Effect of prolonged stay at high altitude on platelet aggregation and fibrinogen levels.

Increased liability for thrombosis has been suspected at high altitude. Platelet function and fibrinogen levels are known to play a major role in thrombogenic disorders. In order to investigate effect of chronic hypobaric hypoxia on platelet function and fibrinogen concentration, a study was conducted on 40 healthy men at sea level and following 3 and 13 months sojourn at high altitude (4100 m-4500 m). A consistent decline in platelet number was observed in high altitude sojourners during 13 months stay at high altitude. Platelet counts decreased by 12% after 3 months of stay at high altitude and by 31% after 13 months stay in comparison to sea level counts (266.01 +/- 65.10 x 10(3)/microl). Mean platelet volume increased from 9.18 +/- 0.83 fl at sea level to 10.8 +/- 0.98 fl after 3 months stay at high altitude and showed a further increase to 12.15 +/- 1.18 fl following 13 months residency in the hypoxic environment. There was significant reduction in maximal platelet aggregation in response to ADP, epinephrine and collagen, the effect being most pronounced with collagen (75%) and least with ADP (24%). Plasma fibrinogen concentration was 53% higher in sojourners after 3 months stay at high altitude and 61% higher after 13 months stay at high altitude. This implies that increased platelet activity may not be responsible for precipitation of thrombotic phenomenon during prolonged stay at high altitude but increased availability of substrate for coagulation could still favor pro-coagulant tendencies.