An optical device to measure blood components by a photoplethysmographic method

The development of the photometric device described here is based on the realization of a photoplethysmography measurement device developed for the German Space Agency DLR. It is well known in biomedical engineering that pulsatile changes of blood volume in tissue can be observed by measuring the transmission or the reflection of light (Roberts 1982 Trans. Inst. Meas. Control 4 101-6). The non-invasive multi-spectral method described here is based on the radiation of monochromatic light, emitted by laser diodes in the range 600-1400 nm, through an area of skin on the finger. After interaction with the tissue the transmitted light is detected non-invasively by photo-diodes. The method makes use of the intensity fluctuations caused by the pulse wave. The ratio between the peak to peak pulse amplitudes measured at different wavelengths and its dependence on the optical absorbability characteristics of human blood yields information on the blood composition. Deferrals between the proportions of haemoglobin and water in the intravasal volume should be detected photo-electrically by signal-analytic evaluation of the signals. The computed coefficients are used for the measurement and calculation of the arterial oxygenic saturation (SaO₂) and the relative haemoglobin concentration change. Results of clinical measurements are presented for a deoxygenation study with ICG-bolus injection (indocyanine green).