Ischemia may be less detrimental than anemia for O₂ transport because of CO₂ transport: A model analysis

Abstract

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.