A systematic review of the prothrombotic effects of an acute change in posture: a possible mechanism underlying the morning excess in cardiovascular events?

BACKGROUND: Epidemiologic studies have demonstrated a morning excess of acute cardiovascular events, which has subsequently been refined to the 2 to 3 h after awakening. It is therefore reasonable to assume that some activity associated with awakening may trigger such cardiovascular events, with the assumption of an upright posture being of potential significance. OBJECTIVE: To identify and systematically review the literature to determine the effects of acute changes in posture on hemorheology and hemostasis. DATA SOURCES: Electronic databases MEDLINE, EMBASE, PSYCHINFO, and CINAHL (earliest date available to February 5, 2006) and conference abstracts were searched. STUDY SELECTION: Studies examining the hemorheologic and hemostatic response to a change in posture from a supine position to an upright position were eligible for inclusion. Two reviewers independently selected the studies for inclusion. RESULTS: The experimental evidence reviewed demonstrates that the assumption of an upright posture is associated with a significant degree of hemoconcentration, which is characterized by an increase in hematocrit and a decrease in plasma volume. Research examining the effect of postural change on hemostasis has been limited, although a small number of studies have demonstrated some significant changes in markers of coagulation, fibrinolysis, and platelet reactivity. CONCLUSIONS: A sudden change in posture may act as a potential trigger for the onset of cardiovascular events occurring in the 3 h after awakening via effects on hemostasis and hemorheology. However, other possible triggering factors such as dehydration, mental stress, and physical exertion must also be considered.