Antithrombin III in patients with prosthetic cardiac valves.

When compared to 40 control subjects and to 13 patients with mitral stenosis (8 of them commissurotomized) antithrombin III (AT III) activity, assessed by a clotting technique, was found to be slightly but significantly depressed in the 18 patients with prosthetic valves in mitral position. Decrease of AT III level occurred mainly in patients with metallic Björk-Shiley prostheses in whom mitral valve replacement had been performed more than 12 months before the investigation. Particularly low AT III activities were noted in the three patients who developed thromboembolic complications with lethal evolution. Impaired AT III activity in a patient with prosthetic cardiac valves might be considered as a marker of ongoing in vivo activation of coagulation. Such a finding should be a warning and an indication for intensifying the antithrombotic therapy.