Abstract
For indirect measurement of both beat-to-beat systolic and diastolic pressures and pressure waveform in the human finger, a new hydraulic servocontrol system, which is to maintain the vascular volume in the unloaded state, has been designed. The proposed system consists of a compression chamber equipped with an occluding cuff and a transmittance photoelectric plethysmograph, an electromagnetic shaker, and a volume servo circuit. The shaker connected to a diaphragm actuator is used for controlling the chamber pressure (cuff pressure). The vascular volume change in the finger is detected by the photoelectric plethysmograph. The output signal from the plethysmograph is fed into the servo circuit to control the cuff pressure, which is clamped at a proper value corresponding to the unloaded vascular volume. At this state the cuff pressure thus automatically controlled follows the intra-arterial pressure. The accuracy of this method has been evaluated by both the in vitro experiment using a vascular model and the animal experiment using the rat's tail. Very close agreement between the simultaneous data from this method and the direct measurement was obtained. Comparisons with direct measurement w ere also carried out successfully in four normotensive and six hypertensive subjects.
