The Measurement of Arterial Pressure in Man. 509 



Now Mac William and Melvin* have studied the behaviour of an excised 

 artery compressed in a tube containing water with external pressures from 

 zero up to diastolic pressure. They find that with pressures, say, from zero 

 up to 50 mm. Hg, the distal manometers, systolic and diastolic, give practically 

 unchanged readings. It is only when the external pressures become equal 

 to the diastolic, or above it, that any great alteration in the distal manometer 

 reading occurs. We find that this is so in the case of the carotid of a 

 living animal enclosed in a glass compression tube and compressed by water. 

 On the other hand, in the case of the same artery placed on a watch-glass, 

 a much lower external pressure applied with the bag of the pocket sphyg- 

 mometer is sufficient to obliterate the pulse. 



The question now arises, how far do the conditions of an artery embedded 

 in the tissues and surrounded with the tissue vessels resemble those of an 

 excised artery enclosed in a compression chamber full of water ? Were the 

 facts demonstrated by this simple schema applicable, pressures from zero to 

 50 mm. Hg should have no effect upon the diastolic and systolic pressures 

 in the brachial or radial artery distal to the point of compression. 



But such a conclusion is contrary to every-day clinical experience. When 

 an armlet is placed on the upper arm and the radial artery is felt at the 

 wrist, and the pressure in the armlet is raised from zero to 50 mm. Hg, at 

 some stage, varying under different conditions to be detailed later, the pulse 

 beat at the wrist increases perceptibly in force. Accordingly we have 

 here a seeming paradox, augmentation of the pulse produced by an external 

 pressure rising from to 50 mm. Hg — a pressure sufficient to deform an 

 exposed artery lying upon bone and damp out the pulse, but which has no 

 effect on an artery in the simple schema of Mac William and Melvin. 



The phenomenon in question is obtained with a varying degree of facility 

 in different people. In a patient whose systolic pressure was 130 and 

 diastolic 80 mm. Hg, the increase in the force of the pulse beat became 

 perceptible at the wrist when the external pressure in the armlet on the 

 upper arm was raised to 35-40 mm. Hg. After taking exercise for three or 

 four minutes, when the heart had been made to beat violently and the pulse- 

 pressuref range increased in extent, a condition approaching to the findings 

 in aortic disease, an increase in the force of the pulse beat became apparent 

 with 10 mm. of external pressure in the armlet. 



In a case of aortic disease with a large pulse-pressure range, the increase 

 in the force of the pulse beat became apparent with 5-10 mm. of external 

 pressure in the armlet. 



* 1 Heart,' vol. 5, p. 153 (1914). 



t The difference between the diastolic and systolic pressures. 



