Oxygen diffusion in a network model of the myocardial microcirculation.

Oxygen supply was studied in a three-dimensional capillary network model of the myocardial microcirculation. Capillary networks were generated using one common strategy to locate the capillary branchings and segments, arterioles and venules. Flow paths developed with different capillary flow velocities. All pressure-flow relationships were linear. The model includes free diffusion of oxygen within tissue slices that are perpendicular to the main capillary orientation. Oxygen pressure distributions were calculated and correlated to oxygen delivery by small capillary segments. It was shown that intercapillary diffusion is important for reducing PO2 heterogeneity. The absence of this feature leads to an oxygen distribution that has similar heterogeneity characteristics as the capillary flow heterogeneity. Such situations may also occur during simulated, elevated metabolic activity in a network model that allows intercapillary diffusion. On the basis of our simulations of metabolic vasoactivity we concluded that the venous PO2 is a misleading quantity to indicate tissue oxygenation. The venous PO2 in our model was not a good measure for the mean tissue or capillary PO2, and for the low oxygen pressures that exist at some locations in the network model. Moreover, the venous PO2 may remain constant despite considerable changes on the tissue PO2 distribution induced by metabolic activity.