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Decompression' Sickness: Building Understanding

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Decompression' Sickness: Building Understanding L -* 5 Faceplate lO(2): 11-13, 79 ’ DECOMPRESSION’ SICKNESS: BUILDING AN UNDERSTANDING Paul K. Weathersby, Ph. D. Lieutenant, MSC, USN Naval Medical Research Institute A NMRI researcher discusses the kind of reasoning that has been used to design decompression schedules, and presents his own view of how well the various theoretical concepts agree with proven events in a diver’s body. ivers are well acquainted with the U.S. Navy de- D-compression tables. How these tables developed n n. and how their development compares with the reasoning behind decompression tablcs of other navies and private organizations is not as widely known. Decompression sickness (DCS) is a rare example of human diseisz in that prevention of the disease can be approached by the use of involved theoretical calcula- tions. The theoretician follows several steps: First, he decides on the cause of the disease; he then breaks down the cause of the disease into aspects that can be solved separately; finally, he reassembles these partial solutions into a complete package that can dcscribe the entire disease process and its prevention. Decompression calculation proccdures are thus de- veloped from particular sets of ideas about events in the body that may cause the variety of DCS symptoms. Each set of ideas, whether based on fact, assumption, or guess, is known as a “decompression theory.” Search for the DCS Villain Possible causes of DCS, proposed within the last cen- tury, are listed in Table 1. When workers became stricken with DCS, or “the bends,” during the caisson construc- tion of the St. Louis bridge in the 1870s, some doctors concluded that diver exhaustion was to blame. But simple physical exhaustion has failed to answer most of the questions raised by DCS, although exercise while under pressurc still appears to contribute to it. Pneumatic caissons were used widely in the Iote 19th century to construct bridge pilings. Work spaces were filled with com- pressed oir to prevent wotrr seepage. Upon returning to the sur- face, ho wver, workers uere often slricken with decompression sickness. Doctors collrd it “caisson sickriess“ ond orlributed it to physic01 exhoustion. Todoy, gos bubble theories prrdominate, but reseorchers disogree on [Ire precise proccss through which tlie gas leads to DCS. FACEPLATE 11 REPOSITORY l.L.L&U&M 6OXNo.IRB f ~~~~~~ Institutional Review BoapdLDE, P. IRM No. 80P-109 COLLECTION rnccinn Cirknp<< Uusli es if9 Raqiogcjjive Gases Pn ny1 n -- ‘I . most ‘practical’ diving experience does not contribute to our understanding of which (decompression sickness) theories work better than others.” More recently, several groups of diving researchers tissues. Gas transport is frequently described from two have based the prevention and treatment of DCS on simplified viewpoints, the blood perfusion model and drugs and on procedures that affect blood cells, blood the tissue diffusion model. The more common blood vessel walls, or the movement of fluids among various perfusion model states that gas transport is limited by spaces in the body. These researchers could be said to rate of local blood flow because the gas molecules can follow a biologicul approach to DCS prevention. exchange very quickly between small blood capillaries Today, most researchers follow a physical approach and the tissue spaces near them. The tissue diffusion to DCS; that is, they believe that DCS occurs after the model states that blood can supply or remove gas quick- physical process of gas formation in the body. Accord- ly, so that the process of diffusion of gas molecules ing to this belief, DCS can be avoided if the formation between or into tissue cells limits the overall rate of gas and growth of gas bubbles is avoided or, at least, mini- exchange. Evidence from laboratory experiments has not mized. Furthermore, this school of thought attributes yet resolved which simplified approach applies to human the changes in blood cells, body fluids, and blood diving. The final answer may lie between these two ex- vessels to biochemical events that occur only after tremes; already, several theories contain mixtures of the bubbles appear. perfusion and diffusion elements. Emphasis on the gas bubble as primary villain in DCS Symptom-Provoking Events. The second element of has encouraged researchers who are mathematically any gas bubble theory requires that a decision be made inclined to try to solve DCS problems without worrying on what makes bubbles “bad.” For much of this century, about the complicated biological processes that underlie attention was focused on the idea of bubble formation. the disease (which is the usual mode of operation in The tactic in decompression schedule calculation was to medical research). The processcs that describe how gases avoid a critical ratio of inert gas supersaturation (that move through liquids and how bubbles form and grow in is, the ratio of the dissolved gas partial pressure in the supersaturated liquids are fairly well understood. At body to the hydrostatic pressure at the diver’s particular least in simple physical situations, these processes can be depth). This consideration was presumed to prevent described with precise mathematical expressions. bubble formation, and it is the basis for the present U. S. Navy tables. In recent years, however, evidence has accu- Elements of Gas Bubble Theory mulated that during many dives, divers’ bodies contained Elements to be considered when one evaluates DCS gas bubbles without the divers noticing any DCS symp- bubble theories are detailed in Table 2. The major ele- toms. Therefore, several theories allow for the formation ments to be addressed are (1) the rate ofgus exchange of bubbles but try to prevent the bubbles from growing (Le., at what rates and in which locations does gas move to a certain critical size, or deforming certain tissue in the body during a dive?), and (2) the symptom- structures, or exceeding a certain number. provoking event(s) (i.e., what property of the gas bubble(s) causes DCS symptoms?). Why So Many Bubble Theories? Rate of Gas Exchange. Gas exchange rates probably Gas theories for calculating decompression tables are depend, in a rather complicated way, on the properties shown in Table 3. At the risk of oversimplification, I of gas molecules as well as on the properties of body have listed for each theory (1) the number of “tissues” TABLE 1 TABLE 2 POSSIBLE CAUSES OF DECOMPRESSION SICKNESS - - ELEMENTS OF GAS BUBBLE THEORY ABOUT DECOMPRESSION SICKNESS 1. Exhaustion 1. Rate of gas exchange: Blood perfusion 2. Blood cell aggregationlclotting Diffusion through tissue 3. Fluid shifts Statistical 2. Symptom-provoking event(s): 4. Chemical changes in blood vessels Initial formation of bubble 5. Gas bubbles Growth to critical size Biology following appearance Location le FACEPLATE . ... ... - .. I -- TABLE 3 DECOMPRESSION TABLES CALCULATED BY DIFFERENT GAS BUBBLE THEORIES Source 4cTissues” Gus Rate Bubble Event Haldane (1908) 5 Perfusion Formation US Navy - 6-9 Perfusion Formation (Workman, 1956) -.- Royal Navy 1 Diffusion Formation (Hempleman, 1967) Swiss 6 PerfIDiff Formation (Buhlmann, 1969) -- “Thermodynamic” 2 Diffusion Growth (Hills, 1966) Hawaii 1 Perf/Diff Number (Yount, 1978) (mathematically distinct sets of gas exchange rates); Commanders Edward Flynn and Kristopher Greene are (2) the procedtire foi calculating rates deperiding upon’ preparing to measure gas exchange in divers performing the choice of blood perfusion or tissue diffusion models, at different levels of exercise. Dr. Louis Homer is devel- or a combination of both; and (3) the choice of bubble oping the mathematical formulations necessary to corn- event that leads to DCS. Obviously, theories are avail- pare special combinations of perfusion and diffusion gas able to cover most of the possible combinations. exchange conditions with careful animal and human The reasons for having a wide selection of gas bubble measurements. My colleagues and I are measuring gas theories of DCS are simple: Very little experimental in- exchange over prolonged periods in many tissues of formation is available for deciding which set of assump- animals, and we have developed a statistical description tions matches the events occurring in a diver’s body. The of gas exchange rates that avoids choosing between per- gaps in our knowledge exist for many reasons. The fusion and diffusion assumptions. perfusion-diffusion controversy still rages despite evi- dence that both assumptions are incorrect. We still do Conclusions not have methods to detect or mcasure sizes of bubbles Decompression theories are not scientific theories in most parts of the body. In addition, pain, the most in the sense of the law of gravity; rather, they are common symptom of DCS, is difficult to describe bio- mathematical predictions based on unverified logically, much less mathematically. Finally, in spite of assumptions. the inherently low incidence of DCS during thousands Decompression theories do provide calculation of dives using common decompression schcdules, we methods for summarizing the experience of divers; cannot eliminate the possibility that chance alone may they also provide starting values for developing account for the diffcrcnces in safety reported in dives new schedules. using diffcrent dccompression tables. Thus, most “prac- Theoretical calculations of new decompression tical” diving expericncc docs not contribute to our schedules usually cannot be accepted immediately; understandingof which thcorics work better than othcrs. they are rcviscd to provide adequate safety. Several projects at thc Naval Medical Research Insti- It remains for researchers to establish how fast and tutc arc ainicd at plugging thcsc gaps in our knowlcdgc. where gas exchange occurs and the specific pro- Comtnandcr John Hailenbcck has becn ablc to follow I ccsscs through which the gas leads to DCS.@ thc progression of biological evcnts in spinal cord DCS that occurs aftcr bubblcs bccomc visible. Comniandcr Thomas Berghage has amascd a large body of informa- 7he author is gratefiif to his colleagues at the h’aval Medical tion on thc procedurcs that lcad to serious symptoms of Research Institute, especial& to Surgeon Captain E E.
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