Effect of isovolemic hemodilution on microvascular perfusion in rat skeletal muscle during a low flow state

The objective was to evaluate the hemodynamic effect of hemodilution during a state of experimentally reduced microvascular flow. We measured red blood cell velocity (VRBC) and density of capillaries with moving and stationary red cells (CDPER, CDSTAT) at the surface of extensor digitorum longus muscle in 26 rats anesthetized with pentobarbital. The low flow state was achieved by reducing the femoral blood pressure to 30 mmHg via a partial occlusion of the abdominal aorta. Hemodilution with a 6% PVP-40 (polyvinylpyrrolidone) solution (average hematocrit reduction from 47 to 22%) was introduced either 90 minutes before or 30 minutes after the onset of low flow state. When compared with the control state, this state was associated with significantly reduced VRBC (from 0.14 to 0.016 mm/s) and CDPER (from 32.5 to 18.7 cap/mm of test line), and significantly increased CDSTAT (from 4.5 to 13.8 cap/mm), percentage of capillaries with low VRBC, 0-0.05 mm/s, (from 35 to 94%) and VRBC heterogeneity (coefficient of variation increased from 91 to 152%). Pre-hemodilution improved microvascular perfusion dramatically during the low flow state. VRBC and CDPER increased significantly by 387 and 51%, while CDSTAT, percentage of low VRBC capillaries, and heterogeneity decreased significantly by 52, 51 and 32%. Post-hemodilution, on the other hand, showed no improvement as none of these parameters changed significantly from the low flow state. We conclude that, based on this rat model, hemodilution is more suitable as a preventive measure, rather than as a treatment, of microcirculatory disorders associated with compromised flow.