Thermal balance and survival time prediction of man in cold water.

Metabolic rates and rectal temperatures were continuously monitored for humans immersed in cold ocean water (4.6--18.2 degrees C) under stimulated accident conditions. The subjects wore only light clothing and a kapok lifejacket while either holding-still or swimming. While holding-still, metabolic heat production (Hm,kcal-min--1) was inversely related to water temperature (Tw, degrees C) according to the equation Hm equals 4.19 minus-0.117 Tw. This temperature response pattern is shown to be similar to that for exposure to air of the same temperature when air velocity is just over 5 m.p.h. (2.24 m/s). The thermogenic response was one-third efficient in balancing the calculated heat loss in cold water, resulting in hypothermia at a rectal temperature cooling rate (C, degrees C-min--1) dependent on water temperature (Tw, degrees C) according to the relation C equal 0.0785 - 0.0034Tw. Although swimming increased heat production to 2.5 times that of holding-still at 10.5 degrees C water temperature, cooling rate was 35% greater while swimming. A prediction equation for survival time (ts, min) of persons accidentally immersed in cold water (Tw, degrees C) has the form ts equal 15 + 7.2/(0.0785-0.0034Tw), based on the findings of this study, and it is compared to pre-existing models.     

Effect of behavioral variables on cooling rate of man in cold water.

Five different behaviors of man while in cold ocean water (9-10 degrees C) were assessed for their effect on rate of progress into hypothermia. With subjects wearing lifejackets, two thermally protective behaviors were studied which reduce exposure to the water of areas of body surface with high relative heat loss potential. One was huddling of three persons and the other a self-huddle behavior (HELP or Heat Escape Lessening Posture). These two behaviors resulted in significant reductions of rectal temperature cooling rate of 66 per cent and 69 per cent, respectively, of that of a control behavior. With no flotation available, two survival swimming behaviors (treading water and drownproofing) were shown to result in significant increases in cooling rate to 134 per cent and 182 per cent, respectively, of the control behavior. Potential swimming distance of subjects wearing a life-jacket was 0.85 miles in water near 12 degrees C before predicted incapacitation by hypothermia. It was concluded that behavioral variables can be of major importance in determining survival time in cold water through modulation of cooling rate associated with other variables such as fatness, body size, and clothing.     

Effects of physical conditioning in man on thermal responses to cold air

Thermal responses of ten relatively unfit young men (six lean, four obese) were determined during duplicate exposures of two hours each in air of 10°C. Mean heat production ( ), rectal (T_re), esophageal (T_es), skin (T_s), and subcutaneous (T_sc) temperatures were continuously measured during each exposure. All subjects were then physically conditioned for nine weeks, which increased their work capacity and maximal oxygen intake. On duplicate retesting, , T_re and T_es remained essentially similar to preconditioning values. Mean skin temperature (T_s) of both groups was lower following conditioning, due primarily to lower skin temperatures of the chest, thigh, forearm and upper arm. Toe temperature was unchanged. The T_sc to T_s gradients for the forearm and upper arm were greater in both groups after conditioning. Increased vasoconstriction and a different peripheral circulation pattern in the cold may result from physical conditioning.

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Zusammenfassung Die Wärmereaktionen von 10 relativ untrainierten jungen Männern (6 schlank, 4 fett) wurden im Doppelversuch während 2 Stunden Exponierung bei 10°C untersucht. Die Messungen der mittleren Wärmebildung ( ), rektalen (T_re), Oesophagus (T_es), Haut (T_s) und subkutanen (T_sc) temperaturen erfolgten fortlaufend während jeder Exponierung. Danach wurden alle Personen 9 Wochen körperlich trainiert. Dies führte zu einem signifikanten Anstieg der Arbeitsleistung und maximalen O₂-Aufnahme. Bei doppelter Wiederholung der Untersuchung ergaben sich für , T_re und T_es gleiche Werte wie vorher. Nach dem Training was T_s bei beiden Gruppen tiefer, als Folge tieferer Temperaturen über der Brust, den Oberschenkeln und den Armen. Die Zehentemperatur war unverändert. Die T_sc — T_s Grandienten waren am Vorder- und Oberarm in beiden Gruppen nach dem Training grösser. Eine erhöhte Vasokonstriktion und veränderte periphere Zirkulation in der Kälte ist wahrscheinlich Folge des Trainings.

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Resume La réaction à la chaleur de 10 jeunes gens relativement peu entraînés (6 maigres et 4 gras) a été examinée par l'exposition de 2 × 2 heures à 10°C. On a mesuré en permanence durant chaque exposition la production moyenne de chaleur ( ) et les températures rectale (T_re), de l'oesophage (T_es), de la peau (T_s) et subcutanée (T_sc). Ensuite, toutes ces personnes subirent un entraînement durant 9 semaines. Ceci eut pour conséquence une augmentation significative des aptitudes au travail musculaire et de l'absorption maximum de O₂. De nouvelles mesures répétées deux fois ont donné les mêmes valeurs que précédemment pour , T_re et T_es. Après l'entraînement, les deux groupes présentaient des valeurs inférieures pour T_s en raison de températures plus basses sur la poitrine, la cuisse et les bras. La température des orteils n'avait pas changé. Le gradient T_sc — T_s avait augmenté chez les deux groupes au bras et à l'avant-bras. Une constriction vasculaire plus élevée et une modification de la circulation périphérique sont probablement les conséquences d'un certain entraînement.

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This investigation was supported by Contract No AF 41 (609)-2696 from the Aerospace Medical Division (AFSC) Department of the Air Force.     

Effect of altitude exposure on thermoregulatory response of man to cold.

The thermoregulatory responses to 10 degrees C (for 3 h) were investigated in 1) 12 natives from sea level (lowlanders) at 150 m, and on arrival at 3,350 and 4,340 m; 2) 6 of these during a 6-wk sojourn at 4,360 m, and on return to sea level; and 3) 5 natives from each of the two altitudes (highlanders) in their respective habitat, and after descent to 150 m. The cold-induced increase in the rate of O2 consumption (Vo2) of the lowlanders was significantly smaller at both altitudes than at sea level. It did not recover substantially during the 6 wk at altitude, but was restored to its initial rate on return to sea level. By contrast, visible shivering activity was augmented on arrival at altitude. It persisted throughout the 6 wk there, but was greatly depressed on return to sea level, despite the increased Vo2. Mean skin temperatures (Tsk) stabilized in the cold at significantly higher values at altitude. Rectal temperature (Tre) decreased similarly at all altitudes. Vo2 of the highlanders in the cold was significantly greater at sea level than at their resident altitudes, although shivering activity was less intense; Tsk stabilized at significantly lower levels at 150 m than at either altitude. These results indicate that altitude exposure reduces the calorigenic response of man to cold, and that this effect is not moderated by acclimatization to altitude, yet is reversible immediately on descent to sea level. The component of cold thermogenesis which appeared to be reduced by altitude exposure was nonshivering thermogenesis rather than visible shivering.     

Heat balance precedes stabilization of body temperatures during cold water immersion.

Certain previous studies suggest, as hypothesized herein, that heat balance (i.e., when heat loss is matched by heat production) is attained before stabilization of body temperatures during cold exposure. This phenomenon is explained through a theoretical analysis of heat distribution in the body applied to an experiment involving cold water immersion. Six healthy and fit men (mean +/- SD of age = 37.5 +/- 6.5 yr, height = 1.79 +/- 0.07 m, mass = 81.8 +/- 9.5 kg, body fat = 17.3 +/- 4.2%, maximal O2 uptake = 46.9 +/- 5.5 l/min) were immersed in water ranging from 16.4 to 24.1 degrees C for up to 10 h. Core temperature (Tco) underwent an insignificant transient rise during the first hour of immersion, then declined steadily for several hours, although no subject's Tco reached 35 degrees C. Despite the continued decrease in Tco, shivering had reached a steady state of approximately 2 x resting metabolism. Heat debt peaked at 932 +/- 334 kJ after 2 h of immersion, indicating the attainment of heat balance, but unexpectedly proceeded to decline at approximately 48 kJ/h, indicating a recovery of mean body temperature. These observations were rationalized by introducing a third compartment of the body, comprising fat, connective tissue, muscle, and bone, between the core (viscera and vessels) and skin. Temperature change in this "mid region" can account for the incongruity between the body's heat debt and the changes in only the core and skin temperatures. The mid region temperature decreased by 3.7 +/- 1.1 degrees C at maximal heat debt and increased slowly thereafter. The reversal in heat debt might help explain why shivering drive failed to respond to a continued decrease in Tco, as shivering drive might be modulated by changes in body heat content.