The clot thickens: Understanding coagulation in our patients
The haemostatic system can be broken down into 3 main components, primary haemostasis, secondary haemostasis and fibrinolysis.
When the endothelium of a vessel becomes injured, the vessels initially contract to reduce blood flow to the affected Damage to the endothelium activates primary haemostasis which involves the interaction between platelets and the subendothelial matrix to form a temporary platelet plug. In order for this plug to form, the patient needs enough platelets and the ability for these platelets to stick together. Fibrinogen and von Willebrand’s factor are two of the substances required to make this happen. Meaning a patient with von Willebrand’s disease has enough platelets, but they cannot clump together to stop haemorrhage. The platelet plug is unstable and very short-lived but provides a framework for a blood clot to form around allows secondary haemostasis to occur.
Secondary haemostasis involves several enzymatic reactions which we commonly refer to as the intrinsic, extrinsic and common pathways, eventually forming fibrin which stabilises the clot. The intrinsic coagulation pathway is activated by blood being exposed to the damaged endothelium. This causes the activation of factor XII, which in turn activates factor XI, which causes the activation of factors IX and VIII. The extrinsic pathway is kicked off by haemorrhage outside the vessel as it is activated by exposure of blood to tissue factor (a protein present on the outside of extravascular tissues). This causes the activation of factor VII. The intrinsic and extrinsic pathways merge to form the common coagulation pathway. This pathway involves the activation of factors X, V, prothrombin, and XIII, which leads to the conversion of fibrinogen to the fibrin blood clot.
Fibrinolysis is the final process in haemostasis and is how the clots are broken down in the body when they are no longer required in order to restore proper blood flow. Plasminogen is converted to plasmin, which is the enzyme responsible for the breakdown of the fibrin clot.
There are many haemostatic diseases that can be seen in practice including thrombocytopenia, platelet function disorders (aka thrombocytopathia), inherited coagulation factor deficiencies and acquired coagulopathies.
Thrombocytopenia is a decrease in circulating platelet levels and can occur due to four mechanisms. These are either a decrease in platelet production from the bone marrow, increased platelet loss/consumption (haemorrhage/DIC/sepsis), increased platelet destruction (IMTP) or platelet sequestration. Patients with thrombocytopenia typically present with evidence of bleeding, and signs associated with haemorrhage. These include petechiation, ecchymoses, scleral haemorrhage, melena, epistaxis, haematuria, lethargy, anorexia, weakness and collapse. Diagnostic tests that should be performed on thrombocytopaenic patients include a complete blood count, blood smear examination and manual platelet estimate, infectious disease testing (e.g. 4Dx), +/- chest X-rays, +/- abdominal ultrasound (to identify an underlying cause where immune mediated thrombocytopaenia is suspected). Treatment of thrombocytopaenic patients includes suppressing the immune system (in cases of IMTP), vincristine to stimulate platelet release, blood products if severe haemorrhage has occurred, platelet transfusion, intravenous fluid therapy, antiemetics, and gastroprotectant agents as necessary, based on the individual patient and their clinical signs.
Thrombocytopathia is platelet dysfunction, these patients have enough platelets, but they are not functioning properly. There are several causes including inherited defects (von Willebrand's), the administration of certain medications (NSAIDs, aspirin, synthetic colloids, β-lactam antibiotics) and hepatic disease. The clinical signs are the same as thrombocytopaenia. Diagnostic tests commonly performed include platelet count, which should be normal or demonstrate mild-to-moderate reductions where haemorrhage has recently occurred, coagulation times activated partial thromboplastin time (APTT) and prothrombin time (PT), which should be normal and buccal mucosal bleeding time which should be prolonged.
Coagulopathies are disorders of clotting or secondary haemostasis. They can be seen due to congenital clotting factor deficiencies, or acquired, due to toxin ingestion or hepatic dysfunction. The most common inherited coagulopathies include haemophilia A, haemophilia B and factor XII deficiency. Acquired coagulopathies are normally to do with the vitamin K-dependent clotting factors. Deficiencies in these can be seen due to toxicity (usually anticoagulant rodenticides), or hepatic disease. Clinical signs include cavitary haemorrhage, haematoma, GIT, muscle and joint haemorrhage. The most common diagnostics performed in the coagulopathic patient are clotting times (APTT and PT) and specific coagulation factors where a deficiency is suspected. These are usually submitted to an external laboratory. Coagulopathic patients can present very unstable due to haemorrhage, with severe signs of hypovolaemia. Treatment may require fluid resuscitation to maintain perfusion until a blood product is sourced/administered, respiratory support/oxygen therapy, plasma and/or packed red blood cell transfusions.
In terms of nursing care, all of our haemostasis disorder patients must be handled incredibly carefully due to their risk of haemorrhage. In addition to general nursing care, specific areas to consider include no jugular venepuncture (peripheral veins only, with pressure bandages), no neck leads (harness only), no IM/SC injections, no invasive procedures (risk of bleeding), no dry food, gentle handling, close monitoring of vital signs for signs of transfusion dependency.
- In this presentation I aim to describe the normal physiology of haemostasis, including primary and secondary coagulation pathways. I will be discussing the common bleeding and clotting disorders that we may see in our veterinary patients and how to recognise the common signs of bleeding such as petechia, ecchymosis and internal haemorrhage. I'll cover the diagnostics tests that are routinely performed in our haemostasis patients and how these are interpreted. Finally, the very important nursing considerations when caring for a patient with a bleeding disorder will be covered.