Red Blood Cells Preserve Platelet Function and Coagulation From The Effects Of Acidemia

Despite a large body of literature regarding Acute Coagulopathy of Trauma and Massive Transfusion, there is little consensus on the appropriate diagnostic approach to establish the cause of associated uncontrolled bleeding. Thromboelastography (TEG) is widely used to characterize trauma-associated bleeding. However, the use of whole blood in this assay may obscure important changes in blood component hemostatic function. Blood varies in hematocrit from a typical level of 40% in large vessels to 10% in capillaries. Patients with persistent bleeding may present with prolonged prothrombin time (PT)/activated partial thromboplastin time (aPTT) measured in plasma, but unremarkable TEG tracings measured in whole blood drawn from a large vein. As one of the only ubiquitously accepted causes of a bleeding coagulopathy associated with trauma, acidemia was modeled in TEG to investigate 1) the differential coagulopathy detection obtained by separation of blood components before TEG testing, 2) the range of acidemia effects at various levels of the vascular tree through variable hematocrit testing, and 3) effects of replacing RBCs with plasma versus normal saline.

Male donor blood (n=5) was drawn into citrate and the contact pathway was immediately further inhibited with corn trypsin inhibitor. TEG cups were preloaded with appropriate volumes of [morpholino]ethanesulfonic acid buffered saline (MBS) for pH 7.4, pH 7.1, pH 6.8 and were recalcified (15mM). R-time parameters were not significantly different when whole blood (WB) or red blood cells combined with platelet poor plasma (RBC/PPP) were activated to clot with tissue factor (1:5000 dilution induced a normal 4 minute clot time) in the simulated acidemia, whereas platelet poor plasma (PPP) showed a significant delay at pH 7.1 (p<0.0001) and pH 6.8 (p<0.0001) when compared to pH 7.4. Shear Elastic Modulus Strength showed only significant losses in platelet rich plasma (PRP: pH 7.4 vs pH 7.1; p=0.013, pH 7.1 vs pH 6.8; p=0.015) but not WB, RBC/PPP, or PPP.

Hematocrit levels were chosen to model typical changes from large to small vessels, in the presence of 200,000 platelets/microliter, where the volume of RBCs was replaced with plasma. Results indicate that zero hematocrit conditions result in a significant R-time delay (pH 7.4 vs pH 7.1; p=0.0306, pH 7.1 vs pH 6.8; p<0.0001) whereas no significant delays occur at 20%, 30%, and 40% hematocrit. Utilization of normal saline for RBC replacement did not alter any of the 20%, 30%, or 40% hematocrit R-time findings yet zero % hematocrit samples (pH 7.1; 12.9+/-1.64 minutes, pH 6.8; 34.98+/-4.03 minutes) were significantly delayed (p<0.0001) when compared to plasma replacement of RBC (pH 7.1; 8.22+/-0.70 minutes, pH 6.8;13.98+/-1.54 minutes).

Subsequent biochemical evaluation of whole blood (n=5) activated to clot in conditions of acidemia (pH 7.4, pH 7.1, pH 6.8) showed no significant delay or reduced peak of thrombin generation, no significant difference in factor V activation or fibrinopeptide A cleavage.

Data indicate that 1) separation of blood components allows a more sensitive parsing mechanism for TEG evaluation of coagulation in acidemia, 2) simulated capillary blood containing low hematocrit demonstrates the anti-coagulant effects of acidemia obscured in whole blood from large vessels with normal hematocrit, 3) resuscitation with normal saline may exacerbate microvascular bleeding in the acidemic patient, and 4) RBCs prevent delays in clot initiation and loss of platelet force generation in the presence of acidemia. This work provides a mechanism to explain the observation that increased hematocrit reduces microvascular bleeding.