Читать книгу Principles of Dentoalveolar Extractions - Seth Delpachitra - Страница 13

Any tissue trauma will result in bleeding from the local vasculature supplying the tissues. The immediate physiologic reaction is haemostasis, which involves reactive vasospasm, formation of a platelet plug, and activation of the coagulation cascade.

Reactive vasospasm occurs in the seconds to minutes following damage to the blood vessels. This is mediated through neurologic mechanisms, as well as the local release of endothelin. It rapidly reduces blood loss from trauma. In surgery, exogenous vasoactive medications such as adrenaline utilise this response to improve visual access to the surgical field by reducing blood flow.

Damaged endothelial cells result in a conformational change in von Willebrand factor expressed on the cell surface. Von Willebrand factor interacts with glycoprotein Ib on circulating platelets, resulting in activation and aggregation of the platelets, forming links to fibrinogen via the GpIIb/IIIa receptor. This leads to the formation of the platelet plug. Antiplatelet medications inhibit aspects of platelet plug formation and increase the risk of bleeding during surgical procedures.

The coagulation cascade is a series of successive reactions that occur in order to activate thrombin and form a stable fibrin clot (Figure 1.2). There are two pathways in this cascade: intrinsic and extrinsic. The intrinsic pathway is activated within the vascular system through exposure to endothelial collagen, whilst the extrinsic pathway is activated by tissue trauma and release of intracellular tissue factor. Anticoagulant medications and coagulopathies increase the tendency to bleed by inhibiting aspects of the coagulation cascade, and awareness of these effects may be clinically relevant in surgical planning.

Figure 1.2 The coagulation cascade.

Coagulation studies used clinically can assess the function of either the intrinsic, the extrinsic, or the shared common pathway. Prothrombin time screens for factors II, V, VII, and X and fibrinogen; these are all part of the extrinsic pathway, which is used to guide treatment for patients treated with warfarin. Warfarin inhibits vitamin K‐dependent factors common to both pathways, but because factor VII has the shortest half‐life, the extrinsic pathway is used to determine coagulability. The partial thromboplastin time will screen for factors in the intrinsic pathway affected by medications such as heparin and low‐molecular‐weight heparin.