Dynamic effects of calcium on in vivo and ex vivo platelet behavior after trauma

BACKGROUND Mobilization of intra and extracellular calcium is required for platelet activation, aggregation, and degranulation. However, the importance of alterations in the calcium-platelet axis after injury is unknown. We hypothesized that in injured patients, in vivo initial calcium concentrations (pretransfusion) predict ex vivo platelet activation and aggregation, viscoelastic clot strength, and transfusion of blood products. We additionally hypothesized that increasing calcium concentrations ex vivo increases the expression of platelet activation surface receptors and platelet aggregation responses to agonist stimulation in healthy donor blood. METHODS Blood samples were collected from 538 trauma patients on arrival to the emergency department. Standard assays (including calcium), platelet aggregometry (PA) and thromboelastometry (ROTEM) were performed. In PA, platelet activation (prestimulation impedance [Ω]) and aggregation responses to agonist stimulation (area under the aggregation curve [AUC]) with adenosine diphosphate (ADP), thrombin receptor-activating peptide, arachidonic acid (AA), and collagen (COL) were measured. Multivariable regression tested the associations of calcium with PA, ROTEM, and transfusions. To further examine the calcium-platelet axis, calcium was titrated in healthy blood. Platelet aggregometry and ROTEM were performed, and expression of platelet glycoprotein IIb/IIIa and P-selectin was measured by flow cytometry. RESULTS The patients were moderately injured with normal calcium and platelet counts. Higher calcium on arrival (pretransfusion) was independently associated with increased platelet activation (prestimulation, Ω; p < 0.001), aggregation (ADP-stimulated, AUC; p = 0.002; thrombin receptor-activating peptide-stimulated, AUC; p = 0.038), and clot strength (ROTEM max clot firmness; p < 0.001), and inversely associated with 24-hour transfusions of blood, plasma, and platelets (all p < 0.005). Up-titrating calcium in healthy blood increased platelet activation (prestimulation, Ω; p < 0.001), aggregation (ADP, AA, COL-stimulated AUCs; p < 0.050), and expression of P-selectin (p = 0.003). CONCLUSION Initial calcium concentrations (pretransfusion) are independently associated with platelet activation, aggregation, clot-strength, and transfusions after injury. These changes may be mediated by calcium driven expression of surface receptors necessary for platelet activation and aggregation. However, the therapeutic benefit of early, empiric calcium repletion in trauma patients remains undefined. LEVEL OF EVIDENCE Prognostic, level V.