Comparison of different oxygen exchange models

A functional distribution of coronary volume can be estimated from the response of arterio-venous O₂ content difference (AVO₂) to a flow step. However, the results depend on the assumed O₂ exchange model. The previously used model consisted of a single mixed compartment with O₂ exchange in series with an unmixed compartment without O₂ exchange (reference model). The purpose of this study is to provide an estimate of the errors made in the volume estimations by not taking into account factors as flow heterogeneity, different mixing sites or Krogh-like O₂ exchange. The approach is indirect: the response of the AVO₂ to a flow step has been calculated with alternative O₂ exchange models in which the factors mentioned are incorporated. These transients are fitted with the reference model. The resulting estimated volumes are different from the volumes assumed in the alternative models. Large differences are obtained with some of the alternative models, e.g. the model with Krogh characteristics. However, these models seem unrealistic because capillary pO₂ is higher than venous pO₂. Only small differences in volume are obtained with the more realistic models. Therefore, these results indicate that the coronary volumes are approximated well by the estimations obtained with the reference model. These volume estimations were 9.9 and 3.8 ml 100 g⁻¹ for the O₂ exchange vessels and the distal venous volume, respectively.