Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
3.0.0 DECOMPRESSION 3.1.0 General When a diver is at depth, the inert gas of his breathing medium goes into solution in his blood and tissues. On subsequent ascent, an excess of this dis- solved gas can cause formation of bubbles and result in decompression sickness. Prevention of decompression sickness requires either limitation of the amount of dissolved gas by restriction of the diver's depth and time of exposure, provision of decompression stops to permit elimination of the gas on ascent, or both. Decompression sickness itself is a serious potential problem. Its mani- festations range from local pain to severe central nervous system involvement, and its sequelae can include permanent injury or death. Recompression is the only effective treatment, and adequate therapy can require as long as 38 hours in a recompression chamber. Where treatment facilities are distant or lacking, de- compression sickness must be avoided at all costs. Assuring proper decompres- sion imposes formidable limitations on all kinds of diving, and these limitations are particularly serious in military applications of SCUBA. The limits of the diver's air supply or the circumstances of the dive may make normal stage de- compression extremely difficult or impossible; avoiding the necessity for stage decompression may make the practical working time for deep diving impossibly short. The situation is difficult enough even when adequate, applicable decom- pression tables are available for the time of diving involved. Where they are not available, the problems are almost insuperable. In some of the urgent military applications of SCUBA, existing techniques for managing decompression are en- tirely inadequate. Several different types of dive can be encountered and these have different implications as to decompression procedures. Several modes of decompression are available and there are a number of approaches to reduction of the decom- pression time required. 3. 2. 0 Standard Air Diving A "standard" dive can be defined as one in which the diver descends to a known depth, remains there for a known period of time and ascends. Most con- ventional air-hose diving and some SCUBA diving falls in this category and all of the Navy's current decompression tables are designed primarily for it. 3.2.1 Regular Decompression Regular decompression implies that all of the decompression stops are made in the water and that the same breathing medium is used throughout. The Navy Standard Decompression Table (Air) applies to these conditions. This is an adequate table for the purposes for which it was designed, namely standard air dives with conventional suit and helmet and with a recompression chamber near- by. It approaches adequacy for standard diving with SCUBA, but certain short- comings become evident even in this connection. For example, the table pro- vides only rather large and irregular increments of diving time. This means that 13
a diver who overstays a given increment by even one minute must, to be safe, take the decompression specified for the next increment. This may be much longer than necessary, but on-the-spot interpolation is neither practical nor safe. The added time is seldom of much concern to the air-hose diver. He has ample air supply, surface personnel are in control, and the communication is good. The SCUBA diver is not so fortunate, and the extra time may place him at a very serious disadvantage. Smaller increments of bottom time would be very valuable in SCUBA diving. On the other hand, a SCUBA diver who follows the standard decompression table precisely may not invariably receive adequate decompression. The table was not intended to be 100% safe in preventing bends, since allowing for all individ- uals and all conditions would impose a large burden of unnecessary decompression on the majority. An occasional case of decompression sickness is acceptable if facilities for prompt and adequate recompression are available. In SCUBA diving, this situation is more likely to be the exception than the rule, and precise control of the diver's depth and time is also less likely than in conventional diving. For these reasons, certain portions of the table should probably be made somewhat more conservative for SCUBA use. At the same time, however, it might be pos- sible to liberalize other parts of the table without compromising safety, and any safe reduction of decompression time would be of obvious advantage to the SCUBA diver. Within certain limits of depth and exposure time, no decompression stops are required. The standard table specifies a "no stop" time for each 10-foot in- crement of depth from 40 to 130 feet. These limits are of particular interest to the SCUBA diver since avoiding the necessity for stops greatly simplifies the div- ing procedure. Whether the "no stop" times indicated in the present table are optimal is of obvious importance. 3. 2. 2 Surface Decompression Surface decompression allows the diver to spend a considerable portion of his decompression time in a recompression chamber rather than in the water. This is possible, within certain limits, if the diver can be put in a chamber im- mediately upon surfacing. Although it has seldom been exploited in SCUBA div- ing and would not often be possible, this procedure would have great advantages in some SCUBA operations. 3. 2. 3 Oxygen Decompression Oxygen decompression involving a shift to pure oxygen on ascent at a depth where this can be done safely (current procedure is to begin oxygen breath- ing at 50 feet), shortens decompression by hastening the elimination of inert gas. A highly successful combination of surface decompression and oxygen decom- pression is provided by the Navy's "Surface Decompression Table Using Oxygen". This has proved both safe and extremely advantageous in routine use, and it should be applicable to SCUBA diving without modification wherever a chamber with oxygen-breathing facilities is immediately accessible. The same table could probably be applied to the use of oxygen in underwater decompression with SCUBA, but it should be tested specifically for such use. The saving of time 14
would amply justify the trouble of shifting the breathing medium in many circum- stances. 3. 3. 0 Repetitive and Multilevel Air Diving ("Irregular Dives") The decompression procedures discussed above are designed not only for "standard" diving but also primarily for dives separated by considerable inter- vals of time. The only current provision for repetitive diving (successive dives made at relatively short intervals) consists of a single rule-of-thumb. This states that if more than one dive is made within a 12-hour period, the decompres- sion for each succeeding dive shall be that specified for the combined bottom- times of all exposures during that period and for the actual depth of the latest dive. In many types of SCUBA operation, it may be necessary or highly desira- ble for a diver to make several dives at very short intervals. In such cases, the existing rule appears to be prohibitively over-safe for many combinations of depth and time, but it may well be unsafe in others. A better means of setting limits and determining proper decompression In repetitive diving is badly needed. An approach to this is provided by tables set forth by French investigators, but these appear to be neither as complete as might be desired nor as well-tested as neces- sary for outright adoption. A similar but even more difficult problem is presented by "multilevel" dives, those which involve spending a variety of times at a number of different depths during the course of a single exposure. The SCUBA diver's freedom of movement in all directions and the nature of his assignments make such dives frequent and often essential. Even if the exact length of time at each depth could be tabulated, present information would not permit truly appropriate decompres- sion to be specified. Decompressing for the deepest depth and the total time, which is about the only safe rule which can be followed at present, would be so over-safe and restrictive in many situations that the desired operations would be rendered impossible. There is probably no fully satisfactory solution to the problem of decom- pression in multilevel diving, but it is probable that more realistic rules could be formulated for certain patterns of dive. If so, many operations could be planned to fit these patterns. The idea of a "decompression computer" to be worn by the diver appears most promising. Suggestions along these lines involve a system which would respond to ambient-pressure changes and exposure times in a manner analogous to that of the solution and elimination of nitrogen in the tis- sues. By providing a warning when serious super saturation became imminent, such an aid would permit the diver to govern his ascent. Considering the varia- bility in susceptibility to bends, no mechanical device could be expected to pro- vide more than a safe approximation of optimum decompression. However, even this could be a decided improvement for both multilevel and repetitive diving. 15
3.4. 0 Mixed Gas Diving 3. 4.1 Nitrogen-Oxygen Diving In certain military applications, the decompression required in air div- ing would be impractical even with the best possible techniques and tables. Con- sequently, some method of reducing decompression requirements to their abso- lute minimum is needed. The most obvious method of accomplishing this reduc- tion is to limit the partial pressure of inert gas to which the diver is exposed. This can be done by increasing the proportion of oxygen in the breathing mixture. However, the oxygen level must be kept within safe limits relative to oxygen poisoning. Calculations based on known tolerance to oxygen exposure at various depths have indicated that the use of "high-oxygen mixtures" could safely permit marked extension of "no stop" decompression limits and great reduction of de- compression time. However, in preliminary studies with nitrogen-oxygen mix- tures, toxic symptoms appeared in much shorter time than when the same partial pressure of oxygen is encountered with pure oxygen as the breathing medium. Subsequent studies indicated that carbon dioxide retention, due to insufficient pul- monary ventilation, was a common occurrence in divers breathing nitrogen- oxygen mixtures during work at depth. Such hypo ventilation may explain the ap- parently increased susceptibility to oxygen toxicity and the occurrence of carbon dioxide intoxication. These findings suggest serious limitations in the potential- ities of nitrogen-oxygen diving and have important implications in other types of diving as well. This phenomenon requires much further study. 3.4. Z Helium-Oxygen Diving The technique of helium-oxygen diving is already well developed and has been used successfully for over 15 years. The procedure involves using oxygen percentages scaled according to the depth; oxygen decompression is an integral part of the technique, and surface decompression is employed routinely. The present helium-oxygen technique could be adapted to SCUBA diving with very lit- tle change, but several physiological problems must be solved before the adap- tion would be fully satisfactory. The oxygen-exposure limits specified in cur- rent helium-oxygen techniques are almost certainly not safe enough for working dives of significant duration. However, helium does not appear to share the re- spiratory depressant effects observed with nitrogen-oxygen mixtures. If it does not, the partial pressure limits for oxygen in helium-oxygen mixtures could probably be similar to those for exposure to pure oxygen. In any event, specific investigation is required. Present helium-oxygen decompression technique requires shifting the breathing medium to pure oxygen at 50 feet on ascent, and there are no decom- pression stops shallower than 40 feet. For SCUBA use it might be more desira- ble to shift to oxygen at 40 feet and to spend the remainder of decompression time at a succession of shallower stops. However, shifting at 40 feet would in- crease the decompression time required, and the tables would have to be com- pletely recalculated and retested in either case. In some situations, shifting to oxygen might prove impractical. If so, new tables permitting decompression on 16
the original breathing mixture would be required. It appears inevitable that a SCUBA dive made on a helium-oxygen mix- ture will require longer decompression than an identical dive made with the cor- responding nitrogen-oxygen breathing medium unless the dive is deeper and longer than is at present anticipated in SCUBA operations. It is possible that the decompression specified by the present helium-oxygen tables is more than ade- quate in the short, shallow range of diving. This possibility will certainly need investigation if it appears likely that the use of helium-oxygen must take over the role intended for nitrogen-oxygen mixtures. 3.4.3 Multiple Inert-Gas Mixtures A recurrent proposal concerns the -use of a combination of two or more inert gases as a diluent for oxygen. While this does not appear likely to produce any marked saving of decompression time, it might permit balancing the dis- tinctive properties of nitrogen and helium in a beneficial manner. This possibil- ity deserves further theoretical and experimental study. 3.4.4 Possible Refinements of the "Mixed Gas" Principle The above discussion of oxygen-nitrogen or oxygen-helium mixtures as- sumes the use of a breathing medium of reasonably constant percentage composi- tion throughout the dive or at least until the beginning of decompression. While this appears to be the simplest approach to the reduction of decompression time, it is probably not the most effective. Several alternative methods which have been proposed offer potentially fruitful subjects for consideration and research: a. Constant Partial Pressure of Inspired Oxygen. Maintenance of a given partial pressure of oxygen regardless of the diver's depth would keep the exposure to an inert gas close to the level required to avoid oxygen poisoning at a particular depth. The ultimate ap- proach of this sort might involve automatic reduction of the oxygen tension as time progressed, permitting a higher mean oxygen ten- sion for the dive and thus a minimum uptake of inert gas. b. Exertion-governed Oxygen Percentage. A man is most susceptible to oxygen poisoning when he is working. During rest or minimal exertion, his tolerance is considerably greater. Consequently, a relatively high mean oxygen partial pressure could be tolerated if a drop in the oxygen level could be assured during periods of exertion. c. Interrupted Oxygen Exposure. Interrupted oxygen exposure (2.1. 2) offers the possibility of extending tolerance to high oxygen pres- sures if interruptions are provided at appropriate intervals. In this way, the mean oxygen tension for a dive could be very high indeed and the inert-gas exposure correspondingly limited. All of these possibilities of extending the potentialities of nitrogen-oxygen diving pose serious physiological problems concerned with both oxygen tolerance 17
and the intricacies of inert gas exchange and decompression. Unfortunately, tak- ing positive advantage of the characteristics of such systems would require much more information than is now available concerning oxygen tolerance and inert gas exchange in the presence of fluctuating tensions. The ultimate value of such sys- tems cannot yet be assessed with confidence. It is possible, however, that a mod- erate amount of experimentation could at least allow a significant gain over pres- ent approaches, obviating the need for certain more laborious decompression studies. 3. 5. 0 Derivation of Decompression Tables Development of a decompression table which is safe but not excessively conservative is a difficult enterprise at best. It is rendered more difficult by each additional variable and by each departure from "standard" practice, and both of these factors loom large in SCUBA decompression problems. The most crucial factor in the derivation of tables is the quality of pre- dictions which can be made on the basis of theoretical calculations and previous experience. It is obviously impossible to base a table entirely upon an accumu- lation of accidental instances of bends. The investigator must have some method of deriving working hypotheses which can then be tested. If the basis of these is faulty, a large burden is thrown upon actual experimentation. This, in turn, is extremely laborious and time consuming because of the extent of inter- and intra- individual variability and the ill-defined influences of a variety of factors. Many experimental dives must be made before the appropriateness of a given part of the table can be considered certain. If the basic theory is faulty, it will also be difficult to interpret and apply the results of experimentation. Available methods of computing decompression tables obviously leave much to be desired. The theoretical basis appears to be quite inadequate. As- sessment of some of the proposed methods of handling the decompression prob- lem mentioned above offers a good example of the existing difficulty. Consider- ing them with the aid of present methods and theories might provide insight into their implications and probable value. However, the conclusions would have to be very tentative, subject to actual experiments and, in some cases, these ex- periments would have to be very extensive. In the case of many uncertain but po- tentially fruitful possibilities, such consideration and investigation is a practical impossibility at present. In spite of the need for basic information, further work in this area can- not wait for basic research to be accomplished. The immediate needs are too great. Such research should certainly progress simultaneously with the primi- tive, laborious efforts which its lack makes necessary. Research in the area of decompression sickness has thus far been directed primarily toward the mechan- isms of bubble formation. Studies related to the treatment of decompression sickness, e. g., the reabsorption of gas bubbles, are also required. 3. 6. 0 Air Embolism and Related Problems As used in diving, the term "air embolism" refers to a type of accident which may occur on ascent but which is an entity distinct from decompression IS
sickness. The central nervous system is almost invariably involved. Most frequently, the accident has been associated with relatively rapid ascent without breathing apparatus, as in the "free ascent" method of submarine escape and in emergency ascent following failure of SCUBA. It does not occur on ascent from "breathholding" dives unless the diver has had access to additional air at depth. The accident is extremely rare, but it has followed routine ascents with conven- tional submarine escape and deep-sea diving equipment and even ascents in a dry-pressure chamber. Usually air embolism is clearly associated with breathholding or inade- quate exhalation during ascent. It is attributed to distention of the lungs by the pressure of expanding pulmonary gas. This presumably tears lung tissue and forces gas into the circulation. Embolization of the cerebral circulation by bub- bles is considered the immediate cause of symptoms. Victims occasionally show minimal central nervous system defects, but profound neurological involvement including unconsciousness and convulsions is much more common in recognizable cases. At least some favorable response generally occurs on prompt recom- pression and many have been dramatically and completely relieved by exposure to increased ambient pressure, presumably because this compresses the offend- ing bubbles to non-symptomatic size. If such treatment is delayed or omitted, death is the usual outcome in cases which display severe symptoms. Frank rupture of the lungs, mediastinal and/or subcutaneous emphysema, and possibly pneumothorax without gross pulmonary rupture, may be associated with air embolism. Any of these may occur alone under similar circumstances. Current assumptions concerning the mechanism of air embolism appear justifiable and have in part been verified experimentally. However, they are not completely satisfying. Many details are obscure. As a result, it is difficult to reach firm conclusions about the best method of making emergency ascents or about the exact procedure for optimum treatment. 19
