Similar Hemostatic Responses to Hypovolemia in Hemorrhage and Lbnp Reveals a Hyperfibrinolytic Subset of Baboons

Background: Lower body negative pressure (LBNP) has been established in human studies as an alternative method to blood removal (HEM) to study central hypovolemia. In a baboon model, LBNP also mimics the cardiovascular response of HEM. Hemostatic responses, including platelet activation, coagulation and fibrinolysis in response to LBNP and HEM in this baboon model are reported here. A previous meeting report from this study focused on hemostatic interactions; this report expands the assay of fibrinolytic proteins and includes detailed post-hoc analysis of interactions.

Methods: Thirteen anesthetized baboons were exposed to progressive central hypovolemia by HEM and, four weeks later, by LBNP. Whole blood cell counts were determined; pro-coagulant and fibrinolytic activity was evaluated by plasma markers, thrombelastography, flow cytometry and platelet aggregometry at baseline (BL), intermediate hypovolemia (MID), presyncope or maximal hypovolemia (MAX) and after recovery (REC).

Results: Baseline values for HEM (day 0) and LBNP (day 28) were indistinguishable for most analyzed parameters; however platelet number, maximal clotting (MA), Protein C, TAT, TAFI-activity, plasma renin, angiotensin and epinephrine were all significantly different (p<0.05). Central hypovolemia induced by both methods activated coagulation; TEG revealed decreased clotting time (R) and increased maximal clotting (MA) at MID, MAX and REC over BL. No differences during developing central hypovolemia were observed between HEM and LBNP. Platelets displayed activation markers on flow cytometry; absolute numbers and functional aggregometry were unchanged. At MAX hypovolemia, there was a modest hemodilution for HEM and a modest hemoconcentration for LBNP; red blood cell count decreased 5.8%±5.5%, p=0.0007 with HEM and increased 5.3±5.9%, p=0.0056 with LBNP. Coagulation factors were significantly increased with LBNP at MAX compared to HEM; fibrinolysis proteins were more varied. No hyper-fibrinolysis developed with progressive hypovolemia by either method; instead hyper-fibrinolysis already existed at BL with significant variance in TEG fibrinolysis at 30 min (LY30) values. Post hoc analysis separated study subjects into low lysis (LY30_BL<2) or high lysis (LY30_BL>2) whose fibrinolytic state matched at both HEM and LBNP BL. High lysis samples (n=8) showed a strong correlation (r=0.95; p<0.001) of tissue plasminogen activator (tPA) to LY30 at BL that was absent in low lysis samples (n=6). Thrombin activatable fibrinolysis inhibitor (TAFI) activity correlated with tPA levels for low lysis subjects (r=0.88) with no correlation for high lysis subjects (r=.08).

Conclusions: Central hypovolemia resulted in activation of coagulation by either LBNP or HEM; thus LBNP is a powerful model for studying hemorrhage. This study also revealed a subset of baboons with baseline activation of fibrinolysis which was strongly coupled to tPA and uncoupled from TAFI activity. The methods demonstrated hemodilution with blood removal and hemoconcentration with LBNP, as has been observed in humans, but these modest changes had no significant effect on coagulation parameters. Unexpectedly, this study demonstrated that the effects of blood removal were sustained 28 days later on baseline parameters. Overall, these results in large primates suggest that baseline differences between apparently healthy and superficially homogeneous individuals may determine outcomes in studies of hemostasis.