130 THE CIRCULATION OF THE BLOOD 



tained in laboratory animals by direct measurement through, the use of 

 cannulse inserted into the vessels. Both the systolic and the diastolic 

 pressure can be measured with equal facility and accuracy. Since the 

 technic for making the systolic measurements was described at a much 

 earlier date than that for the diastolic, it has until recently been the 

 habit with a great part of the medical profession to be satisfied with 

 systolic readings alone. This is most unfortunate, because the knowledge 

 which such information gives us is incomparably inferior to that which 

 can be obtained by gauging the diastolic pressure. Until we have learned 

 more about the dynamics of circulation, it would be profitless to go 

 into any details as to the reasons for this statement, but it will soon 

 become self-evident. Suffice it for the present to state that the diastolic 

 pressure is the more important because it gives us the load ivhich the ves- 

 sels and aortic valves must constantly bear, and the resistance which must 

 be overcome prior to the opening of these valves at the beginning of 

 systole. Moreover, it helps us to gauge the peripheral resistance. 



The first step in the technique of blood-pressure measurements in man 

 is the placing of an armlet or cuff around the arm or leg. This armlet 

 consists of a rubber bag at least 12 cm. broad and covered on its outer 

 surface by cloth or leather. The bag is connected by tubing with a pres- 

 sure gauge and a pump. The pressure gauge may be either an ordinary 

 mercury manometer (Fig. 26) or one of the numerous gauges built on 

 the aneroid principle that are now on the market. For measuring the 

 blood pressure in the vessels of the upper extremities, the armlet should 

 be applied around the fleshy part of the upper arm and for the lower limbs 

 around the thigh. For accurate reading of both pressures in the arm the 

 following procedure should be followed. Having applied the armlet, the 

 pulse is palpated at the radial artery, and the pressure in the arm- 

 let then raised until the pulse can no longer be felt, at which moment 

 the pressure in the manometer is noted. The cuff is then slowly 

 decompressed and the pressure noted at which the pulse reappears. 

 These two readings of systolic pressure should be close together, but 

 they will not usually agree exactly for reasons which will be explained 

 immediately. They give us the palpatory systolic index, as it is called. 

 The pressure is now lowered about 15 mm. Hg, and a stethoscope is 

 placed in front of the bend of the elbow over the artery and as close up 

 to the cuff as possible. With each heartbeat a distinct sound like a pistol 

 shot will be heard. The decompression is now continued slowly, and as 

 the pressure falls the sounds will be heard to become louder and prob- 

 ably somewhat murmurish in quality. At a certain pressure this loud 

 character of the sound will suddenly become much less marked, and the 

 murmurish quality if present will disappear. This point corresponds to 

 the diastolic pressure, which is now read off from the manometer. 



