a1 (From the Lister Institute of Preventive Medicine.)
1. The time in which an animal or man exposed to compressed air becomes saturated with nitrogen varies in different parts of the body from a few minutes to several hours. The progress of saturation follows in general the line of a logarithmic curve and is approximately complete in about five hours in man and in a goat in about three hours.
2. The curve of desaturation after decompression is the same as that of saturation, provided no bubbles have formed.
3. Those parts of the body which saturate and desaturate slowly are of great importance in reference to the production of symptoms after decompression.
4. No symptoms are produced by rapid decompression from an excess pressure of 15 pounds, or a little more, to atmospheric pressure, i.e. from two atmospheres absolute to one. In the same way it is safe to quickly reduce the absolute pressure to one-half in any part of the pressure scale up to at least about seven atmospheres: e.g. from six atmospheres (75 pounds in excess) to three (30 pounds), or from four atmospheres to two.
5. Decompression is not safe if the pressure of nitrogen inside the body becomes much more than twice that of the atmospheric nitrogen.
6. In decompressing men or animals from high pressures the first part should consist in rapidly halving the absolute pressure: subsequently the rate of decompression must become slower and slower, so that the nitrogen pressure in no part of the body ever become more than about twice that of the air. A safe rate of decompression can be calculated with considerable accuracy.
7. Uniform decompression has to be extremely slow to attain the same results. It fails because it increases the duration of exposure to high pressure (a great disadvantage in diving work), and makes no use of the possibility of using a considerable difference in the partial pressure of nitrogen within and without the body to hasten the desaturation of the tissues. It is needlessly slow at the beginning and usually dangerously quick near the end.
8. Decompression of men fully saturated at very high pressures must in any case be of very long duration: and to avoid these long decompressions the time of exposure to such pressures must be strictly limited. Tables are given indicating the appropriate mode and duration of decompression after various periods of exposure at pressures up to 90 pounds in excess of atmospheric pressure.
9. Numerous experiments on goats and men are detailed in proof of these principles.
10. The susceptibility of different animals to compressed-air illness increases in general with their size owing to the corresponding diminution in their rates of circulation.
11. The average respiratory exchange of goats is about two-thirds more than that of man; they produce about 0·8 gram. of CO2 per hour per kilogramme of body weight.
12. The mass of the blood in goats is six and a half or seven and a half per cent. of the “clean” body weight.
13. The individual variation among goats in their susceptibility to caisson disease is very large. There is no evidence that this depends directly on sex, size or blood-volume: there is some evidence that fatness and activity of respiratory exchange are important factors.
14. Death is nearly always due to pulmonary air-embolism, and paralysis to blockage of vessels in the spinal cord by air. The cause of “bends” remains undetermined; there are reasons for supposing that in at least many cases they are due to bubbles in the synovial fluid of the joints.
15. In our experiments bubbles were found post-mortem most freely in the blood, fat and synovial fluid; they were not uncommon in the substance of the spinal cord, but otherwise were very rarely found in the solid tissues.
Lieut. and Inspector of Diving, R.N.