Background Residual mural thrombus on severely damaged arterial wall is very thrombogenic.We tested the hypothesis that direct thrombin inhibition will block thrombus growth on fresh thrombus better than indirect thrombin inhibition, cyclooxygenase inhibition, or both.Methods and Results A fresh mural thrombus was formed by directly perfusing fresh porcine blood for 5 minutes over severely damaged arterial wall at a high shear rate in a well-characterized ex vivo perfusion system. The average platelet (P) and fibrinogen (F) deposition (D) achieved in 5 minutes were 382+-32 x 106platelets/cm2and 296+-36 x 1012fibrinogen molecules/cm2, respectively. Thrombus growth on the fresh mural thrombus was quantitated by directly perfusing blood from pigs with Indium-111-labeled platelets and Iodine-125-labeled fibrinogen for an additional 5 minutes over the preformed mural thrombus. Treatment included recombinant hirudin (1 mg/kg per hour IV) as a probe for thrombin, aspirin (5 mg/kg IV) as a platelet inhibitor of cyclooxygenase, heparin (moderate, 100 IU/kg per hour IV; high-dose, 250 IU/kg per hour IV) as an indirect thrombin inhibitor, and heparin (100 IU/kg per hour) plus aspirin (5 mg/kg IV). Thrombus growth as measured by labeled PD (x 106/cm2) and FD (x 1012molecules/cm2) was mildly but not significantly reduced by aspirin (1034+-92 and 436+-78, respectively) compared with baseline (1113+-67 and 545+-52, respectively). Inhibition of thrombus growth with heparin was dose dependent. A regression analysis showed an inverse correlation of PD and FD with mean plasma heparin concentrations (r=-.81, P=.0001 and r=-.49, P=.0007, respectively). Recombinant hirudin led to a profound inhibition of thrombus growth (PD, 30+-12; FD, 109+-21), which was significant compared with all groups, even the highest dosage of heparin (250 IU/kg per hour).Conclusions Specific thrombin inhibition markedly inhibits platelet and fibrinogen deposition onto fresh mural thrombus at a high shear rate.Aspirin alone or in combination with heparin has little effect on evolving thrombosis. Heparin dose dependently reduces thrombus growth, but even the highest dosage is less effective than hirudin. Thrombin appears to be the primary activator of platelets by fresh thrombus. (Circulation. 1994;90:2432-2438.)