Protamine sulfate: crouching clot or hidden hemorrhage?

Protamine sulfate is the agent most frequently used to reverse heparin-mediated anticoagulation, serving as a therapeutic “crouching clot” in vascular and cardiothoracic surgical settings for decades. However, protamine has also been posited to be a source of coagulopathy, or “hidden hemorrhage” for more than 20 years. Protamine has been demonstrated in vitro to decrease thrombin activity, decrease activation of factor VII by tissue factor, enhance tissue-type plasminogen activator– mediated fibrinolysis, decrease factor V activation by activated factor X or thrombin, and attenuate platelet function by inhibition of glycoprotein Ib–von Willebrand factor activity. Concordantly, in vivo protamine-mediated anticoagulation without identification of a specific mechanism(s) has been demonstrated in canine and murine models. Although these investigations support the concept of a protamine-mediated coagulopathy, and editorials urge careful administration of protamine with concurrent monitoring of heparin presence, a definitive study implicating protamine as a source of post–cardiopulmonary bypass bleeding has yet to be performed. In this issue of Anesthesia & Analgesia, Bolliger et al. present us with a multimodal approach to further define the relative hierarchy of the hematological mechanisms that underlie protamine-mediated attenuation of hemostasis. This well-established group utilized standard methods (e.g., prothrombin time) combined with thrombinoscopy (a highly sensitive measure of thrombin generation) and thromboelastometry to test blood obtained from normal subjects and various other factor-deficient plasmas exposed to a range of clinically encountered protamine concentrations. First, the protamine-mediated decrease in thrombin generation was diminished by increasing platelet count from 50,000 to 200,000 platelets/ L. Furthermore, experiments in platelet poor plasma supported the concept that protamine inhibited the intrinsic pathway, specifically factor V activation as previously reported, based on the ability of exogenous factor VIII concentrate to restore thrombin generation in protamine-exposed samples of normal and factor V–deficient plasma. The authors did not find a similar response when recombinant, activated factor VII was added. Thus, it appeared that protamine inhibited the intrinsic (or contact protein) coagulation pathway of coagulation to a greater extent than the extrinsic (or tissue factor-factor VII–dependent) coagulation pathway. Although these conclusions seemed quite reasonable, there existed a body of work that demonstrated that inhibition of tissue factor–mediated activation of factor VII by protamine was far more important than inhibition of the intrinsic pathway in platelet-poor plasma. In this investigation, one of the present authors (VGN) demonstrated a nearly complete cessation of tissue factor–initiated coagulation by protamine, whereas the same concentration only delayed the onset of nearly normal coagulation in intrinsic pathway samples as determined by thrombelastography. Furthermore, increasing tissue factor concentration completely restored both clot growth and resistance to fibrinolysis parameter values in protamine-exposed samples compared with unexposed plasma. During review, Bolliger et al. were of the opinion that small concentrations of tissue factor were used by Nielsen, perhaps in the range of 0.6 pM. Such small concentrations of tissue factor were thought to be easily inhibited by protamine, whereas the large concentrations of tissue factor (2–5 pM) used by Bolliger et al. would not be inhibited by protamine, thus the observations by Bolliger et al. would primarily be secondary to inhibition of factor V. However, it should be noted that the thrombelastographic response in plasma has already been described with up to 45 pM tissue factor. The growth kinetics of these tissue factor–activated plasma clots were actually slower than those published in the work concerning protamine-mediated inhibition of coagulation, supporting the notion that protamine inhibited clot formation with a concentration 45 pM, much larger than 0.6 pM. Furthermore, thrombelastographic experiments defining the coagulation kinetic profiles of factor-deficient plasma demonstrated decreased coagulation in factor V–deficient plasma after activation with similar tissue factor concentrations, a sharp contrast to the essential cessation of thrombin generation in factor V–deficient plasma in response to 5 pM tissue factor noted by Bolliger et al. Taken as a whole, it seems that whereas Bolliger et al. have demonstrated inhibition primarily of the intrinsic pathway, Nielsen’s data support the concept that the extrinsic pathway is more inhibited by protamine. Whether the differences between these 2 bodies of work are From the Department of Anesthesiology, Drexel University College of Medicine, Philadelphia, Pennsylvania.