Hypobaric Hypoxia Induces Hypercoagulability Due to an Increase in Microparticles

Studies investigating the role of hypoxia on coagulation, either by going to high altitude or after a long-haul flight, reveal contrasting results. Previous work from our group suggests that the cellular part of the blood is involved in causing a prothrombotic phenotype. Therefore this study investigated the effect of hypobaric hypoxia on blood coagulation, focusing on the role of cellular components.

After approval from the local medical ethics committee, 16 healthy participants (aged 20 to 50 years old) were included in this study. Exclusion criteria were a history of cardiovascular disease or pulmonary disease, impaired mobility, and medication known to interfere with coagulation. Participants ascended by cable car to 3,883 meters above sea level, after acclimatizing to altitude for 6 days. At 50 meters and 3,883 meters above sea level, blood was drawn and vital signs (SpO₂, heart rate) and the Lake Louise acute mountain sickness questionnaire (LLQ) were recorded. The following tests were performed on whole blood: hemoglobin (Hb), hematocrit (Ht), leucocyte count (L), platelet count (PC) and mean platelet volume (MPV); thrombin generation (TG) in whole blood (WB-TG) with 0.5 pM tissue factor (TF); platelet activation test (PAc-T) triggered by adenosine diphosphate (ADP), thrombin receptor activating peptide (TRAP) and collagen related peptide (CRP) in end concentrations of respectively 10 µM, 30 µM and 1 µg ml^-1. The remainder of the blood was centrifuged to obtain platelet rich plasma (PRP) (once for 15 minutes at 230 g) and platelet poor plasma (PPP) (twice for 10 minutes at 2,821 g). PRP was used to test PRP-TG (1 pM TF) and plasma was used for PPP-TG (1 pM TF) and microparticles (MP)-TG (0 pM TF). The paired t-test with p <0.05 was used to determine statistical significance within participants.

At 3,883 meters above sea level, oxygen saturation decreased and heart rate increased significantly. LLQ scores revealed mild acute mountain sickness (AMS) symptoms. One participant was withdrawn from the study at 3,030 meters due to moderate AMS. After applying TG in PPP, PRP and whole blood, we found that peak height and endogenous thrombin potential (ETP) were increased. Interestingly, we found a decrease in platelet activation and a decreased MPV. To find an explanation for the increased TG in the different media, we performed a TG assay specially designed to detect microparticles. As with the PPP-TG, PRP-TG and WB-TG, we found an increase in ETP and peak height, proving increased content of MPs.

In conclusion, we found that exposure to hypobaric hypoxia increased TG in PPP, PRP and in whole blood. In contrast we found that platelet activation was decreased, indicating that platelets do not play a role in hypoxia-induced hypercoagulability. The increase in peak and ETP in the MP-sensitive TG assay indicates that MPs play a major role in hypoxia-induced hypercoagulability.