Microvascular Blood Flow of Sickled Erythrocytes A Dynamic Morphologic Study

Abstract

Resistance to capillary flow in the sickle cell disorders is related to decreased deformability of erythrocytes. Increased blood viscosity leads to initiation of the ‘vicious cycle’ with subsequent stasis, hypoxia, sickle transformation and vaso-occlusion. In this study, impedance to the microcirculation of sickle cells was demonstrated by microcinematography of sickle cell flow in the capillary bed of an experimental animal. Male Wistar rats weighing 150 gm were anesthetized and 60–80% exchange transfusion was carried out with 6–10 ml of fresh whole blood from patients with sickle cell anemia. Mesenteric capillary flow observed by light microscopy was recorded by cinematography and videotape with the animal in both oxygenated and deoxygenated states. Dynamic morphologic observations included the following: 1. Oxygenated AA and SS cells were capable of deformation into elongated, teardrop, cup or parachute shapes in order to traverse vessels of 10 μm or less at high and moderate flow rates. Intermittent bolus flow was observed, and red blood cell aggregates formed at low flow velocity were dispersed by the force of the bolus flow. 2. Critical vessel radius for SS discocytes was larger than that for AA cells. 3. Sudden increase in viscosity, which occurred when SS discocytes were unable to pass through critical size capillaries, retarded the already slow flow to a standstill long enough to permit hypoxic SS cells to transform into sickle shapes within 20–30 s. 4. Rigid reversible and irreversible sickle cells induced capillary blockades resulting in a marked increase in viscosity. 5. The occlusive sickle cell blockade could be dislodged by a sudden increase in bolus flow induced by reoxygenation or epinephrine, which was sufficient to overcome cellular yield stress and disperse cell aggregrates.