CHAPTER XLIX 



OXYGEN UNSATURATION OF THE BLOOD. CYANOSIS THE 

 THERAPEUTIC VALUE OF OXYGEN 



OXYGEN UNSATURATION OF THE BLOOD 



The blood leaving the lungs, i. e., arterial blood, is 95 per cent satu- 

 rated with oxygen. Its oxygen unsaturation is therefore said to amount 

 to 5 per cent. The venous blood is, of course, unsaturated with oxygen 

 to a greater degree, which varies between 22.7 and 3.3 per cent, depend- 

 ing on the activity of the tissues.* 



The blood is transferred under albolene to prevent contact with air, 

 and is mixed with neutral oxalate. Samples are analysed immediately 

 for the percentage of oxygen actually present. Another sample is re- 

 moved from under the albolene and saturated with oxygen by exposing 

 it to air, after which it is also analysed. The analyses are performed by 

 the method of Van Slyke. Suppose it is found that the venous blood 

 gives 14 per cent 2 and the total 2 capacity is 20, then the venous 

 oxygen unsaturation is 30 per cent. The hemoglobin content may also be 

 used to determine the total 2 capacity. 



The determinations can also be made by using the differential manometer of Bar- 

 croft (page 395). In Ihis case a sample of the blood is removed by means of a pipette 

 from the albolene and discharged under weak ammonia water in the bottle of the dif- 

 ferential manometer. After allowing for temperature changes the bottle is shaken, 

 Avhich causes the blood to become laked and saturated with oxygen, which it takes from 

 the confined atmosphere in the bottle and manometer. This causes shrinkage, the de- 

 gree of which is noted on the manometer. After readjusting the levels in the manom- 

 eter a few drops of a saturated solution of potassium ferricyanide is then mixed with 

 the laked blood (without opening the bottle, see page 403). This expels the O 2 and 

 creates a pressure which is measured in the manometer. The ratio between the first 

 and second readings equals the unsaturation of the blood, and the second reading, mul- 

 tiplied by a factor for the apparatus, gives the oxygen content.! 



Suppose the shrinkage of the manometer in the first operation is 25 mm. and the 

 positive pressure in the second, is 150, and the factor for the instrument is 0.13, then, 



100 x 25 

 ation is 



per cent (150 X 0.13). 



the percentage unsaturation is "' = 16.6 per cent and the total O 2 capacity 19.5 



loO 



*The sample of arterial blood is collected by the method of Hurter, which is described in suf- 

 ficient detail by Stadie; 88 the venous blood must be collected without causing stasis. 



tThe method is that described by Barcroft for obtaining data from which the dissociation curve 

 may be plotted (page 396). It will be found described in simplified form in the Jour. Lab. and 

 Clin. Med., 1919, iv, No. 9. 



443 



