Indirect Measurement of Instantaneous Arterial Blood Pressure in the Human Finger by the Vascular Unloading Technique

For the indirect measurement of beat-to-beat systolic and diastolic pressure in the human finger, a new hydraulic servocontrol system was designed to maintain the vascular volume in the unloaded state. The servocontrol system consists of a compression chamber equipped with an occluding cuff and a photoelectric plethysmograph, an electromagnetic shaker, and a volume servo circuit. The shaker connected to a diaphragm actuator is used for controlling the cuff pressure. The vascular volume change in the finger is detected by the photoelectric plethysmograph. The plethysmographic signal 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 controlled cuff pressure follows the intraarterial pressure. The accuracy of this method was evaluated using an in vitro vascular model of the finger. Comparisons with direct measurement were carried out successfully in four normotensive and six hypertensive subjects.