130 THE CIRCULATION OF THE BLOOD 



the auditory systolic index. It will be found to give a systolic pressure 

 a little higher than that obtained by palpation of the artery at the wrist. 

 The sound being now absent, the pressure in the cuff is lowered until 

 the sound reappears, and the point at which this occurs should almost 

 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 the failure of the auscultatory method, we have to fall back 

 upon the palpatory method for systolic pressure; and for the measure- 

 ment of diastolic, we have to use the method known as the oscillator]/, 

 which until recent years was the only one used 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 Avhich it is Avell to bear in mind. These have been investi- 

 gated by MacWilliam, 1 Leonard Hill, 2 and Erlanger. 3 With regard to 

 the systolic pressure the most important of these 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) Dis- 

 crepancies are often noted between the systolic readings on compres- 



