Factors affecting the direct red cell effect on thrombosis: Hematocrit dilution and injury patterns.

Background: Red blood cell (RBC) aggregation can be initiated by calcium and tissue factor, which may independently contribute to microvascular and macrovascular thrombosis after injury and transfusion. Previous studies have demonstrated that increased blood storage duration may contribute to thrombotic events. The aims of this study were to first determine the effect of blood product components, age, and hematocrit (HCT) on the aggregability of RBCs, followed by measurement of RBC aggregability in two specific injury models including traumatic brain injury (TBI) and hemorrhagic shock.

Methods: Human whole blood (WB) units were obtained following the standard 21-day storage period. Whole blood was separated into components including RBCs, platelet-rich plasma (PRP), and platelet-poor plasma (PPP) via serial centrifugation and diluted to a standardized HCT on Days 2 and 23 following isolation. Finally, WB was collected from murine models of TBI and hemorrhagic shock at sequential, postinjury timepoints. Whole blood and component groups were analyzed for RBC aggregability with calcium and tissue factor initiated electrical impedance aggregometry.

Results: At both timepoints, nondiluted HCT RBCs demonstrated similar aggregability to standardized-HCT RBCs when diluted with phosphate buffered saline (PBS). Red blood cells diluted with PRP and PPP demonstrated significantly higher aggregation than RBCs diluted with PBS at both timepoints. Reconstitution with PRP and PPP demonstrated similar aggregability. Murine RBCs demonstrated increased aggregation at the 4-hour postinjury timepoint following TBI and decreased aggregation at the 1-hour postinjury following hemorrhagic shock.

Conclusion: Neither hemoconcentration or age of donated blood products affect the calcium and tissue-factor dependent aggregability of RBCs. Further, RBC aggregation is increased in the presence of plasma, not platelets-indicating a potential role for plasma in regulating RBC aggregation. Finally, injury patterns including TBI and hemorrhagic shock may influence hypercoagulability or coagulopathy via change in RBC aggregability.