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CARDIOVASCULAR ANESTHESIOLOGY

Blood Coagulation: Hemostasis and Thrombin Regulation

Kenichi A. Tanaka, MD, MSc^*, Nigel S. Key, MD, and Jerrold H. Levy, MD^*

From the *Division of Cardiothoracic Anesthesia and Critical Care, Department of Anesthesiology, Emory University School of Medicine, Atlanta, Georgia; and Division of Hematology, Department of Internal Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

Address correspondence and reprint requests to Kenichi A. Tanaka, MD, MSc, Division of Cardiothoracic Anesthesia, Department of Anesthesiology, 1364 Clifton Road NE, Atlanta, GA 30322. Address E-mail to ktanaka{at}emory.edu.

Abstract

Perioperative bleeding is a major challenge particularly because of increasing clinical use of potent antithrombotic drugs. Understanding current concepts of coagulation is important in determining the preoperative bleeding risk of patients, and in managing hemostatic therapy perioperatively. The serine protease thrombin plays pivotal roles in the activation of additional serine protease zymogens (inactive enzymatic precursors), cofactors, and cell-surface receptors. Thrombin generation is closely regulated to locally achieve rapid hemostasis after injury without causing uncontrolled systemic thrombosis. During surgery, there are major disturbances in coagulation and inflammatory systems because of hemorrhage/hemodilution, blood transfusion, and surgical stresses. Postoperative bleeding often requires allogeneic blood transfusions, which support thrombin generation and hemostasis. However, procoagulant activity and inflammation are increased postoperatively; thus, antithrombotic therapy may be required to prevent perioperative thrombotic complications. There have been significant advances in the management of perioperative hemostasis and thrombosis because of the introduction of novel hemostatic and antithrombotic drugs. However, a limitation of current treatment is that conventional clotting tests do not reflect the entire physiological processes of coagulation making optimal pharmacologic therapy difficult. Understanding the in vivo regulatory mechanisms and pharmacologic modulation of thrombin generation may help control bleeding without potentially increasing prothrombotic risks. In this review, we focus on the regulatory mechanisms of hemostasis and thrombin generation using multiple, simplified models of coagulation.