Summary
The objective of the study was to investigate possible diurnal rhythms in coagulation
tests during a continuous intravenous infusion of unfractionated heparin. Six volunteers
participated in the study, which was divided in a treatment (500 U heparin/h for 30
h) and a control experiment. Under basal conditions, no rhythm was found in coagulation
tests. During heparin treatment, APTT, thrombin clotting time and anti-Xa activity
showed a greater anticoagulant effect at night, with a striking decrease in the morning.
In a search for the explanation of this phenomenon we looked for diurnal variations
in the urinary excretion of heparin, in the plasma concentrations of antithrombin
III and platelet factor 4, and in the effect of heparin added to the plasma samples
in vitro. None of these studies provided the explanation.