The pathophysiology of DIC: When the hemostatic system malfunctions

To diagnose disseminated intravascular coagulation and achieve the best outcome, you must first understand how the hemostatic system should work and what causes it to go awry in patients with this serious bleeding disorder.

Disseminated intravascular coagulation (DIC), also known as consumptive coagulopathy or defibrination syndrome, is an acquired disorder of the hemostatic system that results in the pathologic activation and disequilibria of normal hemostasis and fibrinolysis, leading to potentially fatal consequences. This syndrome is common in critically ill veterinary patients and is always secondary to an underlying disorder that increases systemic thrombin and plasmin activities.1-9

The potential clinical consequences of DIC are systemic microvascular thrombosis with secondary organ damage or failure, hemorrhagic tendencies, shock, and death.1-5,9-11 These consequences make establishing a rapid diagnosis a must, so appropriate therapy can be initiated quickly to optimize a patient's response.

The principal goal of DIC therapy is to remove the primary disorder serving as the procoagulant stimulus. Until the primary disorder is eliminated, DIC will persist. Eliminating the primary disorder is rarely accomplished quickly, so the treatment regimen is initially supportive. To establish a correct diagnosis and rational therapeutic decisions regarding DIC, clinicians must be familiar with the normal hemostatic system and the pathophysiology of DIC.

NORMAL HEMOSTASIS

The hemostatic system controls endothelial damage by generating a platelet plug and fibrin clots (coagulation) to minimize blood loss. The system also uses fibrinolysis to slowly dissolve clots to maintain organ perfusion. Physiologic inhibitors in the plasma maintain balance between coagulation and fibrinolysis by localizing coagulation activity and preventing systemic coagulation. Any hemostatic system disorder can lead to hypocoagulation (hemorrhage), hypercoagulation (thrombosis), or both (DIC).1,6,12

The hemostatic system is traditionally divided into three categories based on the types of disorders encountered in clinical medicine and on in vitro laboratory testing: primary hemostasis, secondary hemostasis, and fibrinolysis.

Primary hemostasis

Platelet adherence and activation initiate hemostasis. Von Willebrand's factor and membrane glycoproteins mediate platelet adherence to damaged endothelium or collagen.6,12-14 Once adhered, the platelets become activated to aggregate, secrete, and contract. Activated platelets secrete serotonin and thromboxane A2, which initiate smooth muscle contraction of the vessel wall.6,12-14 This contraction, plus platelet contraction through cytoplasmic microtubules, forms the platelet plug. However, for hemostasis to continue, secondary hemostasis must follow.13

Secondary hemostasis

Secondary hemostasis, or the activation of coagulation factors, begins when blood is exposed to negatively charged surfaces of damaged endothelium (intrinsic coagulation system) or extravascular tissues (extrinsic coagulation system).6,13 The common endpoint of intrinsic and extrinsic coagulation is the conversion of prothrombin to thrombin—the critical step in fibrin clot formation.


Figure 1. A Simplification of Hemostasis
Intrinsic coagulation
When blood is exposed to damaged endothelium, the enzyme kallikrein activates factor XII. Kallikrein is produced from a complex formed by prekallikrein, high-molecular-weight kininogen, and activated factor XII. Once activated, factor XII activates factor XI. Activated factor XI then activates factor IX, and the process continues, involving a total of 10 serum proteins, calcium, and phospholipids (Figure 1).13

When blood is exposed to damaged endothelium, the enzyme kallikrein activates factor XII. Kallikrein is produced from a complex formed by prekallikrein, high-molecular-weight kininogen, and activated factor XII. Once activated, factor XII activates factor XI. Activated factor XI then activates factor IX, and the process continues, involving a total of 10 serum proteins, calcium, and phospholipids (Figure 1).13

Extrinsic coagulation
When blood is exposed to extravascular tissue, factor VII contacts calcium and a cell membrane factor (factor III, tissue thromboplastin, or tissue factor) and induces coagulation.13 Tissue factor is abundantly present on extravascular tissues and certain neoplastic cells. Endothelial cells, monocytes, and macrophages can express tissue factor in response to cytokines, especially interleukin-1 and tumor necrosis factor alpha (TNF-alpha), and to endotoxin.5,9,15-17 Activation of factor X leads to the common pathway and, ultimately, fibrin clot formation (Figure 1).