1 70 RESPIRATION 



undue rise in their CO2 pressure. But owing to the combination of 

 these two conditions the fall in oxygen pressure and rise in CO2 

 pressure will both be moderate until the slowing of circulation is 

 excessive : for the oxygen will fall along the steep part of the 

 dotted curve in Figure 21, while the CO2 pressure will rise along 

 the thick line in Figure 26. This means that a great diminution in 

 the charge of oxygen in the haemoglobin, and consequently a very 

 considerable cyanosis, will be possible with a comparative small 

 fall in the oxygen pressure or rise in the CO2 pressure. Hence 

 cyanosis due to slowing of the circulation is not in itself such a 

 serious indication as cyanosis due to failing saturation of the blood 

 with oxygen, although of course indicative of possible more 

 serious failure of the circulation. 



When fall of arterial blood pressure is due to defective filling 

 of the large veins leading to the heart, benefit may be expected 

 from the intravenous injection of suitable saline solution, as this 

 will tend to fill up the veins, and to bring about adequate filling 

 of the heart. A simple salt solution tends, however, to leak out 

 again very quickly from the circulation. To remedy this defect 

 Bayliss^''^ has introduced the plan of adding gum to the salt solu- 

 tion, the gum fulfilling the same function in preventing leakage 

 as the proteins normally present in blood plasma. This procedure 

 has proved very successful, and avoids the risks and practical 

 difficulties associated with transfusion of blood or liquids con- 

 taining proteins. For the reasons already pointed out, the dilution 

 of the blood by the saline injection does not cause anoxaemia. 



As will be pointed out in Chapter X, failure in the venous return 

 to the heart may be due to deficient pressure of CO2 in the systemic 

 capillaries, owing to excessive washing out of CO2 in the lungs ; 

 and this excessive washing out may be secondary to arterial anox- 

 aemia. Arterial anoxaemia and deficiency of CO2 may also be the 

 i cause of failure of the heart muscle. It is probable, therefore, that 

 1 in many cases the vicious circle may be more effectively broken by 

 ' administration of oxygen or even CO2 than by injection of gum^ 

 j saline solution or transfusion of blood ; but in other cases injection 

 I or transfusion would quite clearly be required. 



" Bayliss, Intravenous Injection in Wound Shock, 19 18. 



