicate that people can swim in cold water for a distance ranging between about 800 and 1500 m before being incapacitated by the cold. The average swimming duration for the studies was about 47 min before incapacitation, regardless of the swimming ability of the subjects. Recent evidence shows that people have a very accurate idea about how long it will take them to achieve a given swimming goal despite a 3-fold overestimation of the absolute distance to swim. The subjects were quite astute at deciding their swimming strategy early in the immersion with 86% success, but after about 30 min of swimming or passive cooling, their decision-making ability became impaired. It would therefore seem wise to make one's accidental immersion survival plan early during the immersion, directly after cessation of the cold shock responses. Additional recommendations for self-rescue are provided based on recent scientific evidence.

Estimating changes in mean body temperature for humans during exercise using core and skin temperatures is inaccurate even with a correction factor

Changes in mean body temperature (DeltaT(b)) estimated by the traditional two-compartment model of "core" and "shell" temperatures and an adjusted two-compartment model incorporating a correction factor were compared with values derived by whole body calorimetry. Sixty participants (31 men, 29 women) cycled at 40% of peak O(2) consumption for 60 or 90 min in the Snellen calorimeter at 24 or 30 degrees C. The core compartment was represented by esophageal, rectal (T(re)), and aural canal temperature, and the shell compartment was represented by a 12-point mean skin temperature (T(sk)). Using T(re) and conventional core-to-shell weightings (X) of 0.66, 0.79, and 0.90, mean DeltaT(b) estimation error (with 95% confidence interval limits in parentheses) for the traditional model was -95.2% (-83.0, -107.3) to -76.6% (-72.8, -80.5) after 10 min and -47.2% (-40.9, -53.5) to -22.6% (-14.5, -30.7) after 90 min. Using T(re), X = 0.80, and a correction factor (X(0)) of 0.40, mean DeltaT(b) estimation error for the adjusted model was +9.5% (+16.9, +2.1) to -0.3% (+11.9, -12.5) after 10 min and +15.0% (+27.2, +2.8) to -13.7% (-4.2, -23.3) after 90 min. Quadratic analyses of calorimetry DeltaT(b) data was subsequently used to derive best-fitting values of X for both models and X(0) for the adjusted model for each measure of core temperature. The most accurate model at any time point or condition only accounted for 20% of the variation observed in DeltaT(b) for the traditional model and 56% for the adjusted model. In conclusion, throughout exercise the estimation of DeltaT(b) using any measure of core temperature together with mean skin temperature irrespective of weighting is inaccurate even with a correction factor customized for the specific conditions.

Plasma dopamine and noradrenaline variations in response to stress

Dopamine (DA) the precursor of noradrenaline (NA) has been shown to have many functions such as its influence on endorphins activity and its association with hedonic impact, anxiety and depression. However with regard to the sympathetic nervous system activity, the role of DA has merely been considered as being the precursor of NA. We have shown in a previous study a positive correlation between the resting plasma level of NA and those found during exposure to a physical stress. No explanation was proposed to explain this finding. Enhanced sympathetic nervous system activity has been shown to increase the secretion of NA as well as DA. It is not known however if the secretion of DA during exposure to stress parallels that of NA. What are the interactions between the two amines and also between values at rest and during exposure to stress? For that reason a test was used which consisted of blowing cold wind (4 degrees C at 60 km/h) on the face of a group of subjects and measuring plasma concentration of the two amines before, during and after the test. For a given individual, the increase of either plasma NA or DA in response to the cold wind stress parallels the resting plasma concentrations of these two amines respectively. Low level of one amine at rest coincides with low increase during the stress. Furthermore the results have shown that when the plasma level of either one of these two amines is high in response to stress, the values of the other amine are small; both amines are not high or low at the same time. The literature suggests that dopamine beta hydroxylase (DBH), the enzyme which catalyzes the synthesis of NA from DA, may provide an explanation to our findings. Lower DBH activity of this enzyme would result in a lower NA and a greater DA storage and secretion. Further investigation is needed to verify this possibility.

Prediction of facial cooling while walking in cold wind

A dynamic model of cheek cooling has been modified to account for increased skin blood circulation of individuals walking in cold wind. This was achieved by modelling the cold-induced vasodilation response to cold as a varying blood perfusion term, which provided a source of convective heat to the skin tissues of the model. Physiologically-valid blood perfusion was fitted to replicate the cheek skin temperature responses of 12 individuals experimentally exposed to air temperatures from -10 to 10 degrees C at wind speeds from 2 to 8 ms(-1). Resultant cheek skin temperatures met goodness-of-fit criteria and implications on wind chill predictions are discussed.

A three-compartment thermometry model for the improved estimation of changes in body heat content

The aim of this study was to use whole body calorimetry to directly measure the change in body heat content (DeltaH(b)) during steady-state exercise and compare these values with those estimated using thermometry. The thermometry models tested were the traditional two-compartment model of "core" and "shell" temperatures, and a three-compartment model of "core," "muscle," and "shell" temperatures; with individual compartments within each model weighted for their relative influence upon DeltaH(b) by coefficients subject to a nonnegative and a sum-to-one constraint. Fifty-two participants performed 90 min of moderate-intensity exercise (40% of Vo(2 peak)) on a cycle ergometer in the Snellen air calorimeter, at regulated air temperatures of 24 degrees C or 30 degrees C and a relative humidity of either 30% or 60%. The "core" compartment was represented by temperatures measured in the esophagus (T(es)), rectum (T(re)), and aural canal (T(au)), while the "muscle" compartment was represented by regional muscle temperature measured in the vastus lateralis (T(vl)), triceps brachii (T(tb)), and upper trapezius (T(ut)). The "shell" compartment was represented by the weighted mean of 12 skin temperatures (T(sk)). The whole body calorimetry data were used to derive optimally fitting two- and three-compartment thermometry models. The traditional two-compartment model was found to be statistically biased, systematically underestimating DeltaH(b) by 15.5% (SD 31.3) at 24 degrees C and by 35.5% (SD 21.9) at 30 degrees C. The three-compartment model showed no such bias, yielding a more precise estimate of DeltaH(b) as evidenced by a mean estimation error of 1.1% (SD 29.5) at 24 degrees C and 5.4% (SD 30.0) at 30 degrees C with an adjusted R(2) of 0.48 and 0.51, respectively. It is concluded that a major source of error in the estimation of DeltaH(b) using the traditional two-compartment thermometry model is the lack of an expression independently representing the heat storage in muscle during exercise.

The Snellen human calorimeter revisited, re-engineered and upgraded: design and performance characteristics

The measurement of whole body heat loss in humans and the performance characteristics of a modified Snellen whole body air calorimeter are described. Modifications included the location of the calorimeter in a pressurized room, control of operating temperature over a range of - 15 to + 35 degrees C, control of ambient relative humidity over a range of 20-65%, incorporation of an air mass flow measuring system to provide real time measurement of air mass flow through the calorimeter, incorporation of a constant load 'eddy current' resistance ergometer and an open circuit, expired gas analysis calorimetry system. The performance of the calorimeter is a function of the sensitivity, precision, accuracy and response time characteristics of the fundamental measurement systems including: air mass flow; thermometry and hygrometry. Calibration experiments included a calibration of the air mass flow sensor, the response of the thermometric measurement system for dry heat loss and the response of the hygrometric measurement system for evaporative heat loss. The air mass flow system was evaluated using standard differential temperature procedures to demonstrate linearity and sensitivity of the device. A novel procedure based on differential hygrometry was developed to ascertain the absolute calibration of air mass flow by resolving the unique system coefficient K. The results of the hygrometric calibration demonstrate the air mass flow response of the system is linear over the range of air mass flows from 6 to 15 kg min(-1). R(2) was 0.995. The average half response time (tR50) was 14.5 +/- 2.1 s. Similarly the results of the thermometric calibration demonstrate that the response of the apparatus is linear over the range of power input measured (coefficient of linearity R(2)=0.9997) with a precision of 0.72 W and an accuracy to within 0.36 W. The average (tR50) over all conditions was 6.0 +/- 1.9 min. In summary, modifications brought to the Snellen calorimeter have significantly improved the precision, accuracy and response time characteristics of the previous system while extending its operating range.

Heat stress of helicopter aircrew wearing immersion suit

The objectives of the present study were to define the lowest ambient air and cabin temperatures at which aircrews wearing immersion protection are starting to experience thermal discomfort and heat stress during flight operations, and to characterize during a flight simulation in laboratory, the severity of the heat stress during exposure to a typical northern summer ambient condition (25 degrees C, 40% RH). Twenty male helicopter aircrews wearing immersion suits (insulation of 2.2 Clo in air) performed 26 flights within an 8-month period at ambient temperatures ranging between -15 and 25 degrees C, and cabin temperatures ranging between 3 and 28 degrees C. It was observed based on thermal comfort ratings that the aircrews were starting to experience thermal discomfort and heat stress at ambient and cabin air conditions above 18 degrees C and at a WBGT index of 16 degrees C. In a subsequent study, seven aircrews dressed with the same clothing were exposed for 140 min to 25 degrees C and 40% RH in a climatic chamber. During the exposure, the aircrews simulated pilot flight maneuvers for 80 min followed with backender/flight engineer activities for 60 min. By the end of the 140 min exposure, the skin temperature, rectal temperature and heart rate had increased significantly to 35.7 +/- 0.2 degrees C, 38.4 +/- 0.2 degrees C and between 110 and 160 beats/min depending on the level of physical activity. The body sweat rate averaged 0.58 kg/h and the relative humidity inside the clothing was at saturation by the end of the exposure. It was concluded that aircrews wearing immersion suits during the summer months in northern climates might experience thermal discomfort and heat stress at ambient or cabin air temperature as low as 18 degrees C.

Insulation disks on the skin to estimate muscle temperature

This study examined the use of insulation disks placed on the skin to estimate muscle temperature in resting subjects exposed to a thermoneutral (28 degrees C) ambient environment. The working hypothesis was that the skin temperature under each insulation disk would increase to a value corresponding to a specific muscle temperature measured by a control probe at 0.8+/-0.2, 1.3+/-0.2, 1.8+/-0.2, 2.3+/-0.2, and 2.8+/-0.2 cm below the skin surface. Eight subjects sat for 120 min while lateral thigh skin temperatures and vastus lateralis muscle temperature were directly measured. Vastus lateralis temperature was estimated non-invasively using two 5 cm diameter foam neoprene disks which were placed on top of the skin temperature probes (from time 60 to 120 min) located at 15.3 and 26.3 cm superior to the patella. The disks at the two locations were 3.2 and 4.8 mm thick, respectively. The placement of the 3.2- and 4.8-mm disks on the thigh for a minimum of 15 and 20 min, respectively, resulted in an increase in skin temperature under the disks which corresponded to the lateral thigh muscle temperature measured directly and invasively at 0.8+/-0.2 and 1.3+/-0.2 cm, respectively, below the skin.

Facial cold-induced vasodilation and skin temperature during exposure to cold wind

One purpose of this study was to characterize the facial skin temperature and cold-induced vasodilation (CIVD) response of 12 subjects (six males and six females) during exposure to cold wind (i.e., -10 to 10 degrees C; 2, 5, and 8 m/s wind speed). This study found that at each wind speed, facial skin temperature decreased as ambient temperature decreased. The percentage of subjects showing facial CIVD decreased significantly at an ambient temperature above -10 degrees C. A similar CIVD percentage was observed between 0 degrees C dry and 10 degrees C wet (face sprayed with fine water mist) at each wind speed. No CIVDs were observed during the 10 degrees C dry condition at any wind speed. The incidence of CIVD response was more uniform across facial sites when there was a greater cold stress (i.e., -10 degrees C and 8 m/s wind). Another objective of the study was to examine the effect of the thermal state of the body (as reflected by core temperature) on the facial skin temperature response during rest and exercise. This study found that nose skin temperature was significantly higher in exercising subjects with an elevated core temperature even though there was no significant difference in face skin temperature between the two conditions. Therefore, this finding suggests that acral regions of the face, such as the nose, are more sensitive to changes in the thermal state of the body, and hence will stay warmer relative to other parts of the face during exercise in the cold.

Safe cooling limits from exercise-induced hyperthermia

We evaluated the cooling rate of hyperthermic subjects, as measured by three estimates of deep core temperatures (esophageal, rectal and aural canal temperatures), during immersion in a range of water temperatures. The objective of the study was to compare the three indices of core temperature and define safe cooling limits when using rectal temperature to avoid the development of hypothermia. On 4 separate days, seven subjects (four males, three females) exercised for 45.4+/-4.1 min at 65% V(O2)max at an ambient temperature of 39 degrees C, RH: 36.5%, until rectal temperature (T (re)) increased to 40.0 degrees C (39.5 degrees C for two subjects). Following exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14 and 20 degrees C until T (re) returned to 37.5 degrees C. When T (re) reached normothermia during the cooling period (37.5+/-0.05 degrees C), both esophageal (T (es)) (35.6+/-1.3 degrees C) and aural canal (T (ac)) (35.9+/-0.9 degrees C) temperatures were approaching or reaching hypothermia, particularly during immersion in 2 degrees C water (T (es)=34.5+/-1.2 degrees C). On the basis of the heat loss data, the heat gained during the exercise was fully eliminated after 5.4+/-1.5, 7.9+/-2.9, 10.4+/-3.8 and 13.1+/-2.8 min of immersion in 2, 8, 14 and 20 degrees C water, respectively, with the coldest water showing a significantly faster cooling rate. During the immersion in 2 degrees C water, a decrease of only 1.5 degrees C in T (re) resulted in the elimination of 100% of the heat gained during exercise without causing hypothermia. This study would therefore support cooling the core temperature of hyperthermic subjects to a rectal temperature between 37.8 degrees C (during immersion in water >10 degrees C) and 38.6 degrees C (during immersion in water <10 degrees C) to eliminate the heat gained during exercise without causing hypothermia.

Confounding factors in the use of the zero-heat-flow method for non-invasive muscle temperature measurement

This study evaluated a zero-heat-flow (ZHF), non-invasive temperature probe for in- vivo measurement of resting muscle temperature for up to 2 cm below the skin surface. The ZHF probe works by preventing heat loss from the tissue below the probe by actively heating the tissue until no temperature gradient exists across the probe. The skin temperature under the probe is then used as an indicator of the muscle temperature below. Eight subjects sat for 130 min during exposure to 28 degrees C air. Vastus lateralis (lateral thigh) muscle temperature was measured non-invasively using a ZHF probe which covered an invasive multicouple probe (which measured tissue temperature 0.5 cm, 1 cm, 1.5 cm, and 2 cm below the skin) located 15 cm superior to the patella (T (covered)). T (covered) was evaluated against an uncovered control multicouple probe located 20 cm superior to the patella (T (uncovered)). Rectal temperature and lateral thigh skin temperature were also measured. Mean T (uncovered) (based on average temperatures at the 0.5 cm, 1 cm, 1.5 cm, and 2 cm depths) and Mean T (covered) were similar from time 0 min to 60 min. However, when the ZHF was turned on at 70 min, Mean T (covered) increased by 2.11 +/- 0.20 degrees C by 130 min, while T (uncovered) remained stable. The ZHF probe temperature was similar to T (covered) at 1 cm and after time 85 min, significantly higher than T (covered) at the 0.5 cm, 1.5 cm, and 2 cm depths; however from a physiological standpoint, the temperatures between the different depths and the ZHF probe could be considered uniform (< or =0.2 degrees C separation). Rectal and thigh skin temperatures were stable at 36.99 +/- 0.08 degrees C and 32.82 +/- 0.23 degrees C, respectively. In conclusion, the non-invasive ZHF probe temperature was similar to the T (covered) temperatures directly measured up to 2 cm beneath the surface of the thigh, but all T (covered) temperatures were not representative of the true muscle temperature up to 2 cm below the skin because the ZHF probe heated the muscle by 2.11 +/- 0.20 degrees C during its operation.

Influence of personality traits on plasma levels of cortisol and cholesterol

The literature reports many organic malfunctions that are associated with elevated plasma cortisol and cholesterol levels. The present investigation was concerned with the influence of personality on plasma levels of cortisol and cholesterol. To that effect these variables were determined in a group of 20 subjects who answered the Big-Five Inventory for measurements of personality traits. It was found that: among the 5 personality traits, extraversion was positively correlated to plasma levels of cortisol and cholesterol while the correlation was negative for neuroticism. The positive correlation between extraversion and plasma cortisol and cholesterol, as well as with the responses to stress as shown in a previous study, are similar to findings previously reported on type A individuals. Further studies are needed with a larger group of subjects to conclude to a direct causal relationship between extraversion and the high levels of plasma cortisol and cholesterol, or a predisposition to some organic malfunctions as is the case for type A.

Study on the correlation of the autonomic nervous system responses to a stressor of high discomfort with personality traits

The present study investigated Eysenck's predictions concerning the correlation of personality to arousal at higher levels of stress. Twenty young adults were exposed to a physical stress causing great discomfort, specifically a cold wind (4 degrees C at 60 km/h) exposure to the face for 3 min. Autonomic nervous system (ANS) responses were measured by continuous heart rate and plasma catecholamine determinations before, during and after the test. At the end of the test, the participants gave a rating of discomfort on a 0 to 10 scale. The personality traits were assessed with the Big-Five Inventory test (BFI). Results indicated that higher levels of trait extraversion were positively correlated with discomfort ratings and with the increased heart rate and the noradrenaline responses. Neuroticism was negatively correlated to discomfort and the autonomic responses. These findings tend to support Eysenck's theory on the role of personality on arousal at higher levels of stress. It is also proposed that the better tolerance to this severe stress observed with neuroticism is correlated to a certain habituation process caused by light to moderate arousal frequently experienced by participants with this personality trait during their daily activities.

Warming by immersion or exercise affects initial cooling rate during subsequent cold water immersion

INTRODUCTION

We examined the effect of prior heating, by exercise and warm-water immersion, on core cooling rates in individuals rendered mildly hypothermic by immersion in cold water.

METHODS

There were seven male subjects who were randomly assigned to one of three groups: 1) seated rest for 15 min (control); 2) cycling ergometry for 15 min at 70% Vo2 peak (active warming); or 3) immersion in a circulated bath at 40 degrees C to an esophageal temperature (Tes) similar to that at the end of exercise (passive warming). Subjects were then immersed in 7 degrees C water to a Tes of 34.5 degrees C.

RESULTS

Initial Tes cooling rates (initial approximately 6 min cooling) differed significantly among the treatment conditions (0.074 +/- 0.045, 0.129 +/- 0.076, and 0.348 +/- 0.117 degrees C x min(-1) for control, active, and passive warming conditions, respectively); however, secondary cooling rates (rates following initial approximately 6 min cooling to the end of immersion) were not different between treatments (average of 0.102 +/- 0.085 degrees C x min(-1)). Overall Tes cooling rates during the full immersion period differed significantly and were 0.067 +/- 0.047, 0.085 +/- 0.045, and 0.209 +/- 0.131 degrees C x min(-1) for control, active, and passive warming, respectively.

DISCUSSION

These results suggest that prior warming by both active and, to a greater extent, passive warming, may predispose a person to greater heat loss and to experience a larger decline in core temperature when subsequently exposed to cold water. Thus, functional time and possibly survival time could be reduced when cold water immersion is preceded by whole-body passive warming, and to a lesser degree by active warming.

Selection of military survival gears using thermal manikin and computer survival model data

This study presents a practical example of the selection of protective equipment for 12-h cold survival on land and at sea using computer model and manikin data. The thermal immersion manikin was exposed to 19 realistic survival scenarios to estimate the thermal resistance of different survival systems. The computer survival model used specific environmental limits and anthropometric data from the target population in addition to the estimated manikin thermal resistance values to generate survival times. The results showed that the required 12-h survival time criteria were met for all dry land scenarios (> 2 Clo), but not for wet land or water scenarios ( < 1 Clo). Those data provided the basis for the selection of survival equipment and the development of survival strategies for aircrew.

Effects of carbohydrate availability on sustained shivering II. Relating muscle recruitment to fuel selection

The purpose of this study was to quantify how shivering activity would be affected by large changes in fuel metabolism (see Haman F, Peronnet F, Kenny GP, Doucet E, Massicotte D, Lavoie C, and Weber J-M, J Appl Physiol 96: 000-000, 2004). Adult men were exposed to 10 degrees C for 2 h after a low-carbohydrate diet and exercise (Lo) and after high-carbohydrate diet without exercise (Hi). Using simultaneous metabolic and electromyographic (EMG) measurements, we quantified the effects of changes in fuel selection on the shivering activity of eight large muscles representing >90% of total shivering muscle mass. Contrary to expectation, drastic changes in fuel metabolism [carbohydrates 28 vs. 65% of total heat production (Hprod), lipids 53 vs. 23% Hprod, and proteins 19 vs. 12% Hprod for Lo and Hi, respectively] are achieved without altering the EMG signature of shivering muscles. Results show that total shivering activity and the specific contribution of each muscle to total shivering activity are not affected by large changes in fuel selection. In addition, we found that changes in burst shivering rate ( approximately 4 bursts/min), relative contribution of burst activity to total shivering ( approximately 10% of total shivering activity), and burst shivering intensity ( approximately 12% of maximal voluntary contraction) are the same between Lo and Hi. Spectral analysis of EMG signals also reveals that mean frequencies of the power spectrum remained the same under all conditions (whole body average of 78 +/- 5 Hz for Lo and 83 +/- 7 Hz for Hi). During low-intensity shivering, humans are therefore able to sustain the same thermogenic rate by oxidizing widely different fuel mixtures within the same muscle fibers.

Response to thermal stress and personality

Tolerance to cold and heat exposure shows large variations for which there is still insufficient explanation. On the other hand the relationship between the responses to mental stress and individual personality is well documented. The aim of this study was then to find if personality traits have some influence on the responses to environmental temperature exposure. A group of 20 young adults were exposed for 90 min to cold (10 degrees C) while skin temperature (Ts), oxygen consumption and discomfort rating were recorded. In a second experiment they were exposed to heat (40 degrees C) for 90 min when the sweat rate and the discomfort rating were recorded. Prior to these tests the Big Five Personality Test was used to measure the personality traits of the subjects. The results show significant negative correlation between neuroticism and the O(2) consumed, the discomfort rating and Ts for the test in the cold, while extraversion was positively related to O(2) consumption but not to Ts and discomfort rating. In response to heat, neuroticism predominance was associated with greater discomfort, reduced tolerance and diminished sweat rate. The discomfort rating, in this case, was negatively related to extraversion. It is proposed that the reduced O(2) consumption in the cold and the lower rate of sweating in the heat observed with neuroticism, are caused by enhanced activity of the sympathetic nervous system. Further investigation is required to assess the validity of this proposal. Overall, the present investigation shows that physical environmental stresses, in common with mental stress, could be in some ways related to personality traits.

Finger dexterity, skin temperature, and blood flow during auxiliary heating in the cold

The primary purpose of the present study was to compare the effectiveness of two forms of hand heating and to discuss specific trends that relate finger dexterity performance to variables such as finger skin temperature (T(fing)), finger blood flow (Q(fing)), forearm skin temperature (T(fsk)), forearm muscle temperature (Tfmus), mean weighted body skin temperature (Tsk), and change in body heat content (DeltaH(b)). These variables along with rate of body heat storage, toe skin temperature, and change in rectal temperature were measured during direct and indirect hand heating. Direct hand heating involved the use of electrically heated gloves to keep the fingers warm (heated gloves condition), whereas indirect hand heating involved warming the fingers indirectly by actively heating the torso with an electrically heated vest (heated vest condition). Seven men (age 35.6 +/- 5.6 yr) were subjected to each method of hand heating while they sat in a chair for 3 h during exposure to -25 degrees C air. Q(fing) was significantly (P < 0.05) higher during the heated vest condition compared with the heated gloves condition (234 +/- 28 and 33 +/- 4 perfusion units, respectively), despite a similar T(fing) (which ranged between 28 and 35 degrees C during the 3-h exposure). Despite the difference in Q(fing), there was no significant difference in finger dexterity performance. Therefore, finger dexterity can be maintained with direct hand heating despite a low Q(fing). DeltaH(b), Tsk, and T(fmus) reached a low of -472 +/- 18 kJ, 28.5 +/- 0.3 degrees C, and 29.8 +/- 0.5 degrees C, respectively, during the heated gloves condition, but the values were not low enough to affect finger dexterity.

Muscle temperature transients before, during, and after exercise measured using an intramuscular multisensor probe

Seven subjects (1 woman) performed an incremental isotonic test on a Kin-Com isokinetic apparatus to determine their maximal oxygen consumption during bilateral knee extensions (Vo(2 sp)). A multisensor thermal probe was inserted into the left vastus medialis (middiaphysis) under ultrasound guidance. The deepest sensor (tip) was located approximately 10 mm from the femur and deep femoral artery (T(mu 10)), with additional sensors located 15 (T(mu 25)) and 30 mm (T(mu 40)) from the tip. Esophageal temperature (T(es)) was measured as an index of core temperature. Subjects rested in an upright seated position for 60 min in an ambient condition of 22 degrees C. They then performed 15 min of isolated bilateral knee extensions (60% of Vo(2 sp)) on a Kin-Com, followed by 60 min of recovery. Resting T(es) was 36.80 degrees C, whereas T(mu 10), T(mu 25), and T(mu 40) were 36.14, 35.86, and 35.01 degrees C, respectively. Exercise resulted in a T(es) increase of 0.55 degrees C above preexercise resting, whereas muscle temperature of the exercising leg increased by 2.00, 2.37, and 3.20 degrees C for T(mu 10), T(mu 25), and T(mu 40), respectively. Postexercise T(es) showed a rapid decrease followed by a prolonged sustained elevation approximately 0.3 degrees C above resting. Muscle temperature decreased gradually over the course of recovery, with values remaining significantly elevated by 0.92, 1.05, and 1.77 degrees C for T(mu 10), T(mu 25), and T(mu 40), respectively, at end of recovery (P < 0.05). These results suggest that the transfer of residual heat from previously active musculature may contribute to the sustained elevation in postexercise T(es).

Effect of water temperature on cooling efficiency during hyperthermia in humans

We evaluated the cooling rate of hyperthermic subjects, as measured by rectal temperature (T(re)), during immersion in a range of water temperatures. On 4 separate days, seven subjects (4 men, 3 women) exercised at 65% maximal oxygen consumption at an ambient temperature of 39 degrees C until T(re) increased to 40 degrees C (45.4 +/- 4.1 min). After exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14, or 20 degrees C until T(re) returned to 37.5 degrees C. No difference in cooling rate was observed between the immersions at 8, 14, and 20 degrees C despite the differences in the skin surface-to-water temperature gradient, possibly because of the presence of shivering at 8 and 14 degrees C. Compared with the other conditions, however, the rate of cooling (0.35 +/- 0.14 degrees C/min) was significantly greater during the 2 degrees C water immersion, in which shivering was seldom observed. This rate was almost twice as much as the other conditions (P < 0.05). Our results suggest that 2 degrees C water is the most effective immersion treatment for exercise-induced hyperthermia.

Tissue temperature transients in resting contra-lateral leg muscle tissue during isolated knee extension

This study was designed to evaluate the role of non-active tissue in the retention and dissipation of heat during and following intense isolated muscle activity. Six subjects performed an incremental isotonic test (constant angular velocity, increases in force output) on a KIN-COM isokinetic apparatus to determine their maximal oxygen consumption during single knee extensions (VO2sp). In a subsequent session, a thin wire multi-sensor temperature probe was inserted into the left vastus medialis under ultrasound guidance at a specific internal marker. The deepest temperature sensor (tip, Tmu10) was located approximately 10 mm from the femur and deep femoral artery with 2 additional sensors located at 15 (Tmu25) and 30 (Tmu40) mm from the tip. Implant site was midway between and medial to a line joining the anterior superior iliac spine and base of patella. Esophageal temperature (Tes) temperature was measured as an index of core temperature. Subjects rested in a supine position for 60 min followed by 30 min of seated rest in an ambient condition of 22 degree C. Subjects then performed 15 min of isolated single right knee extensions against a dynamic resistance on a KIN COM corresponding to 60% of VO2sp at 60 degree x sec(-1). Exercise was followed by 60 min of seated rest. Resting Tes was 37 degree C while Tmu10, Tmu25, and Tmu40 were 36.58, 36.55 and 36.45 degree C, respectively. Exercise resulted in a Tes increase of 0.31 C above pre-exercise resting. Tmu of the non-exercising leg increased 0.23, 0.19 and 0.09 degree C for Tmu10, Tmu25, and Tmu40, respectively. While Tes decreased to baseline values within approximately 15 min of end-exercise, Tmu10 reached resting values following approximately 40 min of recovery. These results suggest that during isolated muscle activity, convective heat transfer by the blood to non-active muscle tissue may have a significant role in maintaining resting core temperature.

Effect of modafinil on core temperature during sustained wakefulness and exercise in a warm environment

BACKGROUND

Previous studies have revealed that modafinil elevates resting core temperature during periods of sustained wakefulness. The purpose of this study was to examine the effects of modafinil on core temperature during rest and exercise throughout 40 h of sustained wakefulness in a warm environment.

METHODS

Ten males performed a drug session (three 100 mg doses per day) and a placebo session that involved a control day, 40 h of sustained wakefulness, and a recovery sleep. For 38 h of the sustained wakefulness, subjects were exposed to 30 degrees C with 50% relative humidity. During the afternoon of both days of wakefulness and during the early morning of the second day, subjects performed 2 h of exercise at 60% VO2max while exposed to the warm environment.

RESULTS

The data revealed that rectal temperature (Tre) was elevated at rest 0.15-0.2 degrees C following modafinil ingestion throughout the period of sustained wakefulness. This increase in body temperature at rest was due to an increase in heat production during the first day of wakefulness followed by a lower evaporative heat loss during the second day. During exercise, an inconsistent effect of the drug on Tre was observed throughout the 38-h period. On the first afternoon, the impact of modafinil on Tre was no longer evident after 20 min of exercise. In contrast, during the early morning and afternoon of the second day, the effects of the drug on Tre at rest remained during exercise. For seven subjects who had Tre data for 80 min during all exercise periods, Tre during the placebo session was 38.9, 38.4, and 38.7 degrees C after 80 min of exercise for periods one, two and three, respectively, whereas the corresponding values during the modafinil session were 38.8, 38.7, and 38.9 degrees C.

CONCLUSION

With a greater cumulative dose of the drug, Tre remained elevated throughout the exercise period to an extent similar to the increase observed under resting conditions when compared with the placebo condition.

Ultra-sound imaging for precision implantation of a multi sensor temperature probe in skeletal muscle tissue

A technique for implanting multi sensor temperature probes in muscle tissue was developed to optimize the accuracy of the tissue temperature measurements and the internal localization of the probe. Real time ultra-sound imaging was used to (a) determine the best perpendicular insertion tract, (b) guide the insertion of the probe in order to avoid major blood vessels, and (c) verify the insertion point relative to discernable anatomic reference structures such as arteries and bone.

Combined effect of repetitive work and cold on muscle function and fatigue

This study compared the effect of repetitive work in thermoneutral and cold conditions on forearm muscle electromyogram (EMG) and fatigue. We hypothesize that cold and repetitive work together cause higher EMG activity and fatigue than repetitive work only, thus creating a higher risk for overuse injuries. Eight men performed six 20-min work bouts at 25 degrees C (W-25) and at 5 degrees C while exposed to systemic (C-5) and local cooling (LC-5). The work was wrist flexion-extension exercise at 10% maximal voluntary contraction. The EMG activity of the forearm flexors and extensors was higher during C-5 (31 and 30%, respectively) and LC-5 (25 and 28%, respectively) than during W-25 (P < 0.05). On the basis of fatigue index (calculated from changes in maximal flexor force and flexor EMG activity), the fatigue in the forearm flexors at the end of W-25 was 15%. The corresponding values at the end of C-5 and LC-5 were 37% (P < 0.05 in relation to W-25) and 20%, respectively. Thus repetitive work in the cold causes higher EMG activity and fatigue than repetitive work in thermoneutral conditions.

Relationship between body heat content and finger temperature during cold exposure

The purpose of the present experiment was to examine the relationship between rate of body heat storage (S˙), change in body heat content (ΔHb), extremity temperatures, and finger dexterity. S˙, ΔHb , finger skin temperature (Tfing), toe skin temperature, finger dexterity, and rectal temperature were measured during active torso heating while the subjects sat in a chair and were exposed to −25°C air. S˙ and ΔHb were measured using partitional calorimetry, rather than thermometry, which was used in the majority of previous studies. Eight men were exposed to four conditions in which the clothing covering the body or the level of torso heating was modified. After 3 h, Tfing was 34.9 ± 0.4, 31.2 ± 1.2, 18.3 ± 3.1, and 12.1 ± 0.5°C for the four conditions, whereas finger dexterity decreased by 0, 0, 26, and 39%, respectively. In contrast to some past studies, extremity comfort can be maintained, despite S˙ that is slightly negative. This study also found a direct linear relationship between ΔHb and Tfing and toe skin temperature at a negative ΔHb. In addition, ΔHb was a better indicator of the relative changes in extremity temperatures and finger dexterity over time than S˙.

Factors limiting cold-water swimming distance while wearing personal floatation devices

The influence of body adiposity, arm skinfold thickness, aerobic capacity, and cooling rate were studied in a mock survival swimming situation conducted in water at around 14 degrees C. Seventeen adult participants wore personal floatation devices on top of seasonal clothing and were asked to swim as far as they could, as if attempting to reach shore following an accidental immersion in cold water. Triceps and patellar skinfold thickness showed a significant correlation with distance covered (r = 0.70 and 0.56, respectively), while abdominal skinfold and percent body fat showed no significant correlation. Maximum oxygen consumption (VO2max) was not significantly related to distance covered. There was a negative correlation between body cooling rate during the swimming period and distance covered. A multiple stepwise regression analysis, however, indicated that the only significant contributor to variance in the distance covered was the triceps skinfold thickness (r2 = 0.49). It was concluded that for a healthy subject accidentally immersed in cold water, triceps skinfold thickness is a stronger predictor of the swimming distance covered than body adiposity, VO2max, or the drop in core temperature.

Axon reflexes in human cold exposed fingers

Exposure of fingers to severe cold induces cold induced vasodilatation (CIVD). The mechanism of CIVD is still debated. The original theory states that an axon reflex causes CIVD. To test this hypothesis, axon reflexes were evoked by electrical stimulation of the middle fingers of hands immersed in water at either 5 degrees C or 35 degrees C. Axon reflexes were pronounced in the middle finger of the hand in warm water, but absent from the hand in cold water, even though the stimulation was rated as "rather painful" to "painful". These results showed that axon reflexes do not occur in a cold-exposed hand and thus are unlikely to explain the CIVD phenomenon.

Finger cold-induced vasodilation during mild hypothermia, hyperthermia and at thermoneutrality

BACKGROUND

Exposure of the fingers to severe cold leads to cold-induced vasodilation (CIVD). The influence of ambient temperature on the CIVD-response is well understood and documented, but the response of CIVD to hyperthermia and mild hypothermia has rarely been investigated.

METHODS

To investigate the influence of body thermal status on the CIVD response, eight subjects immersed their right hand in 5 degrees C water for 40 min during mild hypothermia (C), thermoneutrality (N) and hyperthermia (W). The mean skin temperature of the body (Tsk), the esophageal temperature (Tes), the temperature of the volar side of the distal phalanx of each immersed finger (Tfi) and the skin perfusion of the immersed middle finger (Qsk) were continuously measured.

RESULTS

During the W condition the body temperatures were higher (Tes: 38.0+/-0.1 degrees C; Tsk: 37.9+/-0.7 degrees C) than during N (Tes: 36.8+/-0.2 degrees C; Tsk: 31.8+/-0.7 degrees C) and during C (Tes: 36.1+/-0.8 degrees C; Tsk: 21.2+/-1.9 degrees C). Tfi and Qsk were higher during the W condition (Tfi: 16.5+/-2.3 degrees C; Qsk: 133+/-53 perfusion units (PU)) than during N (Tfi: 8.1+/-1.7 degrees C; Qsk: 57+/-39 PU) and during C (Tfi: 6.8+/-1.2 degrees C; Qsk: 22+/-14 PU). The onset time of CIVD was significantly prolonged in condition C (13.0+/-3.8 min) as compared with N (7.2+/-2.2 min).

CONCLUSION

It was concluded that the CIVD response is significantly affected by body core and skin temperatures.

Physiological responses of exercised-fatigued individuals exposed to wet-cold conditions

Thirteen healthy and fit men [age = 27 +/- 8 (SD) yr, height = 177 +/- 5 cm, mass = 75 +/- 7 kg, body fat = 14 +/- 5%, maximal O2 consumption = 51 +/- 4 ml. kg-1. min-1] participated in an experiment designed to test their thermoregulatory response to a challenging cold exposure after 5 h of demanding mixed exercise during which only water was consumed. Subjects expended 7,314 +/- 741 kJ on cycling, rowing, and treadmill-walking machines, performed 8,403 +/- 1,401 kg. m of mechanical work during resistance exercises, and completed 120 inclined sit-ups. Subjects then assumed a seated position in a 10 degrees C air environment while wearing shorts, T-shirt, rain hat, and neoprene gloves and boots. After 30 min the subjects were showered continuously with cold water ( approximately 920 ml/min at 10 degrees C) on their backs accompanied by a 6 km/h wind for up to 4 h. Blood samples were taken from the nondominant arm every 30 min during the exposure and assayed for energy metabolites, hormones, indexes of hydration, and neurotransmitters. Counterbalanced control trials without prior exercise were also conducted. Blood insulin was higher during the control trial, whereas values of glycerol, nonesterified fatty acids, beta-hydroxybutyrate, lactate, cortisol, free triiodothyronine, and thyroxine were lower. Three subjects lasted the maximum duration of 4.5 h for control and fatigue trials, with final rectal temperatures of 36.43 +/- 0.21 and 36.08 +/- 0.49 degrees C, respectively. Overall, the duration of 172 +/- 68 (SD) min for the fatigue trial was not significantly different from that of the control trial (197 +/- 72 min) and, therefore, was not affected by the preexposure exercise. Although duration was positively correlated to body fatness and shivering intensity, the latter was not correlated to any physical characteristic or the fitness level of the individual.

Influence of localized auxiliary heating on hand comfort during cold exposure

There is a need for a hand-heating system that will keep the hands warm during cold exposure without hampering finger dexterity. The purpose of this study was to examine the effects of torso heating on the vasodilative responses and comfort levels of cooled extremities during a 3-h exposure to -15 degreesC air. Subjects were insulated, but their upper extremities were left exposed to the cold ambient air. The effect of heating the torso [torso-heating test (THT)] on hand comfort was compared with a control condition in which no torso heating was applied, but Arctic mitts were worn [control test (CT)]. The results indicate that mean finger temperature, mean finger blood flow, mean toe temperature, mean body skin temperature, body thermal comfort, mean finger thermal comfort, and rate of body heat storage were all significantly (P < 0.05) higher on average (n = 6) during THT. Mean body heat flow was significantly (P < 0.05) lower during THT. There were no significant differences (P >/= 0.05) in rectal temperature between CT and THT. Mean unheated body skin temperature and mean unheated body heat flow (both of which did not include the torso area in the calculation of mean body skin temperature and mean body heat flow) were also calculated. There were no significant differences (P >/= 0.05) in mean unheated body skin temperature and mean unheated body heat flow between CT and THT. It is concluded that the application of heat to the torso can maintain finger and toe comfort for an extended period of time during cold exposure.

The effect of wave motion on dry suit insulation and the responses to cold water immersion

METHODS

Six subjects who were each wearing a dry immersion suit system were immersed for 1 h in 16 degrees C water in a number of different wave conditions, ranging from still water to 70 cm in height. Physiological and physical parameters were measured in order to calculate the total thermal resistance of the suit system and its components.

RESULTS

None of the physiological parameters were affected significantly by the wave conditions, except for skin heat flux, which increased with wave height from 72.0 +/- 1.9 W x m(-2), at 0 cm of height, to 85.5 +/- 2.9 W x m(-2), at 70 cm of height. Wave heights up to 70 cm decreased the insulation (including boundary layer) of the dry suit system by 14%, and the only component of the suit affected by the wave motion was the insulation of the water boundary layer, which decreased by 75%. The body sites that were most affected by wave motion were the head and the trunk, with an average 45% decrement in suit system thermal resistance at those sites at wave heights of 0 to 70 cm. No significant effect was observed at sites on the distal limbs.

CONCLUSION

To simulate open ocean conditions in the laboratory, the standards must take the reduction of suit insulation into account.

A Chronic Arterial Cannula for Blood Sampling in Conscious, Unrestrained Rats

We require repeated blood samples for at least 10 days from conscious, unrestrained rats in our studies investigating the inflammatory response to intra-abdominal infection. Furthermore, we require a means by which other experimental devices (implanted concurrently) can be kept out of reach of the animals for the duration of the study. Under these conditions, previously reported postoperative restraint and cannulation techniques did not allow cannula patency for longer than a few days. We designed a cannula that is constructed from microbore tubing, attached to a modified winged infusion catheter, and sutured over the scapulae of the animal. Our device features a curved, perforated tip to allow its easy insertion into the thoracic aorta via the left carotid artery, and it provides an attachment site for a small, lightweight backpack. This cannula has a small dead-space and accommodates repeated arterial blood sampling for at least 10 days without leading to bacteremia or significant alterations in hematological or immune parameters. With minor modifications, this cannula has potential application in the chronic cannulation of other vessels and ducts in other small animals.

The effect of body temperature on the hunting response of the middle finger skin temperature

The relationship between body temperature and the hunting response (intermittent supply of warm blood to cold exposed extremities) was quantified for nine subjects by immersing one hand in 8 degree C water while their body was either warm, cool or comfortable. Core and skin temperatures were manipulated by exposing the subjects to different ambient temperatures (30, 22, or 15 degrees C), by adjusting their clothing insulation (moderate, light, or none), and by drinking beverages at different temperatures (43, 37 and 0 degrees C). The middle finger temperature (Tfi) response was recorded, together with ear canal (Tear), rectal (Tre), and mean skin temperature (Tsk). The induced mean Tear changes were -0.34 (0.08) and +0.29 (0.03) degrees C following consumption of the cold and hot beverage, respectively. Tsk ranged from 26.7 to 34.5 degrees C during the tests. In the warm environment after a hot drink, the initial finger temperature (T(fi,base)) was 35.3 (0.4) degrees C, the minimum finger temperature during immersion (T(fi,min)) was 11.3 (0.5) degrees C, and 2.6 (0.4) hunting waves occurred in the 30-min immersion period. In the neutral condition (thermoneutral room and beverage) T(fi,base) was 32.1 (1.0) degrees C, T(fi,min) was 9.6 (0.3) degrees C, and 1.6 (0.2) waves occurred. In the cold environment after a cold drink, these values were 19.3 (0.9) degrees C, 8.7 (0.2) degrees C, and 0.8 (0.2) waves, respectively. A colder body induced a decrease in the magnitude and frequency of the hunting response. The total heat transferred from the hand to the water, as estimated by the area under the middle finger temperature curve, was also dependent upon the induced increase or decrease in Tear and Tsk. We conclude that the characteristics of the hunting temperature response curve of the finger are in part determined by core temperature and Tsk. Both T(fi,min) and the maximal finger temperature during immersion were higher when the core temperature was elevated; Tsk seemed to be an important determinant of the onset time of the cold-induced vasodilation response.

Efficacy of forced-air and inhalation rewarming by using a human model for severe hypothermia

We recently developed a nonshivering human model for severe hypothermia by using meperidine to inhibit shivering in mildly hypothermic subjects. This thermal model was used to evaluate warming techniques. On three occasions, eight subjects were immersed for approximately 25 min in 9 degrees C water. Meperidine (1.5 mg/kg) was injected before the subjects exited the water. Subjects were then removed, insulated, and rewarmed in an ambient temperature of -20 degrees C with either 1) spontaneous rewarming (control), 2) inhalation rewarming with saturated air at approximately 43 degrees C, or 3) forced-air warming. Additional meperidine (to a maximum cumulative dose of 2.5 mg/kg) was given to maintain shivering inhibition. The core temperature afterdrop was 30-40% less during forced-air warming (0.9 degree C) than during control (1.4 degrees C) and inhalation rewarming (1.2 degrees C) (P < 0.05). Rewarming rate was 6- to 10-fold greater during forced-air warming (2.40 degrees C/h) than during control (0.41 degree C/h) and inhalation rewarming (0.23 degree C/h) (P < 0.05). In nonshivering hypothermic subjects, forced-air warming provided a rewarming advantage, but inhalation rewarming did not.

Granisetron shows no pro-arrhythmic effect in normal subjects during or after exercise in a hot environment

BACKGROUND

5-HT3 receptor antagonists are being evaluated as possible agents to prevent nausea and vomiting associated with radiation exposure in a non-clinical military setting. Because of concern about potential cardiovascular toxicity and the observation that certain developmental 5-HT3 antagonists produced undesirable effects, all drugs in this class are being carefully scrutinized for possible adverse cardiac effects.

METHOD

In this study, nine subjects underwent ambulatory ECG monitoring for an average of 21.6 h after a 2-mg oral dose of granisetron or placebo in a double-blind crossover protocol. Monitoring included a 3-h period of submaximal exercise in a 40 degrees C environment.

RESULTS

Although isolated ventricular and supraventricular ectopic activity, sinus bradycardia, and pauses were found, there were no sustained arrhythmias observed in either the placebo or granisetron conditions.

CONCLUSION

Although the generalizability of this study is limited by the small number of subjects, these observations add to the body of evidence confirming the lack of cardiovascular toxicity of granisetron.

Influence of granisetron on thermoregulation during exercise in the heat

BACKGROUND

A NATO project group has an interest in selecting an antiemetic agent that not only is effective in the prevention of emesis induced by chemical agents or radiation exposure but also has minimal, if any, side effects. Granisetron is the second candidate drug of a class of selective serotonin antagonists that has been shown to be an effective antiemetic agent for patients receiving radiation or chemotherapy treatment. The present study was designed to evaluate whether a single 2-mg oral dose of granisetron influenced temperature regulation during exercise in a hot and relatively dry environment.

HYPOTHESIS

Based on our previous findings with the other candidate drug, ondanseton, we hypothesized that granisetron would not influence temperature regulation.

METHODS

Nine unacclimatized males performed a drug and placebo trial in a double-blind manner. The sessions involved walking on a treadmill at 4.8 km.h-1 with a 2% elevation for a maximum of 3 h at 40 degrees C and 30% relative humidity while wearing combat clothing.

RESULTS

Granisetron was associated with a small (0.2 degrees C) but significant elevation in mean skin temperature at the beginning and after 2 h of exercise. However, there was no difference between trials for the 1.6 degrees C increase in rectal temperature. Also, body heat gain (406 +/- 97 and 407 +/- 103 k) for the placebo and drug trial, respectively) and whole body sweat rates (0.72 +/- 0.10 and 0.73 +/- 0.10 kg.h-1 for the placebo and granisetron trial, respectively) were not different. Tolerance times also were not different for the placebo (157.4 +/- 16.7 min) and drug (159.4 +/- 20.4 min) sessions.

CONCLUSIONS

For the environmental conditions used in this investigation, we would accept the null hypothesis that a single 2 mg oral dose of granisetron does not influence temperature regulation during exercise.

The effect of postural changes on body temperatures and heat balance

Early studies have demonstrated that rectal temperature (T(re)) decreases and mean skin temperature (Tsk) increases in subjects changing their posture from standing to supine, and vice versa. Such changes have important implications insofar as thermal stress experiments are conducted and interpreted. However, the extent of these changes between steady-state conditions is not known. In addition, it is not known whether thermal balance is also affected by postural changes. To examine these questions, 11 healthy males were exposed to a thermoneutral air environment (28.2-28.5 degrees C and 40% relative humidity) in various postures at rest. Body temperatures, heat losses, and metabolic rate were measured. Subjects wore shorts only and began in an upright posture (standing or sitting at an inclination of 7.5 degrees) on a customized tilt-table. They were tilted twice, once into a supine position and then back to the original upright position. Each tilt occurred after steady state was satisfied based on the subject's circadian variation of T(re) determined previously in a 4.25 h control supine trial. Times to supine steady state following the first tilt were [mean(SE)] 92.6 (6.4) and 116.6 (5.1) min for the standing and sitting trials, respectively. Times to upright steady state following the second tilt were 107.9 (11.4) and 124.1 (9.0) min. Mean steady-state T(re) and Tsk were 36.87 (0.07) and 34.04 (0.14), 37.47 (0.09) and 33.48 (0.14), and 37.26 (0.05) and 33.49 (0.10) degrees C for supine, standing, and sitting, respectively. Thermal balance was attained in all steady-state conditions, and allowing for a decrease in the weighting factor of T(re) for mean body temperature in the upright postures, it also appears that thermal balance was preserved between changes in posture. These results are consistent with no perceived changes by the subjects in their thermal comfort and skin wetness.

Effect of triazolam on responses to a cold-water immersion in humans

Managing alertness of soldiers during sustained operations is a source of serious concern for military unit commanders. A frequently employed strategy is to induce sleep before an operation, especially operations requiring prolonged travel. Sleep-inducing drugs could have an action on thermoregulation through their effect on alertness and a possible direct effect on the brain. The goal of this study was therefore to evaluate the effect of a commonly prescribed triazolam (Halcion) on thermoregulatory responses to cold-water immersion. Eight subjects were immersed twice in 18 degrees C water for up to 90 min in the morning; once after ingesting 0.25 mg triazolam (TRZ) the prior evening, and again after placebo (PLB) treatment. There were no significant differences between trials for mean duration of the immersion, the change in rectal temperature and mean skin temperature. Total metabolic heat production was similar for both conditions: 767 +/- 107 vs. 781 +/- 105 kj.m-2 for TRZ and PLB, respectively. The results should be considered in light of a large variation among the subjects in sensitivity to TRZ, which was unrelated to biometrical characteristics such as surface area-to-mass ratio, lean body mass, % body fat, and physical fitness. Although not statistically significant, there was a trend for a smaller increase in plasma free fatty acid and glycerol concentrations after water immersion with TRZ. The results suggest that the ingestion of a single dose of triazolam 11 h prior to a cold-water immersion is not likely to accelerate the rate of onset of hypothermia. Individual sensitivity, however, may predispose some sensitive subjects to negative effects in this regard.

Role of blood as heat source or sink in human limbs during local cooling and heating

The objective of the present study was to investigate the relative contribution of the convective heat transfer in the forearm and hand to 1) the total heat loss during partial immersion in cold water [water temperature (Tw) = 20 degrees C] and 2) the heat gained during partial immersion in warm water (Tw = 38 degrees C). The heat fluxes from the skin of the forearm and finger were continuously monitored during the 3.5-h immersion of the upper limb (forearm and hand) with 23 recalibrated heat flux transducers. The last 30 min of the partial immersion were conducted with an arterial occlusion of the forearm. The heat flux values decreased during the occlusion period at Tw = 20 degrees C and increased at Tw = 38 degrees C for all sites, plateauing only for the finger to the value of the tissue metabolic rate (124.8 +/- 29.0 W/m3 at Tw = 20 degrees C and 287.7 +/- 41.8 W/m3 at Tw = 38 degrees C). The present study shows that, at thermal steady state during partial immersion in water at 20 degrees C, the convective heat transfer between the blood and the forearm tissue is the major heat source of the tissue and accounts for 85% of the total heat loss to the environment. For the finger, however, the heat produced by the tissue metabolism and that liberated by the convective heat transfer are equivalent. At thermal steady state during partial immersion in water at 38 degrees C, the blood has the role of a heat sink, carrying away from the limb the heat gained from the environment and, to a lesser extent (25%), the metabolic and conductive heats. These results suggest that during local cold stress the convective heat transfer by the blood has a greater role than that suggested by previous studies for the forearm but a lesser role for the hand.

Comparison of forced-air patient warming systems for perioperative use

BACKGROUND

Perianesthetic hypothermia is common and produces several complications, including postoperative shivering, decreased drug metabolism and clearance, and impaired wound healing. Forced-air warming transfers more than 50 W to the body and is an efficient method for either preventing or reversing decreases in core temperature.

METHODS

The authors compared the efficacy of four complete forced-air warming systems: (1) Bair Hugger 250/PACU Patient Warming System with 300 Warming Cover (Augustine Medical, Eden Prairie, MN); (2) Thermacare TC1000 Power Unit with TC1050 Comfort Quilt (Gaymar Industries, Orchard Park, NY); (3) WarmAir 130 Hypothermia System with 140 Warming Tube (Cincinnati Sub-Zero Products, Cincinnati, OH); and (4) WarmTouch 5000 Patient Warming System and 503-0810 CareQuilt (with the connecting hose compressed [short] and extended [long]) (Mallinckrodt Medical, St. Louis, MO). Six minimally clothed male volunteers were studied supine in a 24.5 degrees C environment. Cutaneous heat flux and skin temperature was measured at 14 area-weighted sites using thermal flux transducers. After 20-min control periods, volunteers were warmed for 40 min in each condition. A cotton blanket was placed over each cover. Power units were placed at the foot end of the bed, started cold, and set at maximum temperature and flow settings. All units reached maximum efficiency within 20 min.

RESULTS

Total heat transfer with the Bair Hugger system (95 +/- 7 W) was greater (P < 0.05) than with WarmTouch (short hose 81 +/- 6 W and long hose 68 +/- 8 W), Thermacare (61 +/- 5 W), and WarmAir (38 +/- 6 W) systems. Each cover also was tested on a common power unit (Bair Hugger 200). Total heat transfer was greater (P < 0.05) with the Warming Cover (Bair Hugger) (88 +/- 8 W), followed by the Comfort Quilt (Thermacare) (56 +/- 6 W), CareQuilt (WarmTouch) (50 +/- 7 W), and the Warming Tube (WarmAir) (43 +/- 6 W).

CONCLUSIONS

The advantages of the Bair Hugger system and Warming Cover are evident in areas that are important for heat transfer from the periphery to the body core (chest, axilla, abdomen, and upper legs).

Influence of ondansetron on thermoregulation during exercise in the heat wearing combat clothing

Ondansetron is a selective serotonin antagonist which has been shown to be an effective antiemetic agent for patients undergoing radiation or chemotherapy treatment. The Canadian Forces together with other NATO Countries have an interest in selecting an antiemetic agent that not only is effective in the prevention of emesis induced by chemical agents or radiation exposure, but also has minimal, if any, side effects. The purpose of this study was to examine the influence of a single 8-mg oral dose of the drug on thermoregulation during exercise in a hot (40 degrees C, 30% relative humidity) environment. Ten unacclimatized males performed a drug and placebo trial in single-blind random order. The sessions involved walking on a treadmill at 4.8 km.h-1 with a 2% elevation for a maximum of 3 h. Subjects wore combat clothing during the trials. Total exposure time was similar for the placebo (177 +/- 6 min) and drug (172 +/- 11 min) trials. Also, the rate of sweat production (0.64 +/- 0.1 and 0.66 +/- 0.1 kg.h-1 for placebo and drug, respectively) and body heat gain (303 +/- 112 and 305 +/- 110 kj for the placebo and drug, respectively) were not different between trials. Rectal temperature increased 1.48 +/- 0.40 degrees C for the placebo and 1.47 +/- 0.37 degrees C for the ondansetron trial. Finally, there was no difference in the mean skin temperature response which increased in both conditions to 37.1 +/- 0.5 degrees C. Under the conditions of this experiment, there is no evidence to suggest that the ingestion of ondansetron influences thermoregulation in a hot environment.

Adequacy of food rations in soldiers during an arctic exercise measured by doubly labeled water

To investigate the adequacy of food rations to supply energy needs in cold-temperature environments, caloric expenditure and intake and body composition changes were measured in a group of infantrymen during a 10-day field exercise in the Canadian Arctic. Energy expenditure was measured by the doubly labeled water method (n = 10), and caloric intake was measured by complete food intake records (n = 20). Body composition was determined by isotope dilution (n = 10) and bioelectrical impedence analysis (n = 20) on days 0 and 10. Baseline isotopic enrichment shifts due to geographical relocation were also monitored (n = 5). Mean body weight decreased 0.63 +/- 0.83 (SD) kg over the study period (P < 0.005), although fat-free and fat mass compartment changes were not significant. Baseline isotopic changes were -4.65 +/- 2.54 and -0.48 +/- 0.07 /1000/day for deuterium and 18O, respectively. Mean baseline corrected energy expenditure level was 4,317 +/- 927 kcal/day. Self-reported caloric intakes obtained from food records were 2,633 +/- 499 kcal/day (61.0% of expenditure). Rations packs contained 4,350 kcal/day. Results suggest that 1) food intake was significantly underreported and 2) the energy needs of most subjects were being met by rations and available supplements.