IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-27, NO. 3, MARCH 1980 Indirect Measurement of Instantaneous Arterial Blood Pressure in the Human Finger by the Vascular Unloading Technique KEN-ICHI YAMAKOSHI, HIDEAKI SHIMAZU, AND TATSUO TOGAWA Abstract-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 con- nected to a diaphragm actuator is used for controlling the cuff pressure. The vascular volume change in the rmger is detected by the photoelectric plethysmograph. The plethysmographic signal is fed into the servo cir- cuit to control the cuff pressure, which is clamped at a proper value corresponding to the unloaded vascular volume. At this state the con- trolled cuff pressure follows the intraarterial pressure. The accuracy of this method was evaluated using an in vitro vascular model of the fin- ger. Comparisons with direct measurement were carried out success- fully in four normotensive and six hypertensive subjects. INTRODUCTION A number of studies for the long-term and/or automated indirect measurement of arterial pressure using sphyg- momanometric techniques has been presented [1], [2]. In most of these the systolic and diastolic pressure can be ob- tained only intennittently, because some time is needed for the measurement procedure. There is a need for an instrument by which arterial pressure can be measured continuously and noninvasively. The present paper describes a new method for measuring both beat-to-beat systolic and diastolic pressure and pressure wave- form in the human fmger noninvasively. Using a hydraulic servocontrol technique, an applied counterpressure is made to equalize the intraarterial pressure. Thus, the instantaneous arterial pressure can be indirectly recorded by measuring the counterpressure. METHODS The Instrument The method to be described is based on the indirect unload- ing of the vascular wall [3], [4] . The vascular volume changes associated with the intraarterial pressure are compensated by an applied counterpressure to maintain a constant vascular volume in the unloaded state (the unloaded vascular volume). Manuscript received June 25, 1979; revised August 30, 1979, and October 11, 1979. The authors are with the Institute for Medical and Dental Engineer- ing, Tokyo Medical and Dental University, Tokyo, Japan. In the unloaded state the counterpressure is equal to the intra- arterial pressure. This allows the indirect measurement of the intraarterial pressure. Fig. 1(a) shows a block diagram of the instrument. This is a modification of the system used for the measurement in the rat's tail [5] . The instrument can be divided into a mechanical- sensing system and a servo system. The mechanical system is shown in Fig. 1(b). It consists of a compression chamber (CC) filled with water and a diaphragm actuator (DA) (effec- tive diaphragm area = 11 cm2) connected with an electro- magnetic shaker (SHAKER, G-002, Shinken Company). A finger is placed in the chamber through an occluding cuff (OC), so that the finger segment can be compressed or decompressed by the hydraulic pressure in the chamber (cuff pressure Pa). The cuff is a thin-walled (0.1 mm) translucent polyurethane tube. It is formed so that during the finger compression no cuff tension is developed. Both flanges of the cuff are firmly fixed to both ends of the chamber by a pair of annular disks [AD in Fig. 1(a)]. The effective width of the cuff is 28 mm and its inner diameter is about 22 mm. The diaphragm actuator which is used to control the cuff pressure is connected with the chamber through a 10 mm-long fluid passage of 20 mm inner diameter (compliance of its diaphragm = 0.011 ml/100 mmHg). A retainer plate (RP) firmly fixed to the diaphragm is connected with plunger (P) of the shaker. The maximum displacement, the force, and nat- ural frequency of the shaker are ±2 mm, 5 kg, and 80 Hz, respectively. The position of the plunger is sensed by a photo- electric linear displacement transducer (LT) (frequency re- sponse = -3 dB at 100 Hz). The controlled volume (Se) can be obtained from the output of this transducer. The chamber has a side connection (SC) for a Statham P-37 pressure trans- ducer and a vent (AV) to remove air bubbles. A transmittance photoelectric plethysmograph is used to de- tect the volume change in the finger. Series-connected light emitting diodes (LED's) (TLN103, Toshiba Electric Company) are used as a light source, while parallel-connected phototran- sistors (PT) (TPS603, Toshiba Electric Company) are used as photodetector [see Fig. 1(c)] . The light source (LED) is placed in a cylindrical hole in the upper part of the chamber and in- sulated from the water by a transparent acrylic plate. The photodetector (PT) is fixed directly on the skin on the opposite side of the light source using adhesive tape. The frequency re- sponse of this plethysmograph is uniform up to about 200 Hz. The servocontrol system consists of a differential amplifier 0018-9294/80/0300-0150$00.75 © 1980 IEEE ISO