Methods of Raising a Loiv Arterial Pressure. 



381 



Loss of Blood. 



I will consider first the restoration of arterial pressure after loss of blood. 



Experiments were made to begin with in order to analyse the effects 

 produced by injection of pure saline solutions. 



A cat was anaesthetised by chloroform and ether and afterwards by 

 ure thane intravenously (1 grm. per kilogramme body weight), given slowly. 

 Cannulas were placed in the carotid artery for the registration of the arterial 

 pressure, in the external jugular vein for the introduction of liquids from a 

 burette, and in the femoral artery for the withdrawal of blood. The vagus 

 nerves were cut in order to avoid cardiac and depressor reflexes. The blood 

 pressure at the beginning of the experiment was 158 mm. of mercury. This 

 was reduced to 46 mm. by the removal of 66 c.c. of blood. The same volume 

 of Binger's solution, warmed to 38° C, was then run into the vein. The 

 blood pressure returned for a brief period to 128 mm. ; that is, there was a 

 restoration of three-quarters of the amount by which it had fallen. The size 

 of the heart beats, which had been greatly reduced by the loss of blood, 

 became as large as at first. But this partial recovery was only maintained 

 for about five minutes and by the end of 21 minutes the pressure had fallen 

 again to 80 mm., being only half of the original height. 



In other experiments of the same kind, it was found that the fall of blood 

 pressure resulting from haemorrhage was restored by about two-thirds to 

 three-quarters of the amount by which it had fallen. Of course, by the 

 injection of amounts of saline solution greater than the volume of the blood 

 removed, a temporary rise to the normal value may be obtained. But it is 

 undesirable to augment the total volume of blood on account of the strain 

 on the heart, and it is important to know why the restoration of the blood 

 to its normal quantity, if done by saline injection, does not result in a 

 return to the corresponding initial pressure. 



Downs (12) found that, if the arterial pressure had fallen to three-eighths 

 of the normal by hsemorrhage, injection of saline could only raise it to about 

 two-thirds again. He gives detailed measurements of the relative effects 

 obtained in various degrees of loss of blood. 



As will be seen presently, there are two separate phenomena requiring 

 explanation. "Why is saline solution relatively ineffective in restoring 

 pressure ? And, secondly, why does the pressure actually produced fall again 

 more or less rapidly to a value very little higher than that before the 

 injection. 



In regard to the former problem, we know that, the cardiac output being 

 supposed constant, the height of the arterial pressure depends on the 



