Antithrombin affects hemostatic response to recombinant activated factor VII in factor VIII deficient plasma

Thromboembolic complications can occur with recombinant activated factor VII (rFVIIa) treatment in trauma and surgical patients but they are infrequent in hemophiliacs. Bleeding diathesis in these conditions is often attributed to reduced thrombin generation, which may be improved with rFVIIa. Normally, thrombin that diffuses from local vascular injury sites is quickly inactivated by antithrombin (AT). Evaluating the influence of AT levels on thrombin generation in hypocoagulable FVIII-deficient plasma would be a simple approach to better understand how procoagulant stimuli, such as rFVIIa, might result in postoperative thrombotic complications. We hypothesize that reduced AT concentrations would increase the procoagulant effects of rFVIIa in FVIII-deficient plasma.

Thrombin generation was evaluated in vitro in FVIII-deficient and AT/FVIII-deficient plasma using thrombelastography and a thrombin generation assay (Thrombinoscope). The effect of added rFVIIa on these variables was evaluated.

Delayed thrombus formation based on thrombelastography in FVIII-deficient plasma was predictably reversed by rFVIIa. Improved thrombus formation and responses to rFVIIa were observed when AT levels were 20%-50% of normal. Thrombin generation in FVIII-deficient plasma increased in an inverse relationship to AT levels. Supplemental rFVIIa decreased the lag time of thrombin generation but not the amount of thrombin generated.

Using FVIII-deficient plasma as a model of reduced thrombin generation, we demonstrate that low AT levels enhance in vitro hemostatic responses to rFVIIa. Reduced AT levels in trauma and surgical patients with normal or increased FVIII levels may be considered potentially prothrombotic. Monitoring of AT levels during rFVIIa therapy may thus reduce thrombotic complications in nonhemophiliacs.