The evaluation of blood flow velocity and heart rate by the frequency of oxygen saturation fluctuation in skin tissue

The oxygen in human body is transported by the hemoglobin and blood, and blood flow velocity (BFV) was related to the heart rate (HR); therefore, the oxygen saturation image of human skin can be used to evaluate the HR and BFV of human. When irradiating two different wavelengths light alternately and repeatedly, the frequency of oxygen saturation (SpO₂) fluctuation (f_c) can be obtained from the Fourier transformation of SpO₂ images, and roughly applied to build the relationship between BFV and HR. In the experiment, a high speed light controller was employed to switch two LED light source (red-660 nm and NIR-890 nm). Due to the penetration depth of incident light increases with increasing wavelength, and the deoxy-hemoglobin (Hb) and oxy-hemoglobin (HbO₂) has a relatively high absorptivity in visible and near-infrared (NIR) spectrum, respectively; the intensity of diffuse reflected images of skin can be used to compute the oxygen saturation images of skin tissue. The experiments show the SpO₂ after 3 minutes exercise would decrease 3 % and recover in 100 seconds, and the SpO₂ after 30 minutes exercise increased 1.5 % and became stable in 3 minutes. Simultaneously, the HR and BFV have the linear relationship to the frequency of SpO₂ fluctuation (f_c), so both two physiological indexes can be directly evaluated by SpO₂ images.