ments. These symptoms may not be noticed early enough to terminate the exposure before convulsions occur. Breathing air at atmospheric pressure after the onset of symptoms may restore tolerance and permit the resumption of oxygen breathing. Deep breathing or hyperventilation may help to avoid the onset of convulsions if initiated at the onset of the warning symptoms. Convulsions are the most serious direct consequence of oxygen poisoning. The victim becomes unconscious during the convulsion, followed by semiconsciousness, restless agitation, and random movements. When this occurs with a diver under water, the danger of drowning is great. Gas embolism may be induced by bringing the diver up or decreasing chamber pressure when he is not breathing regularly during the generalized muscular spasms of the convulsive seizure. Removal of the oxygen mask or ventilating with air will be effective in decreasing oxygen until breathing is resumed and pressure can be safely decreased. Figure 11–1 describes the, relationship of the percentage of oxygen in the breathing mixture to partial pressure and depth. If convulsions occur in a decompression chamber, the tender should keep the victim from thrashing against hard objects and injuring himself. Inserting a mouth bit will prevent damage to the victim's tongue from self-inflicted bites. He should then be turned onto his abdomen with the head to one side to aid breathing. The mechanism of oxygen toxicity appears to result from interference with the functioning of the enzyme systems involved with cell metabolism. Since no safe drugs are available for use in preventing oxygen poisoning, the observance of depthtime limits and the avoidance of excessive exertion and carbon dioxide retention are important to prevent its occurrence. 2.4 EFFECTS OF COLD (HYPOTHERMIA) Hypothermia is a condition in which the deep tissue or "core" temperature of the body falls below the normal physiological range, about 97° F (36° C) and is the temperature at which malfunctions in normal physiology begin to occur (Beckman 1963). If the core temperature continues to drop below 36° C, serious consequences usually develop. At about 34° C temporary amnesia may occur, between 30° 32° C cardiac irregularities commence and unconsciousness may result. If the core temperature of a diver should decrease to 32° C therefore, his operational usefulness would cease and the mission should be terminated. Because water has a specific heat, approximately 1000 times greater than air, and a thermal conductivity 25 times greater than air, the body loses heat much faster in water than in air of the same temperature. Fortunately, the thermo-regulatory system of the body is highly sensitive to stimulation in specific areas, i.e., the hands, feet, and head, so that the body's heat generating systems are activated before the core temperature is seriously affected. Thus, the fact that the hands and feet get cold first is an advantage, in that dives are terminated before the core temperature has a chance to drop appreciably. At temperatures below 35.7° C the defense mechanisms of the body are activated. These mechanisms take the form of shivering, which can increase basal body heat production by about 5 to 7 times; or by vaso-constriction which reduces blood flow to periphery thus reducing heat loss. The heat regulating system is so sensitive that moderate changes in surrounding water temperature can produce an alternating vaso-dilation and vaso-constriction. In addition to losing body heat by conductive losses from the skin, there is a significant loss (15 to 24 percent of the total body heat loss) by evaporation from the lungs and by heating the inspired air. This percentage range is dependent on the humidity of the inspired air, i.e., the drier the air the greater the evaporative heat loss. It is obvious from the above that the diver must wear protective clothing when exposed to cold water or when exposed to moderately warm water for long periods. Because of the large individual differences in cold tolerance, each diver must determine the most suitable protection for himself. There are a variety of diving suits available ranging from standard foamed neoprene wet suits, dry suits, to specially heated suits. These are described in Paragraphs 4.8 and 9.3.2. Chilling, even if not severe enough to threaten life, will produce loss of dexterity and sense of touch in the hands, making it difficult for a diver to do useful work or even control his diving equipment. Shivering causes a lack of coordination and may make it difficult for a diver to hold his mouthpiece in place. Ability to think clearly and short term memory may also be seriously affected by cold. It has been well established that dives in cold water must be planned so that the diver's tasks are both simple and few. Because of the effect of cold on short term memory complicated sequences of tasks are extremely difficult to carry out. A diver surfacing from a dive in cold water needs to be rewarmed as rapidly as possible, and should be completely warmed, as demonstrated by sweating, before he dives again. A hot bath or waterheated suit is preferable to a shower for rewarming. WARNING If the Dive Approached No Decompression Limits or Required Decompression, Caution Should Be Exercised in the Application of Extreme Amounts of External Heat as It May Stimulate Bubble Formation. Photo: Paul Stang Exercise to generate internal heat is also helpful to speed up the process. The diver should then change into warm, dry clothing and continue some mild exercise to improve heat production and circulation. Hours of time may be required to restore all the body heat lost. Drinking alcohol is not beneficial. It increases circulation of blood to the skin, and speeds the loss of body heat if the surroundings are cold. Hot, nourishing liquids such as soup are useful. |