Blood Outflow from Capillary under Forced Pressure: Comparison between Electric Circuit Analog and Optical Measurement

Abstract

An electric circuit analog is proposed for blood outflow from a skin capillary when static pressure higher than the highest blood pressure of the artery is applied to the capillary. The proposed electric circuit analog consists of a capacitor and a variable resistor. The resistance of the variable resistor changes depending on the charge stored in the capacitor, which models the increase in blood flow resistance due to less blood in the capillary. In order to evaluate the electric circuit analog, the analytical solution is compared with in vivo data. In vivo data are measured using an inherent optical sensor, which consists of a light-emitting diode and a photodetector mounted on a mechanical actuator that applies static pressure to the skin surface. The change of the received light during pressurization is recorded. A parameter that is related to blood concentration is estimated from the Lambert–Beer law. In vivo data are measured for fingertips of 30 healthy volunteers, and the square errors of the fitting curves derived using an exponential function model, third-order function model, and the proposed electric circuit analog are compared. It is shown that the proposed electric circuit analog gives the best approximation among the three models. This result confirms that the blood outflow from the capillary under forced pressure can be evaluated using the proposed model.