Abstract
The arterial wall is distended outward due to the intraarterial pressure (I. e., blood pressure ; BP). Therefore, pushing back the arterial wall by using an appropriate counterpressure, the counter-pressure will balance with BP at the moment when the arterial distension is completely relieved (I. e., the vascular unloaded state). The vascular unloaded state is detectable from the occurrence of the maximum volume pulsation in an artery during the application of the counterpressure. When the transmural pressure reaches zero (I. e., unloaded), the highest compliance of the vascular wall results in the maximum amplitude of volume pulsations. Based on this principle, Yamakoshi et al. (1980) developed a new method for measuring instantaneous BP. A transmittance photoelectric plethysmograph is attached to the medial or root portion of a finger to detect the vascular volume change. The finger is inserted into an occluding cuff which is installed in a compression chamber filled with water. The chamber is connected with a diaphragm actuator which is further linked to a shaker. The finger can, therefore, be compressed or decompressed by the hydraulic pressure in the chamber (I. e., cuff pressure). First, a reference value for the servocontrol which corresponds to the vascular volume in the unloaded state is detected. This is obtained by performing an open-loop operation for the servo-control system. The cuff pressure is raised gradually using an external signal, and a mean level of the photoelectric plethysmogram is detected at the moment when the maximum amplitude of the pulsating signal is attained. After the reference value is determined, the closed-loop operation is made to measure the continuous BP. The plethysmographic signal is fed to a differential amplifier and subtracted from the reference value. The servocontrol error thus obtained is fed to a power amplifier. Consequently the output signal of the power amplifier drives the shaker which sets the diaphragm actuator going to control the cuff pressure. In this way, any variation in vascular volume due to the cbange in BP is instantaneously compensated for by the servocontrol system. The vascular volume is fixed at the reference value, and the cuff pres- sure follows instantaneously the BP. In this method, beat-to-beat systolic and diastolic BP as well as the BP waveform can be measured. Comparisons with direct measurement have been carried out successfully in many normotensive and hypertensive subjects. Unlike the Riva-Rocci method, the present one does not occlude the arteries of the finger but hold them under the vascular unloaded state. Venous return occurs little by little as the venous pressure in the dista is balanced against the cuff pressure. Therefore, the continuous measurement of BP can be carried out without much discomfort for long periods of time. Bodily movements and/or changes in posture seldomly produce the artifacts of BP recordings because the finger segment is compressed moderately in the cuff, and the frequency response of the whole system [is unform up to more than 60 Hz. In conclusion, the present method seems to be superior to other indirect methods. BP biofeedback experiments should be carried out more easily by use of this method.
