|Department of Commerce||National Oceanic and Atmospheric Administration||Diving for Science and Technology|
Hypothermia is a condition in which the deep tissue or core temperature of the body falls below 95° F (35° C), which is the temperature at which malfunctions in normal physiology begin to occur. If the core temperature drops below 96.8° F (36° C), diving operations should be terminated because the consequences of continuing are serious. If the core temperature falls to 93.2° F (34° C), temporary amnesia may occur and emergency rewarming and medical treatment are required. Between 86° and 89.6° F (30° and 32° C), cardiac irregularities commence and unconsciousness may result.
Because water has a specific heat approximately 1000 times greater than that of air and a thermal conductivity 24 times greater than that of air, the body loses heat much faster in water than in air of the same temperature. Fortunately, the thermoregulatory system of the body is highly sensitive to stimulation from the hands and feet, so that the body's heat generating systems are activated before the core temperature is affected seriously. The fact that the hands and feet get cold first is thus, in this sense, an advantage.
With cold skin and with core temperatures below 96.8° F (36° C), the defense mechanisms of the body are activated. These mechanisms consist of shivering, which can increase basal body heat production by up to five times, and vasoconstriction, which reduces blood flow to the periphery and thus reduces heat loss. Unfortunately, these mechanisms rarely achieve heat balance, so that the diver continues to lose heat.
In addition to losing body heat by conductive loss from the skin, a significant loss (10 to 20 percent of total body heat loss) occurs by evaporation from the lungs. The percentage is dependent on the humidity of the inspired air, since the drier the air the greater the evaporative heat loss. Further, as divers go deeper and their breathing gas becomes more dense, convective heat loss increases. Breathing gas heating is needed beyond depths of 400 feet (122 meters).
Obviously, a diver exposed to cold water or even moderately warm water for long periods must wear protective clothing. Because of large individual differences in cold tolerance, every diver must determine the most suitable protection on an individual basis. A variety of diving suits is available, ranging from standard foamed neoprene wet suits and dry suits to specially heated suits (for detailed descriptions of these suits, see Sections 5.4 and 10.8).
The use of protective equipment, however, creates a complication because the body's defense mechanism is modified by the thermal barrier of the clothing. This complication is only just being recognized as important, and divers should be aware that the faster the rate of heat loss, the smaller the drop in core temperature for a given quantity of heat loss. Furthermore, whether or not a person shivers is strongly influenced by: (1) the rate of body heat loss; (2) the amount of body fat; and (3) the body size. Larger, fatter people are less affected by a given cold exposure and less affected by a given amount of heat loss. For example, because heat transfer is about 100 to 200 times faster in water than in air, the heat that reaches the skin surface is rapidly transferred to the water. Generally, the thicker the layer of subcutaneous fat, the greater the insulation.
During swimming, the increase in energy production resulting from exercise is counterbalanced by the increase in muscle blood flow resulting in greater heat transfer. Thus swimming promotes faster transfer of heat from the core to the periphery, and this heat is in turn lost to the water (Nadel 1984). This is why persons suddenly immersed in cold water or divers becoming cold are better off remaining still than trying to swim. Rapid heat loss provokes strong shivering, so that the diver is warned. Gradual heat loss over a long time often will not cause shivering, yet the accumulated cooling and the likelihood of hypothermia may be even greater, with the likely result of impaired performance. Use of apparently adequate thermal protection in prolonged dives, or repeated dives over several days, may produce long slow cooling and undetected hypothermia even in tropical water. This affects memory and the speed of reasoning and other cognitive functions, thus reducing a diver's effectiveness and possibly endangering him or her. In addition, repeated diving with inadequate thermal protection may lead to an unwillingness to dive again or to disabling fatigue--states that are now known to be associated with being cold (Webb 1985).
Symptoms of Hypothermia
It is easy to recognize that hands and feet are cold by the familiar sensations of discomfort, numbness, pain, and diminished usefulness. On the other hand, loss of body heat is extremely difficult to recognize. Individuals are poor judges of their own thermal state. As body heat is lost, the body approaches hypothermia; recognizing hypothermia in its early stages is a serious problem in diving. Deep hypothermia, meaning a rectal temperature of 95° F (35° C) or lower, is dangerous; at this stage, a diver may become helpless.
Survival in Cold Water
If ship abandonment is necessary, there are procedures that can significantly increase the chances of survival, even in extremely cold water. Records show that ship sinkings, even in the worst cases, usually require at least 15 to 30 minutes. This affords valuable time for preparation. The following procedures should be carried out (U.S. Coast Guard 1975):
At the end of a dive, a cold diver should be rewarmed. This can be accomplished by having the diver drink hot liquids such as soup or coffee, dry off in a warm place, and bathe in warm water. Studies have shown that rewarming in 104° F (40° C) water reestablishes normal body temperature 67 percent faster than rewarming in 100° F (38° C) air (Strauss and Vaughan 1981). Cold divers should not make a second dive on the same day, because it is difficult to know when body heat has been restored. However, if a second dive is necessary, it is advisable to overdo the rewarming until sweating occurs, which indicates that body heat has been restored. Exercising to generate internal heat is also helpful to speed up the rewarming process. The diver should then change into warm, dry clothing and continue some mild exercise to improve heat production and circulation. Several hours may be required to restore all the body heat lost. Drinking alcohol is not beneficial, because it increases circulation of blood to the skin and speeds the loss of body heat in cold surroundings. A diver who is so hypothermic that he or she is helpless, irrational, or lethargic should be rewarmed more vigorously. Ideally, a hot bath should be used, but if none is available, a hot water suit, electric blanket, or inhalation rewarming are suitable methods. A hypothermic diver who is helpless, irrational, lethargic, or unconscious needs medical attention and immediate and vigorous rewarming, by any of the prescribed techniques (see Section 18.8.3 for further discussion of rewarming).
Divers Who Have Been Chilled on Decompression Dives (or Dives Near the Decompression Limit) Should Not Take Very Hot Baths or Showers Because These May Stimulate Bubble Formation