Quinquennium of medicine and surgery : being the first supplementary volume to Green's Encyclopedia and dictionary of medicine and surgery / edited by J.W. Ballantyne.

Date:
1911
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Licence: In copyright

Credit: Quinquennium of medicine and surgery : being the first supplementary volume to Green's Encyclopedia and dictionary of medicine and surgery / edited by J.W. Ballantyne. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    result of caisson disease. The slow satura- tion of the fat and nerve tissues may lead to the process of saturation going on in them while the body, as a whole, is being slowly decompressed. In desaturation the body gives up its nitrogen rapidly at first; then, as the difference between the tissue pressure and atmospheiic pressure becomes less, desaturation goes on more and more slowly. Decompression.—It has been found that caisson disease does not occur unless the pres- sure has exceeded one atmosphere (-1-15 lbs.). It may also be prevented by veiy slow decom- pression, nor does it occur if exposure to high pressure is limited to 10 minutes or so. The practical problem is, how to get the sui-plus nitrogen out of the Ijody as quickly as possible, consistent with the safety of the worker. Two methods are employed, slow decompression and stage decompression, and the two classes of work, diving and caisson work, have to be considered. When slow decompression is em- ployed, Hill recommends that 20 minutes be allowed for each atmosphei'e. The process is necessarily tedious, because there may only be a difFei-ence of one or two pounds between the ^itmosphere and tissue pressures. Stage Decompression.—Starting from the proposition that no symptoms are produced in man by quick decompression after an exposure of any duration to pressure not exceeding 15 lbs.—that is, a decompression of fi'om 2 to 1 atmosphere—Boycott and Haklane conceived that, since halving the pressure always doubles the volume, it would be equally safe to decom- press rapidly from 4 to 2 atmospheres, or from i; to 3, and experiments on animals verified the correctness of their supposition. On this is founded the method of stage decompres- sion, which is now ofiicially adopted for divers. The rationale of safe, quick decompression is, then, (1) never to allow the nitrogen pressure in the tissues to be more than twice the air pressure, and (2) to make the fullest use of the permissible difference of pressure to get the nitrogen out of the tissues. Suppose, for example, that it is necessary to decompress a man saturated at -f- 75 lbs. [90 lbs. absolute]. He can be quickly brought to + 30 lbs. [45 His. absolute]; nitrogen diffuses out rapidly under the stress of this drop of -f-45 lbs., and the saturation of those of his tissues that matter falls to -1-65 ll)s. in a time which can be approximately determined by calculation. . . . The absolute air pressure may now again he reduced to half the tissue pressure, i.e. to ^^^^^ = 40 lbs. absolute or 4-25 lbs. The nitrogen-extracting stress is now only 40 lbs. at first, so that a longei' wait is necessary before the tissue pressure will have fallen another 10 lbs., i.e. to +00 lbs. When this point is reached the air pressure is again leduced to 55-t-15 —-— = 35 lbs. absolute = -f-20 lbs. positive, and the stress is only 30 lbs. Hence the waits become longer and longer as the pres- sure falls. . . . The essential peculiarity of this method of decompression, however, lies in the fact, not that it is done per saltus but that it is quite quick at the beginning and very slow at the end (Boycott). Boycott points out that in slow decompressions there ai'e three fallacies : —(1) No use is made of the possibility of hasten- ing the exit of nitrogen fi'om the tissues by putting on the greatest permissiljle stress. (2) However slow continuous decompression is, the difference between the tissue pressui-e and the atmospheric pressure becomes larger and larger (because desaturation pi'oceeds at a diminishing rate of speed, while in slow decompression the atmospheric pressure falls steadily); hence the body may reach atmospheric pressure with a tissue pressure above -f 15 lbs., whereupon symptoms are likely to occur. (3) In cases of short exposure to high pressure the tissues which saturate slowly will go on taking up more nitrogen during slow decompression, and will ultimately be exposed to a dangerously rapid rate of decompi'ession. Greenwood agrees as to the value of stage decompression in divei-s; he apparently doubts whethei' it has any real advantage ovei' con- tinuous decompression at the rate of 20 minutes per atmosphere in caisson workei's. All vv^orkers agree as to the importance of avoiding hurry- ing the later stages of the process. Paiticulars as to decompression will be found in the Addenda to the Diving Maiuial of the Royal Navy, from which the following example is taken :— If a diver has been working at a depth of 144 feet for between 45 minutes and 1| hours he is directed to ascend rapidly to 50 feet and then pause for 10 minutes; he next comes to 40 feet and again waits for 10 minutes; at 30 feet he stops 20 minutes and at 20 feet 30 minutes, at 10 feet 35 minutes and then leaves the water, the whole ascent occupying 108 minutes. For caisson work decompression can be effected by a sudden drop to half the pressure, followed by slow uniform decompression at the i-ate of 7 or 8 minutes for each pound of pres- sure. During the later stages of decompression, particularly, the workers should exercise their muscles so as to assist in the elimination of nitrogen. Fat men are ill adapted for caisson work. When symptoms arise recompression is the only rational treatment. The diver should be send down again; the caisson worker placed in a compression chamber. LITERATURE.—Gbeenwood. Arris and Gale Lectures, Brit. Med. Joimi., April 18 and 2.5, 1908.—
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