424 THE RESPIRATION 



the external pressure is suddenly lowered: the blood, on leaving the 

 lungs, will contain no excess of gas; when it reaches the tissues it will 

 remove gas until the pressure is equalized, discharge this into the alveoli 

 and return again for more. Other things being equal, it 'will take the 

 same number of minutes to desaturate that it took to saturate, and the 

 parts of the body that will lag behind the others, in being desaturated, 

 are those with a sluggish circulation. 



When the mass movement of the blood is increased by muscular exer- 

 cise, the rate of saturation and desaturation with nitrogen is increased 

 in proportion. During active work the increase in movement of the 

 blood may be four or five times over the normal, so that the tissues of 

 the caisson worker become much more quickly desaturated during decom- 

 pression than the above figures would lead one to expect. 



Application of Foregoing Laws in Practice 



With regard to the application of these principles in the decompression of caisson 

 workers, it is impracticable to occupy as much time as it takes to saturate the body 

 even at comparatively low pressures. If the great dangers attending work in com- 

 pressed air are to be avoided, we must either insist on very gradual decompression 

 or we must show how the dissolved gases may be got rid of by some modification in 

 the decompression procedure. With this object in view, we must determine what 

 difference of pressure may be allowed between the external air and the body without 

 the formation of bubbles. Actual experience shows that there is no risk of bubble- 

 formation, however quick the decompression, after exposure to + 15 pounds pressure 

 (i. e., 2 atmospheres absolute). "Now, the volume of gas capable of being liberated 

 on decompression to any given pressure is the same, if the relative diminution of 

 pressure is the same" (Haldane 3 s). On reduction from 4 to 2 atmospheres, -the same 

 volume of gas will tend to be liberated as on reduction from 2 to 1 atmospheres that 

 is to say, no bubbles will form. The practical conclusion is "that the absolute air 

 pressure can always be reduced to half the absolute pressure at which the tissues arc 

 saturated without risk." Thus, after saturation at 90 pounds absolute pressure (+5 

 atmospheres), a man can be immediately decompressed to 45 pounds (+2 atmospheres) 

 in a few minutes without risk, but from this point on the decompression must be 

 conducted slowly, so as to insure that the nitrogen pressure in the tissues is never 

 more than twice the air pressure. The great advantage of this method is that it makes 

 the greatest possible use of difference of pressure between tissues and blood in order 

 to get rid of the gas that these contain. 



When the decompression from the start is gradual, the desaturation of the tissues 

 will progressively lag behind that of the blood, and the tendency to the liberation of 

 free gas will become greater. In such a case the decompression is far too slow at first 

 and far too rapid later. Theoretically, therefore, the decompression should' be rapid 

 at first and very slow later. 



Before recommending the adoption of this principle of stage decompression in 

 caisson work, Haldane and his coworkers made numerous observations on the incidence 

 of decompression symptoms in laboratory animals. They assert that the stage method 

 is decidedly safer than the uniform method, the advantage being particularly after 

 short exposures. On the other hand, Leonard Hill could make out no definite advan- 

 tage for the stage method. The two methods have also been compared in actual caisson 



