Ischemia may be less detrimental than anemia for O2 transport because of CO 2 transport: A model analysis.

We analyzed the relationship between oxygen delivery (Do2) and Pto2 (tissue Po2). We found an important factor which has not been specified before. In the previous O2 transport model, Do2 was a dependent variable, calculated from hemoglobin, blood flow, Pao2, and the oxygen dissociation curve (ODC). In this study, the model was modified slightly so that the Do2 can be an independent variable. We allowed, instead, one of the three parameters, hemoglobin, blood flow, and Pao2 to be a dependent variable. We compared the brain tissue Po2 under three conditions, hypoxemia, ischemia and anemia, at the same Do2. To further elucidate the mechanism produced by the effect of CO2 transport on the ODC, we studied the effect of the Bohr factor (d log Po2/d pH) and of the gas exchange ratio (Vco2/Vo2) on the O2 transport. Ischemia maintains a slightly higher tissue Po2 than anemia at the same Do2 level. In ischemia the CO2 transport is disturbed, leading a higher draining venous Pco2, which in turn maintains a higher Po2 the capillary, resulting in a higher gradient for Po2 between capillary and the tissue. Between ischemia and anemia, ischemia is less detrimental than anemia. In ischemia, the CO2 transport is disturbed, which in turn maintains a higher Po2 at the capillary.