Analysis of oxygen transport and oxygen utilization combined.
We propose a model which combines oxygen transport system from blood to tissue with oxygen utilization system at the tissue. The model consists of 3 equations; the relationship between tissue P(O)(2) (Pts(O)(2)) and O(2) utilization (Vrc(O)(2)), diffusion from vessel to tissue, and Fick equation. This model has two advantages. First, it is self-consistent. Varying Vrc(O)(2) varies the oxygen transport. Second, it enables to analyze the effects of various factors of oxygen transport/utilization on other factors. We applied this model to the brain tissue. Following values were assumed. Critical tissue P(O)(2) (Pcrit(O)(2)) 2 mmHg; oxygen utilization above this level 3 ml.min(-1).100 g(-1); diffusion coefficient from blood vessel to tissue (D) 0.2 ml.min(-1).mmHg(-1).100 g(-1); cerebral blood flow (CBF) 50 ml.min(-1).100 g(-1); hemoglobin 15 g.100 ml(-1). Hill equation was used for oxygen dissociation curve with n of 2.7 and P50 of 27.0 mmHg. From these, the following values were obtained; Pv(O)(2), Pts(O)(2) and Vrc(O)(2). The changes were analyzed for the 5 input values, Pa(O)(2), CBF, D, P50 and Hb, changing from zero to their respective normal values. A reduction of a single parameter down to 50% of normal barely affected oxygen utilization. A further reduction resulted in significant oxygen utilization. Under conditions studied, a decrease in P50 reduced oxygen utilization faster than that in any other parameters.
Department of Anesthesia, Faculty of Medicine, University of Tokyo, Tokyo, Japan.
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Suwa