At high heparin concentrations, protamine concentrations which reverse heparin anticoagulant effects are insufficient to reverse heparin anti-platelet effects

Combined effects of heparin and protamine on plasma clot structure and platelet function were studied. Anticoagulant effects were monitored as changes in aPTT. Clot structure was defined in terms of fibrin fiber mass/length ratio (μ) and clot elastic modulus (EM). Platelet function was studied utilizing platelet aggregation and platelet force development (PFD) measurements. Heparin (1 U/ml) prolonged the aPTT from 30 to >300 seconds, reduced PFD from 5,100 to 0 dynes, decreased μ (in batroxobin-induced gels) from 1.36 to 1.08 × 10¹³ daltons/cm and decreased clot EM from 9,600 to 2000 dynes/cm². Varying amounts of protamine reversed these effects: 16 μg/ml normalized the aPTT, 20 μg/ml normalized PFD, 32 μg/ml corrected μ, and 20 μg/ml returned EM to baseline. At high heparin concentrations (4 U/ml), protamine concentrations which corrected anticoagulant effects were inadequate to reverse antiplatelet effects. A protamine concentration of 40 μg/ml normalized the aPTT and μ, but 140 μg/ml of protamine was required to reverse heparin suppression of force development and clot elastic modulus. Excess protamine inhibited clotting and platelet function. In plasma containing 1 u heparin/ml, 140 μg protamine/ml reduced PFD by 83%, prolonged the aPTT by 63%, and reduced clot EM by 75%. In heparin free plasma, >75 μg protamine/ml prolonged the aPTT. Thus, platelet function and clot structure are sensitive to protamine during heparin neutralization, and anti-platelet effects of heparin may persist when the aPTT is completely corrected. Excess protamine inhibits platelet function and compromises clot structure.