Ektacytometry has been the primary method for evaluating deformability of red blood cells (RBCs) in both research and clinical settings. This study was designed to test the hypothesis that the flow of RBCs through a network of microfluidic capillaries could provide a more sensitive assessment of the progressive impairment of RBC deformability during hypothermic storage than ektacytometry. RBC units (n = 9) were split in half, with one half stored under standard (normoxic) conditions and the other half stored hypoxically, for up to 6 weeks. RBC deformability was measured weekly using two microfluidic devices, an artificial microvascular network (AMVN) and a multiplexed microcapillary network (MMCN), and two commercially available ektacytometers (RheoScan-D and LORRCA). By week 6, the elongation indexes measured with RheoScan-D and LORRCA decreased by 5.8–7.1% (5.4–6.9% for hypoxic storage). Over the same storage duration, the AMVN perfusion rate declined by 27.5% (24.5% for hypoxic) and the MMCN perfusion rate declined by 49.0% (42.4% for hypoxic). Unlike ektacytometry, both AMVN and MMCN measurements showed statistically significant differences between the two conditions after 1 week of storage. RBC morphology deteriorated continuously with the fraction of irreversibly-damaged (spherical) cells increasing significantly faster for normoxic than for hypoxic storage. Consequently, the number of MMCN capillary plugging events and the time MMCN capillaries spent plugged was consistently lower for hypoxic than for normoxic storage. These data suggest that capillary networks are significantly more sensitive to both the overall storage-induced decline of RBC deformability, and to the differences between the two storage conditions, than ektacytometry.

BACKGROUND Higher hematocrit increases oxygen carrying capacity of blood, but also increases blood viscosity, thus decreasing blood flow through the microvasculature and reducing the oxygen delivery to tissues. Therefore, an optimal value of hematocrit that maximizes tissue oxygenation must exist. STUDY DESIGN AND METHODS We used viscometry and an artificial microvascular network (AMVN) device to determine optimal hematocrit in vitro. Suspensions of fresh red blood cells (RBCs) in plasma, normal saline or a protein-containing buffer, and suspensions of stored RBCs (at week 6 of standard hypothermic storage) in plasma with hematocrits ranging 10 – 80% were evaluated. RESULTS For viscometry, optimal hematocrits were 10, 25.2, 31.9, 37.1 and 37.5% for fresh RBCs in plasma at shear rates of ≤3.2, 11.0, 27.7, 69.5, and 128.5 s−1. For the AMVN, optimal hematocrits were 51.1, 55.6, 59.2, 60.9, 62.3 and 64.6% for fresh RBCs in plasma and 46.4, 48.1, 54.8, 61.4, 65.7 and 66.5% for stored RBCs in plasma at pressures of 2.5, 5, 10, 20, 40 and 60 cmH2O. CONCLUSION Although exact values of optimal hematocrit may depend on specific microvascular architecture, our results suggest that optimal hematocrit for oxygen delivery in the microvasculature depends on perfusion pressure. Anemia in chronic disorders may, therefore, represent a beneficial physiological response to reduced perfusion pressure resulting from decreased heart function and/or vascular stenosis. Our results may help explain why therapeutically increasing hematocrit in such conditions with RBC transfusion frequently leads to worse clinical outcomes.