Extensive thrombus formation with heparin resistance during extracorporeal circulation. A new presentation of familial antithrombin III deficiency.

Extensive thrombus formation during extracorporeal circulation despite the administration of heparin sodium prompted investigation of a 15-year-old boy with a calcified right ventricular thrombus and a history of subacute bacterial endocarditis. In vitro studies confirmed the failure of heparin in standard doses to have an anticoagulant effect. Antithrombin III concentrations were low. The patient's mother, who had no history of thromboembolic disease, was also antithrombin III deficient. Resistance to heparin is a theoretical, but inconsistently documented, feature of antithrombin III deficiency. This deficiency state should be considered whenever heparin resistance is encountered, even in the absence of a personal and family history of thromboses.