[Biological rhythm and thromboembolic disease. Physiological, epidemiological and pharmacological aspects].

Arterial and venous thromboembolic disorders are the leading cause of death in most of the advanced nations. The study of physiologic, epidemiologic and pharmacologic relationships of these disorders to biological rhythms, may lead to a better understanding and perhaps a better treatment. Chronophysiologic studies have shown that hemostatic variables follow circadian rhythms. The level of platelets aggregation and of blood coagulation has been found to be increased in the morning and decreased at night, whereas fibrinolytic activity is lower in the morning than in the evening. Chronoepidemiologic studies demonstrated a morning peak for arterial thromboembolic disorders and an evening peak for cerebral bleedings. These facts might partially be explained by circadian variations in hemostasis and suggest a chronotherapeutic approach in thromboembolic disorders. Unfractionated heparin, because of its antithrombotic effect, is one of the major drugs used to treat this disease. However, with such a treatment venous thrombosis recurs or bleeding complications occur yet in about 30% of patients. Chronopharmacologic studies indicate that anticoagulant effect of heparin is minimum in the morning and maximum at night, following the physiologic circadian variation of blood coagulation. Such results suggest that the heparin doses should be modulated as a function of administration times in order to increase its effectiveness and to minimize both bleeding risk and thrombosis. Further studies are needed to evaluate such a proposal.