An oximeter for measuring hemoglobin concentration and oxygen content

We have developed an oximeter that measures both the total hemoglobin concentration in whole blood and the percentage of the hemoglobin saturated with oxygen. The oximeter uses red and infrared light-emitting diodes to illuminate a capillary tube filled with a sample of whole blood. Light scattered by the blood travels a short distance down the length of the capillary tube and reaches a photodetector, the output of which is amplified, digitized, and fed into a microprocessor. The microprocessor computes the total hemoglobin concentration as a nonlinear function of the infrared light intensity. Oxyhemoglobin saturation is computed from the ratio of the logarithms of the intensities of red and infrared light. Our instrument has the following advantages over existing oximeters: 1) it provides a measurement of total hemoglobin concentration, 2) it is immune to the calibration shifts that fluctuations in total hemoglobin concentration cause in other oximeters, 3) it is accurate over a wide range of oxygen saturation, and 4) the blood samples are not diluted and can thus be preserved for further analysis. A detailed parts list and circuit diagram are presented, and sources of error are discussed.