Annals of biomedical engineering
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Since virtually all the oxygen carried by blood at normal hematocrit is reversibly bound to red blood cell hemoglobin, the distribution of oxygen within the microcirculation can be determined from measurements of hemoglobin concentration and hemoglobin oxygen saturation in vessels of the network. Photometric methods that rely on light absorption and scattering properties of blood are described. Criteria for selecting the wavelengths needed to analyze hemoglobin in the microcirculation are specified. ⋯ Technical aspects of microscope photometry including light sources, microscopy, and detection systems are described with special emphasis on the problem of glare. The importance of in vitro as well as in vivo calibrations is stressed, and several recent applications of a working system are discussed. Current problems as well as future developments of this methodology are delineated as a guide to future work in this area.
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New techniques for determining the hematocrit (Hct) and oxygen saturation (SO2) of whole blood from backscattered light measurements are described. First, theoretical and experimental results are presented which show that the empirical linear relationship between SO2 and the infrared-red backscattered light intensity ratio on which previous instruments have been based is an inadequate description primarily because it does not account for the strong effects of Hct and transducer geometry. Then it is shown that the ratio of backscattered intensities from two appropriately positioned infrared sources can be plotted against the infrared-red intensity ratio to produce a family of calibration curves from which SO2 and Hct can be independently determined. Finally, a practical implementation of an oximetry system which employs a microelectronic catheter-tip optical sensor and a microprocessor-based signal processor is proposed.
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Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. ⋯ Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.