Effect of diameter variability along a microvessel segment on pressure drop

In most microcirculatory studies, the diameter of a blood vessel is characterized by a single, average value, obtained from a projected two-dimensional microscopic view of the vessel. Such diameter values are often used to calculate microvascular hemodynamic variables (e.g., pressure drop along vessels). The validity, and some possible consequences, of the assumption that vessel diameter is constant along vessels were examined for (a) single capillaries of the hamster cremaster muscle and (b) a network model of blood flow in a rat mesenteric network. Vessel diameter was measured in cremaster microvessels of anesthetized golden hamsters (Nembutal, 70 mg/kg ip) at regular intervals along the lengths of individual vessels. The standard deviation of the diameter measurements (ranging from 0.2 to 4.0 microns) in each vessel increased with the average diameter (ranging from 4 to 27 microns, 132 vessels, 12 animals). The coefficient of variation (CV = SD/Mean) was close to 0.1 for vessels larger than 15 microns and up to 0.4 for smaller vessels. In individual isolated vessels, the ratio of calculated pressure drop using the actual diameter measurements (delta P) to pressure drop assuming an average diameter (delta Po) was between 1.05 and 3.0 (at constant volume flow rate); this ratio correlated significantly with the CV for that vessel. Using an iterative mathematical model of network blood flow in a mesenteric network, we investigated the effects on single-vessel delta P/delta P0 of using a single average estimate of diameter for each microvessel segment, compared to the inclusion of variations in each vessel diameter expected from the single-vessel data. The results from the model indicated that single vessel delta P/delta Po values obtained with expected diameter variations in each vessel varied between 0.01 and 100. This implies that the axial variations in vessel diameter that exist in vivo may affect the calculations of vessel pressure drop both in single vessels and in microvascular networks.