An analysis of microcirculatory flow heterogeneity using measurements of transit time.

Heterogeneity of blood flow distribution was measured in capillary networks in cremaster muscles of anesthetized Golden hamsters (nembutal, 70 mg/kg, ip). The relative dispersion of Q/PS, where Q is blood flow, P is permeability, and S is exchange surface area, was estimated; in microvascular terms (and assuming uniform permeability) this ratio reduces to vr/l, where v is plasma velocity, r is vessel radius, and l is vessel length, and where v/l = 1/T, where T is transit time. Distributions of 1/T across complete capillary networks significantly increased in relative dispersion from 68.2 to 97.8% during hyperemia, suggesting an increase in flow heterogeneity with increased inflow. In contrast, relative dispersion of 1/T did not change significantly from rest (72.0%) to hyperemia (66.1%) in capillary segments sampled randomly across the tissue. Other microvascular indices of flow (velocity, cell content) did not reflect the changes in relative flow dispersion shown by the changes in 1/T. The analysis demonstrates that estimates of flow heterogeneity are sensitive both to the selection of an appropriate flow variable and to the manner in which this variable is sampled in the capillary bed.