Chapter 6: Coagulation Monitors

INTRODUCTION

An accurate assessment of a patient’s hemodynamic status is crucial to a safe anesthetic plan. In the presence of ongoing blood loss, one must quickly distinguish between surgical bleeding and coagulation derangements. Initially, this risk is assessed preoperatively with several tests assessing different stages of the clotting cascade. However, given the potential for evolving intraoperative derangements, the clotting ability should also be assessed perioperatively as well. Certain surgical procedures require the induction of a coagulopathic state, such as necessitating the use of extracorporeal blood flow machines.

Coagulopathies result from three etiologies: a failure in primary hemostasis, an incompetent coagulation cascade, and excessive fibrinolysis. Primary hemostasis encompasses platelet plug formation. This process requires functional, circulating platelets and an endothelial defect that exposes platelet-binding receptors. The coagulation cascade reinforces this platelet plug and simultaneously begins the process of deactivating itself. This cascade consists of two pathways, the intrinsic and the extrinsic, that overlap in a common pathway. Fibrinolysis is the process through which the clot breaks down after serving its function in hemostasis.

TESTS OF PRIMARY HEMOSTASIS

The essential elements of primary hemostasis include the concentration and quality of platelets as well as important components that lead to platelet plug formation. Some have a more historical context and are not easily or commonly employed in perioperative use, while others are a part of a standard preoperative workup. Platelet function tests are usually employed when there is evidence of coagulopathies.

Complete Blood Count (CBC)

Measurement of the CBC provides a platelet count but does not assay the functional status of each platelet. Perioperatively, both quantitative and qualitative deficits in platelets contribute to coagulopathies. These deficits are especially prevalent in surgeries involving significant fluid shifts and requiring extracorporeal blood flow such as in extracorporeal membrane oxygenation or cardiopulmonary bypass. Distinguishing between these etiologies aids in identifying the appropriate intervention.

Qualitative deficits have a variety of causes: decreased production, splenic sequestration, increased destruction, or dilution. When splenic sequestration is the underlying cause, platelet levels will often increase in the presence of stressors. In disseminated intravascular coagulation, the platelets will be rapidly consumed, and provide minimal benefit. The risk-to-benefit ratio of administering these products must be weighed. Repeated platelet transfusions run the risk of sensitizing a patient to platelet fragments, impeding future transfusions. As such, platelet transfusions should be reserved for patients with less than 10,000/uL platelets, ongoing blood loss, or invasive procedures.

Bleeding Time

Bleeding time is an older test that can be performed in the absence of a laboratory. There are two variants to this method:

• The Ivy bleeding time is determined by placing a blood pressure cuff on the upper arm and inflated to 40 mmHg. A cut is made on the volar surface of the arm. The cut is blotted ...