Human SS red cell rheological behavior in the microcirculation of cremaster muscle

The hemodynamic behavior of Hb SS erythrocytes introduced into the microcirculation of cremaster muscle (rat and mouse) was studied by intravital microscopy. Simultaneous measurements of intravascular pressure, pressure drop, red cell velocity, hematocrit and oxygen tensions were made to evaluate both regional resistance (arteriovenous) from 2nd order arterioles to venules, as well as the resistance to blood flow in single unbranched arterioles. Following the isovolemic exchange of 2 to 3 blood volumes with SS cells at 25% hematocrit, regional resistance remained within 20% of its control value due to compensatory vasodilation and shunting of blood through pathways parallel to capillaries occluded by sickled red cells. Prolonged exposure to circulating SS cells resulted in a degradation of microvascular function with an attendant four-fold rise in regional resistance. Transient resistance measurements during the passage of a bolus of SS cells through the cremasteric network demonstrated a 100% increase in regional resistance, which was rapidly abated by the subsequent SS cell washout and capillary recruitment. Computations of apparent viscosity in single unbranched arterioles (nominal diameter of 45 μm) revealed a four-fold rise in viscosity following the reduction in intravascular oxygen tension (PO₂) from 40 mm Hg to 7 mm Hg. Observations on the transient increase in viscosity concomitant with a rapid decrease in PO₂, demonstrated a lag time of 2 to 3 s before the elevation of viscosity. To enhance the visualization of these processes, techniques for the fluorescent labelling of Hb SS cells with fluorescein isothiocyanate (FITC) were applied. Visual observations of the movement of SS-FITC red cells by fluorescence microscopy readily revealed the sequestration of sickled cells at the junction between 10 μm transverse arterioles and the smaller true capillaries, and the shunting of labelled cells around obstructed capillaries.