INCREASED PRESSURE 



143 



but above that point the danger rapidly increases, and at two 

 atmospheres the risk is very considerable. 



The causation is due to the fact that the blood absorbs nitrogen 

 from the compressed air, and this gas is retained in simple solution 

 in the blood-plasma solely by the increased pressure. If, now, this 

 pressure be rapidly diminished, the nitrogen is liberated in the form 

 of bubbles in the plasma, and these in their turn block the capillaries 

 and also tend to accumulate in the heart. The other gases, oxygen 

 and carbon dioxide, have nothing to do with the causation, for 

 obvious reasons, because oxygen would be rapidly absorbed by the 

 tissues, and the other gas cannot be increased in the blood affected 

 by compressed air. 



Nitrogen is also held in solution in the tissues, especially in fat, 

 and hence fat people are more susceptible to the disease. 



Time is required to saturate the blood and tissues with nitrogen, 

 and therefore the danger increases with the length of exposure. 



The actual liberation of the bubbles of nitrogen will not take 

 place until decompression is begun. If this is rapid, then the 

 symptoms will be severe ; if slow, they may be few or absent ; but it 

 has been found that danger depends upon relative and not absolute 

 reduction. Thus, two atmospheres may safely be reduced to one, 

 four to two, six to three, but three atmospheres cannot be reduced 

 to one without danger. 



Decompression should therefore be based upon reduction to one- 

 half of that at which work has been done, and after that there is a 

 considerable pause, during which muscular exercises are performed 

 to rid the body of the nitrogen; and, again, a half -decompression, 

 and so on, but each interval must be considerable. 



As the symptoms are due to bubbles of gas, they are extremely 

 varied, but pain and paralysis are the most marked, and should be 

 treated by immediate recompression, followed by slow decompression. 



The medical examination of persons about to work on these lines 

 is most necessary, and all fat persons and those with any organic 

 lesion should be rejected. 



DIMINISHED PRESSURE. 



The attention of tropical workers has been drawn of late to the 

 effects of diminished atmospheric pressure as exemplified by able 

 accounts of the mountain sickness of the Andes. 



Normal atmospheric pressure in alveolar air, which is saturated with aqueous 

 vapour, is 760 millimetres of mercury, of which 47 are due to the pressure 

 of the aqueous vapour, and therefore the rest is due to air gases, of which 

 oxygen, which is present in a proportion of 14-2 per cent., is responsible for 

 10 1 and carbon dioxide for 40 millimetres. 



If the oxygen percentage in inspired air falls from 20-9 to 12 per cent,, 

 hyperpnoea takes place; if to 10 per cent., cyanosis appears and increased 

 frequency of the heart; and at 5 per cent, there is loss of consciousness. 



If the atmospheric pressure is reduced to 580 millimetres, the want of oxygen 

 begins to tell upon the breathing, and the carbon dioxide pressure in alveolar 

 air is also diminished. 



