Pulse Wave and the Measurement of Arterial Pressure. 353 



(3-5 mm. Hg) than at the beginning of the experiment. In one case, when 

 the application of the ice had been so long that the skin of the arm had become 

 red in patches, the pressure in the upper arm was lowered 20 mm. Hg. 



It is a remarkable fact that the readings obtained from the cold upper 

 arm should be lower than those obtained from the flushed forearm. 



The tentative explanation we offer of these results is as follows. The 

 vessels in the tissues of the upper arm under the influence of the ice are 

 constricted and largely ex sanguined, thus the artery is less well supported 

 by the resonance of the pulse in these tissues, and is therefore deformed by a 

 lower pressure than is required in the flushed forearm, where the resonance 

 of the systolic wave is greater. At the same time the cold contracted 

 artery in the forearm conducts the crest of the wave better to the flushed 

 forearm since it is less labile. The readings obtained from the cold upper 

 arm show that the wall of the artery, even though contracted by cold, does 

 not offer such a resistance to compression as to influence the readings. We 

 (L. H. and M. F.) reached the same conclusion by methods we devised for 

 testing the readings we obtained in cases of high blood-pressure.* 



Conclusions. 



We conclude that the armlet or Leonard Hill's small bag, applied to the 

 radial artery, give, under ordinary conditions, accurate readings of systolic 

 pressure, the obliteration of the pulse being taken as the index. This is 

 because the artery is surrounded by pulsing tissues and cannot be deformed 

 until the systolic pressure is overcome in these tissues. The artery is equally 

 compressed on all sides by a pulsating fluid pressure and the conditions are 

 the same as when it is compressed in a glass T filled with Einger's fluid. In 

 the dorsalis pedis, the temporal, or aberrant radial artery, where lying on 

 bone and tendon, the pulse is obliterated by a pressure of the small bag much 

 lower than the systolic pressure. This is because the lumen of the labile 

 arterial wall is deformed more easily under these conditions from the circular 

 to an elliptical shape, and the resistance to the passage of the pulse wave 

 thereby increased. 



The higher the diastolic pressure the greater must be the pressure of the 

 bag to produce the required deformation. As the amplitude of the pulse 

 wave depends so much on the size of the lumen, it seems probable that the 

 higher readings obtained in cases of aortic regurgitation are due in part to the 

 lumen of the aorta, iliac and femoral arteries being relatively wider than that 

 of the subclavian and brachial arteries. The pulsating (resonating) support 

 given to the former arteries by the relatively massive abdominal organs and 

 * ' Brit. Med. Journ.,' January 30, 1909. 



