Fig. 23.—Method of taking blood pressure with a patient in sitting position.
Fig. 24.—Method of taking blood pressure with patient lying down.
The pressure is now allowed to fall, until the palpating finger feels the largest possible pulse wave, which is coincident with the greatest oscillation of the mercury. This is the diastolic pressure. Beyond this point there is no oscillation of the mercury column. The difference between the two is the pulse pressure. Thus the pulse is felt after compression at 120 on the scale, and the maximum oscillation occurs at 80. The systolic pressure is 120 mm., the diastolic is 80 mm., and the pulse pressure is 40 mm.
With the "Tycos" or Faught the arm band is snugly wound around the arm, the bag next to the skin and the end tucked in, so that the whole band will not loosen when air is forced into the bag. The cuff is blown up until the pulse is no longer felt. One should raise the pressure not more than 10 mm. above the point of obliteration of the pulse. The valve is then carefully opened so that the needle gradually turns toward zero. At the first return of the pulse wave felt at the wrist, the needle is sure to give a sudden jump. This is the systolic pressure and is read off on the scale. The needle is now carefully watched until it shows the maximum oscillation. This is the diastolic pressure. The difference between the two is, as above, the pulse pressure.
In taking pressure one should take the average of several, three or four. Moreover, one must not take consecutive readings too quickly and one must be sure that between every two readings all the air is out of the cuff and that the mercury or dial is at zero. It has been repeatedly shown that in a cyanosed arm the systolic pressure is raised so that even slight cyanosis between readings must be carefully avoided.
The only accurate method of determining both the systolic and diastolic pressure, but especially the diastolic, is by the so-called auscultatory method. (See Fig. 25.) The cuff is adjusted in the usual way and one places the bell of a binaural stethoscope over the brachial artery from one to two centimeters below the lower edge of the cuff.[3] Care must be taken that the bell is not pressed too firmly against the arm and that the edge of the bell nearest the cuff is not pressed more firmly than the opposite end. For this purpose, one can not use the ordinary Bowles stethoscope or any of the other much lauded stethoscopes, because the surface of the bell is too large. The diameter of the bell must not be more than twenty-five millimeters, twenty is still better. It is advisable before beginning the observation to locate with the finger the pulse in the brachial artery just above the elbow, so that the stethoscope may be placed over the course of the artery. (Fig. 26.) The first wave which comes through is heard as a click, and occurs at a point on the manometer or dial scale from 5-10 mm. higher than can usually be palpated at the radial artery. This is the true systolic pressure. By keeping the bell of the stethoscope over the brachial artery while the pressure is falling, one comes to a point when all sound suddenly ceases. This is said to be the diastolic pressure. This is incorrect as will be shown later.