Evaluation of photometric methods for quantifying convective mass transport in microvessels.

We evaluated the accuracy of blood (Qb), hemoglobin (QHb) and O2 (QO2) flow determinations in arterioles (30-89 micron ID) and venules (30-140 micron ID) of the hamster retractor muscle by testing flow conservation in straight segments (for Qb and QHb) and at bifurcations (for Qb, QHb, and QO2). We found absolute values of relative discrepancies in flow conservation of greater than 30% in straight sections and of greater than 20% at bifurcations. To understand these results, we evaluated the assumptions implicit in the flow calculations by determining the intraluminal profiles of red blood cell velocity, hemoglobin concentration, and O2 saturation. We found that these assumptions were not generally satisfied since only 59% of velocity profiles, 70% of hemoglobin concentration profiles, and 70% of O2 saturation profiles were symmetric about the vessel axis with the velocity profiles more blunt than would be expected for Poiseuille flow. We reevaluated flow conservation by taking into account the bluntness of the velocity profiles. By selecting vessels that had symmetric velocity and hemoglobin concentration profiles and by including the bluntness of the velocity profile, we obtained a significant improvement in the demonstration of flow conservation.