Evidence for increased perfusion heterogeneity in skeletal muscle during reduced flow

Recently, it has been demonstrated ( D. Cousineau, C. P. Rose, D. Lamoureux, and C. A. Goresky, 1983, Cir. Res., 53, 719–730; K. Tyml, 1986, Microvasc. Res., 32, 84–98) that heterogeneity of microvascular flow depends on tissue metabolism. The objective of this study was to examine the possibility that, independent of metabolism, heterogeneity is also a function of flow. Using an intravital video-microscopic approach, we evaluated heterogeneity in the frog sartorius muscle at different flow rates while maintaining the muscle in the same exercised state. The flow was altered via partial aortal clamping. Exercised state was achieved by direct electrical stimulation. Heterogeneity of flow was evaluated in terms of the coefficient of variation ( CV = SD mean ) computed from simultaneous measurements of red cell velocities in a capillary network. In addition to velocity analysis, the number of perfused capillaries crossing a 1-mm test line on the video monitor was counted. Among 10 networks from eight muscles, the overall preocclusion hyperemic velocity and CV were 0.42 ± 0.15 SD mm/sec and 40 ± 12%, respectively. The overall capillary count was 16.4 ± 3.0 cap/mm. In all networks, increasing clamping reduced the mean hyperemic velocity and increased CV. For reductions to less than 50% of the preocclusion velocity, CV increased significantly ( P < 0.05), up to 88%. In 6 networks only, increasing clamping reduced capillary count, down to 10.2 cap/mm. This reduction was due to flow stoppages in capillaries situated randomly throughout the network. The data demonstrate for the first time that, for the same exercised state, heterogeneity of velocity depends on the mean velocity. This dependence is partially linked to the number of perfused vessels. When the present CV data were compared with our previously obtained resting state data ( Tyml, 1986), it became evident that, at numerically similar low mean velocity, the resting state heterogeneity can be twice as large as the exercised state heterogeneity. The present study supports the view that, in addition to the metabolic component operating at the arteriolar level, a flow-dependent hemodynamic component operating at the capillary level plays a major role in the determination of flow heterogeneity in the microvasculature.