The Measurement of Arterial Pressure in Man. 



515 



say, 200 mm. Hg. in the armlet, a pressure far above the systolic pressure of 

 the blood. It is clear that when an armlet is placed on the upper arm and 

 the pressure raised above systolic pressure, the systolic wave must meet a 

 sudden check at the upper part of the armlet, hence the arteries are here 

 made tense and the sounds produced by their sudden tension are heard. It 

 is possible under these conditions (MacWilliam and Melvin* have observed it 

 in one case) that the sound, if exceptionally loud, may be conducted by bone 

 below the armlet. Their case was one of aortic regurgitation in a healthy 

 student — a good athlete. 



Conclusions. 



1. In the measurement of arterial pressure by means of the armlet and 

 sphygmometer the auditory method gives clear indices of systolic and diastolic 

 pressure. 



The auditory indices are (1) loud throbs heard in the artery below the 

 armlet when the compression is lowered just below the systolic pressure ; 

 (2) a sudden diminution in the sounds when the pressure falls just below 

 the diastolic pressure. We find that these indices depend on the pulsatile 

 flow of blood in and out of the part compressed by the armlet ; the artery is 

 deformed by the compressive force and its wall swings out and in when the 

 pulse wave strikes the deformed part. When the flow of blood into the 

 limb peripheral to the armlet is obstructed the throb is no longer heard, but 

 is replaced by dull sounds caused by the pulse striking the tense arterial wall. 



2. Dull sounds are heard, under ordinary conditions, when the compression 

 is reduced below the diastolic pressure. Such slight compression gives 

 occasion to the pulse to produce the dull sound, by obstructing the venous 

 outflow, and thus raising the diastolic pressure in the arteries and the 

 tension of their walls. 



3. The bigger the systolic wave the less compression is required to make 

 audible the dull sound. 



4. The accuracy of the auditory method depends on the conserving effect 

 which the tissue vessels have on the arterial pulse when the arm is com- 

 pressed. 



5. The method cannot, therefore, be used to give accurate measurements 

 in the case of an artery lying on bone and unsupported by tissue vessels 

 such as the aberrant radial or dorsalis pedis. 



6. Accurate readings can be obtained from these arteries where they lie 

 embedded in tissues, and the reinforcing effect of the tissue vessels comes 

 into play. 



* ' Brit. Med. Journ.,' 1914, A, p. 697. 

 VOL. LXXXVIII. — B. 2 S 



