Die nichtinvasive, pulsspektroskopische Bestimmung der Hämoglobinkonzentration im Blut

Pulse oximetry is a non-invasive, robust method for the determination of the arterial oxygen saturation in blood. In contrast, the hemoglobin concentration in the blood (cHb) is usually determined invasively by drawing a blood sample and using a photometer. Although in the literature, there are a lot of non-invasive methods along the lines of pulse oximetry, none of them are widely adopted in clinical practice. In this study, a non-invasive method for the determination of cHb was developed. On the basis of this method and by comparing it to the well-known method of pulse oximetry, interfering influences are analyzed in order to identify the factors that limit the accuracy of the non-invasive cHb determination. The analysis shows that there were particularly three factors that disturb the measurements: 1) The tissue filters the spectra of the light-emitting diodes (LEDs). Furthermore, LEDs show a thermal wavelength drift. Thus, the non-invasive cHb determination is depending on the LED temperature and the spectral properties of the tissue. In comparison, pulse oximetry is not concerned by these influences to that extent. 2) In contrast to pulse oximetry, measurements show that the developed method only works in transmission, not in remission. As simple model calculations show, it necessarily requires a change in measurement volume induced by the arterial blood pulse. Thereby, the accuracy of the method depends strongly on the precise placement of the sensor and the sensor geometry. 3) The mean path length of the light in the tissue depends on both the hemoglobin concentration and the blood content. Thus, a robust determination of the cHb concentration along the lines of pulse oximetry by using only two measurement wavelengths is impossible. It is shown that the path length can be taken into account by adding a third measurement wavelength. With the extended method based on three measurement wavelengths, a study is conducted on 139 subjects undergoing blood donation or blood transfusion at the Städtisches Klinikum Karlsruhe. Compared to the invasive reference values (laboratory equipment: Sysmex ME and Sysmex KX-21n), the non-invasive, predictive cHb values show a RMS error (RMS = root mean square) of 1.42 g/dl. After eliminating measurements with poor signal quality, RMS error on the remaining 120 measurements is 1.20 g/dl. Thus, the accuracy of invasive spot-check measurements is not fully reached. In this work, the main reasons for the reduced accuracy compared to those of pulse oximetry and invasive methods are discussed.