Thermal problems arising from exposure to cold water pose the major consideration when planning operational dives and selecting equipment. The working diver commonly experiences heat loss during immersion and often expects to be uncomfortably chilled at the end of a dive. Bottom time limits may be determined by the diver's cold tolerance rather than by decompression considerations.
The human body functions effectively within a relatively narrow range of internal temperature. The average, or normal, core temperature of 98.6°F (37°C) is maintained by natural mechanisms of the body, aided by artificial measures such as the use of protective clothing or air conditioning when external conditions tend toward cold or hot extremes. Rewarming before a repetitive dive is as important as allowing for residual nitrogen levels.
When the body temperature is reduced below normal, gas absorption increases. This requires modification of decompression procedures by selecting a decompression table appropriate for the next longer or deeper dive schedule.
3-12.1 Regulating Body Temperature. The metabolic processes of the body constantly generate heat. If heat is allowed to build up inside the body, damage to the cells can occur. To maintain internal temperature at the proper level, the body must lose heat equal to the amount it produces.
Heat transfer is accomplished in several ways. The blood, while circulating through the body, picks up excess heat and carries it to the lungs, where some of it is lost with the exhaled breath. Heat is also transferred to the surface of the skin, where much of it is dissipated through a combination of conduction, convection, and radiation. Moisture released by the sweat glands cools the surface of the body as it evaporates and speeds the transfer of heat from the blood to the surrounding air. If the body is working hard and generating greater than normal quantities of heat, the blood vessels nearest the skin dilate to permit more of the heated blood to reach the body surfaces, and the sweat glands increase their activity.
Maintaining proper body temperature is particularly difficult for a diver working underwater. The principal temperature control problem encountered by divers is keeping the body warm. The high thermal conductivity of water, coupled with the normally cool-to-cold waters in which divers operate, can result in rapid and excessive heat loss.
3-12.2 Excessive Heat Loss (Hypothermia). When cold water enters a dry suit or a wet suit, the diver experiences a sudden drop in skin temperature. If a diver with no thermal protection is suddenly plunged into very cold water, the effects are immediate and rapidly disabling. The diver gasps and his respiratory rate and tidal volume increase. His breathing becomes so rapid and uncontrolled that he cannot coordinate his breathing and swimming movements. This lack of breathing control makes survival in rough, cold water very unlikely.
A water temperature of approximately 91°F (33°C) is required to keep an unprotected, resting man at a stable temperature. The unprotected diver will be affected by excessive heat loss and become chilled within a short period of time in water temperatures below 72°F (23°C). As his body temperature falls, the diver first feels uncomfortable and then, as his body tries to increase heat production in the muscles, shivering begins. If cooling continues, his ability to perform useful work becomes seriously impaired; his sense of touch is dulled and his hands lose dexterity. As shivering intensifies, it brings on a general lack of coordination and a scuba diver may experience difficulty keeping his mouthpiece in place. He soon loses his ability to think clearly and finds it increasingly difficult to concentrate.
At extremely low temperatures or with prolonged immersion, body heat loss reaches a point at which death occurs. Appropriate dress can greatly reduce the effects of heat loss and a diver with proper dress can work in very cold water for reasonable periods of time.
Inhaled gases are heated in the upper respiratory tract. More energy is required to heat the denser gases encountered at depth. Thus, heat loss through the respiratory tract becomes an increasingly significant factor in deeper diving. In fact, respira tory shock can develop if a diver breathes unheated gas while making deep saturation dives at normal water temperature.
3-12.2.1 Internal Temperature Regulation. The body's ability to tolerate cold environments is due to natural insulation and a built-in means of heat regulation. Temperature is not uniform throughout the body. It is more accurate to consider the body in terms of an inner core where a constant or uniform temperature prevails and a superficial region through which a temperature gradient exists from the core to the body surface. Over the trunk of the body, the thickness of the superficial layer may be 1 inch (2.5 cm). The extremities become a superficial insulating layer when their blood flow is reduced to protect the core.
Once in the water, heat loss through the superficial layer is lessened by the reduction of blood flow to the skin. The automatic, cold-induced vasoconstriction (narrowing of the blood vessels) lowers the heat conductance of the superficial layer and acts to maintain the heat of the body core. Unfortunately, vasoconstric-tive regulation of heat loss has only a narrow range of protection. When the extremities are initially put into very cold water, vasoconstriction occurs and the blood flow is reduced to preserve body heat. After a short time, the blood flow increases and fluctuates up and down for as long as the extremities are in cold water. As circulation and heat loss increase, the body temperature falls and may continue falling, even though heat production is increased by shivering.
Much of the heat loss in the trunk area is transferred over the short distance from the deep organs to the body surface by physical conduction, which is not under any physiological control. Most of the heat lost from the body in moderately cold water is from the trunk and not the limbs.
3-12.2.2 Effects of Exercise on Hypothermia. Exercise normally increases heat production and body temperature in dry conditions. Paradoxically, exercise in cold water may cause the body temperature to fall more rapidly. Any movement that stirs the water in contact with the skin creates turbulence that carries off heat (convection). Heat loss is caused not only by convection at the limbs, but also by increased blood flow into the limbs during exercise. Continual movement causes the limbs to resemble the internal body core rather than the insulating superficial layer. These two conflicting effects result in the core temperature being maintained or increased in warm water and decreased in cold water.
Increased heat production requires an equivalent increase in oxygen consumption. The respiratory minute volume of the lungs must increase by the same magnitude. If a diver is breathing nine liters of air per minute at rest in the water and becomes chilled, his heat production may increase three times to compensate for chilling. His respiratory ventilation then increases to 36 liters per minute. In this example, the diver would have the same air consumption at rest keeping warm as when performing moderate work in warm water.
3-12.2.3 Symptoms of Hypothermia. All of these factors work against the diver. Even his body's natural insulation and protective function give way to cold water. The diver's thinking ability becomes impaired and the effect of this impairment on the use of his hands and other motor functions may prevent him from choosing and executing the best procedures to complete a task. In some cases, his survival may be at stake.
The signs and symptoms of dropping body core temperature, from the first noticeable effects to death, are listed in Table 3-1. The treatment for hypothermia is discussed in Volume 5.
Table 3-1. Signs and Symptoms of Dropping Core Temperature.
Table 3-1. Signs and Symptoms of Dropping Core Temperature.
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