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.

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.

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.

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.

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.

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.

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.

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).

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 (DeltaH(b)), extremity temperatures, and finger dexterity. S, DeltaH(b), finger skin temperature (T(fing)), 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 degrees C air. S and DeltaH(b) 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, T(fing) was 34.9 +/- 0.4, 31.2 +/- 1.2, 18.3 +/- 3.1, and 12.1 +/- 0.5 degrees 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 DeltaH(b) and T(fing) and toe skin temperature at a negative DeltaH(b). In addition, DeltaH(b) 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.

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.