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Using the isolated perfused rat hindlimb and the fluorocarbon-transfused rat, we have examined the optical characteristics of the rat skeletal muscle in the near-infrared region. The total contribution of myoglobin and cytochromes to the overall absorbance change was less than 10%. Analyzing transmitted light at 700, 730, and 805 nm, we found linear relationships between the absorbance and the hemoglobin concentrations at hematocrit values from 15 to 50% in the inflowing perfusate. Based on the relationship, we determined the ratio of absorption coefficients at 700, 730, and 805 nm of oxy- and deoxy-hemoglobins of blood in the thigh muscle. The values in thigh muscle were significantly smaller than those in hemoglobin solutions for deoxygenated blood. On the other hand, the values in thigh muscle were larger than those in hemoglobin solutions for oxygenated blood. Solving simultaneous equations by the use of these absorption coefficients, we calculated the changes in the contents of oxy-, deoxy-, and total hemoglobins in the anesthetized rat hindlimb under various conditions. The oxygen saturation of blood determined by our optical method in the thigh muscle was very close to that in the vena cava measured directly with a gas analyzer.
Seiyama et al. (Tue,) studied this question.