November 12, 190S] 



jVA TURE 



41 



if appropriate precautions be taken. Dr. Greenwood 

 has exposed himself to as high a pressure as 92 lb. 

 in an e.xpiriniental pressure chamber, and Lieut. 

 Damant and Gunner Catto ha\-e descended in divinf 

 dresses to a depth of 210 ft., or 94 lb. In these cases 

 the time of exposure to compressed air was verv 

 short, and human experience in caisson work de- 

 finitely shows that it is more dangerous to work for 

 six hours than three, and that exposures of half an 

 hour or so are usually quite free from risk. Prac- 

 tical experience is, however, by no means so clear 

 with regard to the salutary elTect of prolonging the 

 period of decompression. The cause of this is that 

 the re.iUv slow decompressions which are necessary 

 have hardly ever been tried in actual practice, with 

 the result that many practical men are still some- 

 what sceptical as to the truth of Paul Bert's dis- 

 covery. 



The formation of gas bubbles in the living bod' 

 obviously depends on the pressure of the gas dis- 

 solved in the body being greater than the external 

 pressure. It is, however, well known that liquids, 

 and especially sticky liquids such as blood, can hold 

 gas in a state of supersaturation at pressures much 

 in excess of the external pressure without the forma- 

 tion of bubbles, especially if they are not agitated 

 or brought into contact with foreign substances. These 

 phenomena are well illustrated by soda-water after de- 

 compression, noting the effects of shakintr the botil • 

 or adding sugar to the lemon-squash. If the limil 

 of " safe " supersaturation is exceeded, bubbles ar 

 formed. \\"hether this occurs or not in the body wl!' 

 depend, then, on the extent to which the body h.-i- 

 become saturated while under pressure, and \\m 

 point to which the saturation has been reduced 

 during decompression. How much gas is taken up 

 while under pressure and given off during decom- 

 pression depends in the main on the height of th'- 

 pressure, the duration of exposure, the duration ( i 

 decompression, and the activity of the circulation <! 

 the blood, which is the means whereby the air i-. 

 brought from the lungs to the tissues, and subse- 

 quently evacuated from the tissues via the lungs. 



The different parts of the body vary very wideh' 

 in respect of the quantity of blood passing throu!-^!' 

 them in unit time. Those that are freely supplied 

 with blood, such as the kidneys, take up excess g^as 

 very quickly, and in such parts the nitrotren pres- 

 sure soon comes into approximate equilibrium with 

 the nitrogen pressure in the air in the lungs. Bv 

 some ingenious experiments Drs. Hill and Green- 

 wood have shown that this point is reached in as 

 little as ten minutes in the case of the active human 

 kidney. Practical experience show's, however, thnf 

 other parts of the body take much longer — four rr 

 five hours or more — to become saturated with 

 nitrogen at the given excess air pressure, and, 

 further, th.at the importance of these slowlv satur- 

 ating parts as regards caisson disease is mucli 

 greater than that of organs which saturate quicklv. 

 and which, therefore, desaturate correspondingly 

 fast on returning to atmospheric pressure. 



The duration of the exposure to high pressure is. 

 then, of the utmost importance; it should in all 

 cases be reduced to the shortest practicable time. 

 The duration of decompression must, on the other 

 hand, be much extended if accidents are to be 

 avoided. It is also clear that the rate of decom- 

 pression should be adjusted to the height of the pres- 

 sure and the duration of exposure; what is safe 

 after one hour at 60 lb. pressure would be waste of 

 time if the pressure had been 30 lb. or the time of 

 exposure only ten minutes. 



Human experience shows that symptoms practic- 

 NO. 20,^7, VOL. 70] 



ally never occur after decompression from a pressure 

 of 15 lb. in excess of atmospheric pressure, however 

 long the exposure cr however short the period of 

 decompression. In other words, it is safe to reduce 

 the pressure quickly from 30 lb. absolute to 15 lb. 

 absolute. Experiments made at the Lister Institute 

 with the aid of the pressure-chamber presented by 

 Dr. Ludwig Mond showed that the principle that 

 the absolute pressure may always be safely halved 

 held good up to at least 6 atmospheres. A goat, 

 for example, may be quickly decompressed from 

 7.T lb. (90 lb. absolutej to 30 lb. (45 lb. absolute) without 

 ill effects, while a similar rapid drop of 45 lb. from 

 60 lb. absolute to 15 lb. absolute is frequently fol- 

 lowed by severe symptoms. L'nder a pressure differ- 

 ence of 45 lb., nitrogen rapidly leaves the tissues, 

 is carried away by the blood, and diffuses through 

 the lungs without forming bubbles. After a time, 



:el chamber at the Li- 

 for entering, the small air-lock for passing food, Sic, int-. the chaniner, 

 an inspection window, a pressure gauge, and several valves, &c- (Fr.om 



which may be approximatel}' ascertained by calcula- 

 tion, the pressure in the body will have fallen frciii 

 75 lb. to, e.g., 61 lb.; the absolute pressure may 

 again be halved, making the air pressure 23 lb. 

 The difference of pressure inside and outside the bcdv 

 is now only 38 lb., so that nitrogen leaves the bcd>' 

 more slowly than before. Ultimately the pressure 

 difference, which must never be much more than the 

 absolute air pressure, becomes so small that the 

 excess gas escapes very slowly. Consequently, the 

 rate of decompression must be made slow'er and 

 slower as the pressure falls, the final decompression 

 from 15 lb. to atmospheric pressure occupying 

 perhaps as much as 60 minutes. 



It has hitherto been customary to recommend that 

 decompression should be effected at a uniform rnie 

 throughout, such as 20 minutes for each atmosphere 

 of excess pressure. Such ? procedure is altogetIv>r 

 unsound; the rate suggested is needlessly slow for 



