Major circadian fluctuations in fibrinolytic factors and possible relevance to time of onset of myocardial infarction, sudden cardiac death and stroke

Natural inhibitors of endogenous fibrinolysis may displace the hemostatic equilibrium toward thrombosis and favor events such as acute myocardial infarction, sudden cardiac death and stroke, where a thrombotic process is known to occur.1,2 The clinical incidence of these syndromes shows a circadian distribution with highest frequency in the morning.3,5 These observations prompted us to investigate possible circadian changes of blood fibrinolytic activity in normal subjects. Two major components of the fibrinolytic system, tissue-type plasminogen activator (t-PA) and its fast-acting inhibitor (PAI), were measured with specific assays. This study reports markedly reduced fibrinolytic activity during the early morning hours related to increased plasminogen activator inhibition.