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1. 0. 0 THE PROBLEMS, CONCLUSIONS AND RECOMMENDATIONS 1. 1. 0 Introduction The dense, cold, wet, positive-pressure environment of diving presents the individual with a pattern of physiological stresses not encountered in any other military situation. In contradistinction to the crewman of high-altitude air- craft or the submarine, the diver must be exposed to the pressure existent at his working level. This requirement introduces problems which are not readily susceptible to the engineering approach, but require solution through greatly in- creased knowledge of the physiological reactions to the peculiar diving environ- ment. In a field relatively new to quantitative physiology, it appears that inter- relationships of factors present but not important at sea level result in exceed- ingly complex phenomena in diving. The physiological stresses upon the diver vary in type and degree with changes in depth and even with the rate at which change in depth is achieved. At any depth the nature and severity of these stress- es varies with his diving duration. Many unique problems to be overcome are caused by high partial pressures of the gases he must breathe, or even by the normal gaseous product of his own metabolism. No one gas is itself devoid of toxicity; actually each of the respiratory gases appears capable of modifying the adverse effects of the others. Not only the inspired gases, but the effects of hydrostatic forces and water density alter normal respiration, which in turn affects the levels of gases in the blood deliv- ered to the tissues. Through circulatory changes induced by the same factors, further alterations of tissue gas toxicity are brought about. Even alterations of position may conceivably modify the toxicity of a respired gas. Each type of equipment employed to provide a respirable medium underwater introduces its own peculiar emphasis upon one or more of the stresses encountered, further altering the diver's reaction to his environment. The baseline of simple existence underwater upon which performance of useful work is based consists, therefore, of changing interrelationships of such factors as the rate of descent and ascent, depth and duration of diving, hydro- static and inertial forces, individual and composite gas toxicities, the degree of physical exercise, type of breathing apparatus, and temperature, as well as physiological, psychological, and other factors not yet identified. The considerable success in diving thus far has depended primarily upon empirical approximation in large numbers of subjects of the primary barriers to extension of submerged depth and duration, with restriction of diving activi- ties to within the limits of these barriers. At present, attempts are being made to extend safely the limits of tolerance through quantitative studies of the individ- ual physiological problems, and of the interrelationships among them. This work is now only in its early stages but it appears that there is considerable like- lihood of greatly increasing the scope of underwater activity by this approach. This report is intended primarily to present certain important problems in which lack of information is delaying extension of diving capability. The em- phasis placed upon need for further research in particular areas of underwater
physiology is based in part upon information generally available and in part upon recent studies not yet contained in the published literature. 1. 2. 0 Conclusions and Recommendations Naval interest in research in underwater physiology is ultimately directed toward the objectives of enabling divers and underwater swimmers to perform useful work with increased safety at greater depths for longer periods of time and to ensure an uncomplicated return to the surface. This report on the Status of Research in Underwater Physiology attempts to present, in an integrated man- ner, certain important physiological problems interfering with these aims. These problems have most often been considered and investigated separately. The conclusions that must be drawn from this report are that, while some of the problems lend themselves to isolated study, the interrelationships which exist among the several areas render single phases of high-pressure physiology diffi- cult to appraise. The complex overlapping of problems and interrelationship of physiologi- cal reactions makes the listing of specific research recommendations both diffi- cult and undesirable. Therefore only the following rather broad restatement of the problems will be attempted: A. Individual Gases in Respirable Media Considerable advances could be made if the precise mechanisms respon- sible for the toxic effects of oxygen, carbon dioxide, and nitrogen were under- stood. Little information is available concerning the effects of exercise on oxy- gen toxicity and essentially nothing is known about permissible exposures to oxy- gen at shallow depths. Of importance also is an understanding of the basis for individual varia- tion in susceptibility to these effects and of the day-to-day variations in the sin- gle individual. Adequate knowledge in these areas should permit a clear definition of the boundaries within which pure oxygen may be used in diving, provide opportunity for a more accurate prediction of the characteristics of new gas mixtures, and perhaps aid in the selection of personnel for specific diving and swimming assign- ments. B. Use of Mixed Gases When mixtures of gases other than air are used as a breathing mixture in diving, the effects of the individual components appear to be modified to such an extent that the ultimate utility of new combinations cannot be accurately pre- dicted. Nitrogen at high partial pressures appears to reduce pulmonary ventila- tion and respiratory exchange to the extent that carbon dioxide tension is in- creased, which in turn may shorten the latent period for oxygen toxicity. Alternation of high and low partial pressures of oxygen and nitrogen dur- ing a single exposure appear to relieve this situation, but detailed study is needed
to establish satisfactory time and pressure cycles for human subjects at various ambient pressures, both from the standpoint of extending permissible oxygen ex- posure time and the possible reduction in subsequent decompression time. Heli- um does not display the adverse narcotic effects of nitrogen but, in view of cur- rent helium-oxygen decompression tables, may be impractical for use in the re- stricted depth-time range now imposed on self-contained divers by other factors. The potentialities of some combination of helium and nitrogen or other inert gases as an oxygen diluent deserve consideration. No studies of this nature have been undertaken to date. C. Respiration The physical aspects of respiration underwater remain relatively unex- plored. A need for further work in this field is evidenced by the frequent sub- jective observations of air hunger and respiratory fatigue, particularly under conditions of heavy exercise underwater. A study of the flow characteristics of various respirable gases at elevated ambient pressures under conditions of con- stantly changing velocities in both the respiratory airways and breathing appara- tus is indicated. Also needed is a measure of the metabolic requirements in- volved in the rapid displacement of comparatively large volumes of water during high ventilation rates. These data, together with information regarding under- water breathing patterns, are vitally necessary to determine whether or not rad- ical deviations from the present design of self-contained underwater breathing apparatus are indicated. D. Decompression Current decompression tables for deep-sea divers are inadequate for most SCUBA applications. The time intervals of the standard air decompression tables require shortening, and the helium-oxygen decompression tables should be studied for the purpose of establishing suitable increments of time and depth in the limited depth-time ranges for SCUBA dives. Repetitive and multilevel diving with SCUBA also presents a problem. Present requirements for decompression following repetitive diving with air may impose insurmountable limitations on certain SCUBA operations. In single mul- tilevel dives no known tables could apply without a rigid control of the depth-time factors, and even under such controlled conditions they would undoubtedly be either questionable or unduly restrictuve. Both problems require further con- sideration. An analog decompression computer may be the best approach to the multilevel diving problem and study of this approach should be continued. While much time may be spent in the routine testing of decompression tables, the re- sults would be immediately applicable and are urgently needed. Other extant problems in decompression include the re-solution of tissue bubbles and an elucidation of the mechanisms involved in aeroembolism, medi- astinal and subcutaneous emphysema and pneumothorax, with or without frank rupture of the lungs.
E. Temperature It is generally considered that a swimmer, provided with presently avail- able equipment, cannot be maintained in heat balance while submerged in cold water. Studies aimed at extending the useful time of exposure to cold water could profitably proceed along the two lines of improvement in compact, light- weight, flexible exposure suits and the use of physical and/or chemical devices for reducing body heat loss. Special problems include the maintenance of manual dexterity and tactile sense, particularly of the fingers, during exposures to cold water. Circulatory and dietary studies conducted in cold, dry environments should be explored for possible extension to include immersion. Not only the effects of exposure per se but phenomena such as the danger- ous after-drop in body temperature following removal from cold water immer- sion require further study. F. Toxic Contaminants in Compressed Gases In the compression and transfer of high-pressure gases, particularly under field or emergency conditions, contaminants such as oil vapor, carbon monoxide, organic solvents and even particulate matter may be introduced. Re- liable methods of detecting and removing contaminants should be determined for field use. The effects of most of these known possible contaminants have been studied at normal pressures but their possible increased adverse effects or in- fluences on such phenomena as oxygen toxicity and nitrogen narcosis at higher pressures should be investigated. This report describing problems not yet solved in the broad field of underwater physiology extends either directly or indirectly into almost every phase of human physiology. It appears that a comprehensive review of under- water physiology is now needed to guide a systematic approach to many of the problems being encountered in the attempt to extend diving depth and duration.
