Pulse Wave and the Measurement of Arterial Pressure. 349 



to obliterate the pulse varied markedly with the diastolic pressure within 

 the artery : — 





Diastolic pressure. 



Obliteration pressure. 





mm. Hg. 



mm. Hg. 



Head-down 



160 



48 



Horizontal 



134 



28 





125 



20 



The lability of the wall also plays a part, since with the same diastolic 

 pressure a higher pressure is required to obliterate the pulse in the carotid 

 artery of the dog or goat than in that of the cat. 



McQueen, Ingram, and Leonard Hill found that when the pulse in the 

 aberrant radial artery was obliterated by a pressure in the bag of, say, 

 45 mm. Hg, the blood still trickled slowly into the artery. We have made 

 a cut into the exposed carotid of a cat and found that a pressure of 

 26 mm. Hg stopped the visible flow ; a pressure of 20 mm. Hg allowed slow 

 oozing from a very elliptical lumen ; while a pressure of 10 mm. Hg allowed 

 the blood to spout freely through the incision. 



In the above investigations the remarkable fact comes out that the pressure 

 required to deform the artery and to obliterate the pulse wave is considerably 

 below even the internal diastolic pressure of the vessel. To investigate this 

 phenomenon further, we compared the effect of perfusing, with the same 

 pulsating head of pressure, thin rubber tubing (about - 7 mm. thick) and a 

 length of human artery of approximately the same calibre and thickness of 

 wall, and noting the external pressure required to obliterate the pulse wave 

 The results are as follows : — 





Systolic pressure. 



Obliteration of pulse, 

 pressure in bag. 



Rubber 



mm. Hg. 

 140 

 130 



mm. Hg. 

 195 (fig. 3) 

 46 (fig. 4) 



The rubber tube resisted deformation, the labile arterial wall suffered 

 deformation easily. 



2 D 2 



