A blood pressure apparatus with pith-ball attachment indicating diastolic pressure

Of the many factors governing blood pressure, the most important, namely the force of the heart beat, receives least attention. A knowledge of the systolic pressure is of value in warning of certain dangers, but gives no clue as to the amount of work the heart does. In order to estimate this we must know the difference between the pressure during the heart beats and that in the intervals. The maximum pressure is easily ascertained by any one of the numerous sphygmomanometers. Only a few have means, in any way satisfactory, for determining diastolic pressure, and these are prohibitive in price and of tremendous bulk. To overcome these obstacles we have devised a small instrument that can be attached to any sphygmomanometer. This consists in an air chamber communicating by means of a glass tube containing a pith-ball with the tube leading from cuff to manometer. Any increase of pressure in the cuff is equalized in the chamber by a rush of air passing the pith-ball and moving it. With a steady slow exhaust of air the ball moves exactly as the point of Erlanger's lever, but without the fling. An average pulse throws Erlanger's lever about 3 cm., while the pith-ball flies about three times that distance. The movements of the ball are interpreted just as the movements of the lever in Erlanger's instrument. That point of pressure is read at which the oscillations begin to fall off from the maximum.