132 THE CIRCULATION OP THE BLOOD 



exactly correspond to that at which the sound was found to disappear. 

 If the palpatory systolic index is not below the auditory, it indicates 

 that some error has been made in the application of the apparatus, and 

 that the reading of the diastolic pressure will be unreliable. The usual 

 source of error is in the position of the stethoscope. If readjustment of 

 this does not bring the two indices into proper relationship, the auscul- 

 tatory method can not be relied upon for either systolic or diastolic 

 readings. 



In case of failure of the auscultatory method, we have to fall back upon 

 the palpatory method for measurement of the systolic pressure; and for 

 measurement of diastolic, we must use the method known as the oscillatory, 

 which until recent years was the only one known for gauging the dias- 

 tolic pressure. This consists in observing the oscillation of the indicator 

 of the pressure gauge; as the pressure in the cuff falls gradually from 

 below the systolic pressure, these oscillations will be observed to increase 

 in amplitude, until they reach a maximum beyond which with lower 

 pressure they rapidly decline. The pressure in the cuff at the moment 

 when the oscillations are at the maximum represents the diastolic pres- 

 sure. With a mercury instrument it is obviously difficult to employ this 

 method, but with a modern spring instrument it can with a little practice 

 be used with great accuracy and will serve as a valuable check on the 

 diastolic reading as taken by the auscultatory method. 



The procedure may be altered in various ways, there being only one pre- 

 caution to bear in mind ; namely, that the pressure in the cuff should not be 

 applied continuously for more than a few moments of time, for if this 

 is done for long periods, not only will it interfere with the accuracy 

 of the reading, but it may cause considerable discomfort to the patient. 



There are several conditions affecting the accuracy of the readings by each method 

 which it is well to bear in mind. These have been investigated by MacWilliam,i Leon- 

 ard Hill, 2 and Erlanger. 3 The most important conditions affecting the systolic pressure 

 are as follows: (1) The compression cuff should be a wide one (12 cm.), and it 

 should never be applied so that there is any chance of its compressing the artery 

 against a bony surface. This precaution is necessary, since it has been found that 

 much less pressure is required to obliterate any perceptible pulse below, the armlet 

 when the artery is flattened against some hard structure than when it is uniformly 

 compressed in the tissues in which it lies. (2) Discrepancies are often noted between 

 the systolic readings on compression and decompression of the artery; that is, the 

 pulse may reappear on decompression at a lower pressure than that at which it dis- 

 appeared on compression, the difference being most marked when the decompression 

 is done quickly. This difference is owing to the fact that the full force of the pulse 

 does not reach the forearm until all the vessels have become distended with blood. 

 (3) There are often discrepancies in the systolic readings taken from different limbs; 

 thus, it is not uncommon to find that the systolic pressure in the leg is higher than 

 that in the arm even when the observed person is in the horizontal position. These 

 differences are most commonly observed in patients suffering from aortic regurgita- 



