Haematological changes at altitude.

Hypobaric hypoxia is associated with an increase in erythropoesis and an increased thrombotic risk. This is true of long haul air travel, mountaineering expeditions and longer stays at altitude. Studies looking at clotting on mountaineering expeditions are further complicated by the effects of exercise, plasma volume changes and the catecholamine response to hypoxia. This review examines the evidence for changes in clotting factors and functional clotting at altitude and considers the implications of altitude travel for those with pre-existing medical conditions.