Coagulation and Hemostasis

Coagulation and Hemostasis in Dogs and Cats #

Physiology of Hemostasis #

Hemostasis is a critical physiological process that prevents excessive blood loss following vascular injury. It consists of three primary phases:

1. Primary Hemostasis – This phase involves the initial response to vessel injury, where platelets adhere to the exposed subendothelial matrix and aggregate to form a temporary platelet plug. Key components include von Willebrand factor (vWF), platelet activation, and shape change.
2. Secondary Hemostasis – This phase is characterized by the activation of the coagulation cascade, leading to the formation of a stable fibrin clot. It involves intrinsic, extrinsic, and common pathways, with key coagulation factors such as Factor VIII, IX, X, and fibrinogen.
3. Fibrinolysis – The final phase involves clot breakdown to restore normal blood flow. Plasminogen is converted to plasmin, which degrades fibrin into fibrin degradation products (FDPs), including D-dimers.

Coagulation Cascade in Dogs and Cats #

The coagulation cascade consists of:

• Intrinsic Pathway: Initiated by contact with negatively charged surfaces (e.g., Factor XII, XI, IX, and VIII)
• Extrinsic Pathway: Initiated by tissue factor (TF) and Factor VII
• Common Pathway: Involves Factor X, V, II (prothrombin), I (fibrinogen), and XIII

Normal Coagulation Values in Dogs and Cats #

Coagulation parameters are used to assess hemostasis in veterinary patients. Normal values vary slightly between dogs and cats:

Test	Dog (Normal Range)	Cat (Normal Range)
Platelet Count (10^3/μL)	200-500	300-800
Buccal Mucosal Bleeding Time (BMBT) (min)	<4 min	<3 min
Activated Clotting Time (ACT) (sec)	60-120 sec	60-100 sec
Prothrombin Time (PT) (sec)	7-10 sec	8-12 sec
Activated Partial Thromboplastin Time (aPTT) (sec)	12-18 sec	14-20 sec
Fibrinogen (mg/dL)	150-400	100-300
D-dimers (μg/mL)	<0.5	<0.5

Abnormal Coagulation Ranges #

Abnormal coagulation values may indicate various disorders:

• Thrombocytopenia (Platelet count <150,000/μL): May be caused by immune-mediated thrombocytopenia (IMTP), bone marrow disease, or disseminated intravascular coagulation (DIC).
• Prolonged BMBT (>4 min in dogs, >3 min in cats): Suggests platelet dysfunction (e.g., von Willebrand disease, thrombocytopathies).
• Prolonged PT (>12 sec in dogs, >14 sec in cats): May indicate vitamin K deficiency, anticoagulant rodenticide toxicity, or liver dysfunction.
• Prolonged aPTT (>18 sec in dogs, >20 sec in cats): Suggests intrinsic pathway defects (e.g., hemophilia A or B, severe liver disease).
• Elevated D-dimers (>0.5 μg/mL): Indicates fibrinolysis and may be associated with DIC, thromboembolic disease, or sepsis.

Coagulation Disorders in Dogs and Cats #

Several inherited and acquired conditions can affect coagulation:

Inherited Coagulopathies #

• Von Willebrand Disease (vWD): Affects platelet adhesion due to deficient or defective von Willebrand factor. More common in certain dog breeds (Doberman Pinschers, Shetland Sheepdogs).
• Hemophilia A (Factor VIII deficiency): X-linked disorder leading to prolonged aPTT.
• Hemophilia B (Factor IX deficiency): Similar to hemophilia A but rarer.

Acquired Coagulopathies #

• Disseminated Intravascular Coagulation (DIC): A systemic, life-threatening condition characterized by excessive coagulation and fibrinolysis.
• Rodenticide Toxicity: Involves vitamin K antagonists leading to prolonged PT and aPTT.
• Liver Disease: The liver produces most clotting factors; hepatic insufficiency can prolong PT and aPTT.
• Immune-Mediated Thrombocytopenia (IMTP): An autoimmune disorder that destroys platelets, leading to prolonged bleeding.

Monitoring Techniques for Coagulation #

Several diagnostic tests are used to evaluate coagulation in veterinary medicine:

Laboratory Tests #

• Platelet Count: Performed using automated hematology analyzers or manual smear evaluation.
• Buccal Mucosal Bleeding Time (BMBT): Assesses primary hemostasis by making a standardized incision and measuring the time to clot formation.
• Activated Clotting Time (ACT): Measures the time required for whole blood to clot after exposure to a surface activator.
• Prothrombin Time (PT): Evaluates the extrinsic and common pathways.
• Activated Partial Thromboplastin Time (aPTT): Assesses the intrinsic and common pathways.
• Fibrinogen Levels: Measured using heat precipitation or quantitative assays.
• D-dimers and FDPs: Markers of fibrinolysis and useful in diagnosing DIC or thromboembolic disease.

Point-of-Care (POC) Testing #

• Coagulation analyzers such as thromboelastography (TEG) and rotational thromboelastometry (ROTEM) provide real-time assessment of clot formation, stability, and breakdown.
• Viscoelastic Coagulation Testing: Offers dynamic information about clot strength and fibrinolysis.

Treatment of Coagulation Disorders #

Management of coagulation disorders depends on the underlying cause:

• Vitamin K1 Therapy: Used in anticoagulant rodenticide toxicity.
• Fresh Frozen Plasma (FFP): Contains coagulation factors and is used for DIC, hemophilia, and liver disease.
• Cryoprecipitate: Enriched in Factor VIII and von Willebrand factor; used for hemophilia A and vWD.
• Platelet-Rich Plasma (PRP) or Whole Blood Transfusion: Indicated in cases of severe thrombocytopenia or hemorrhage.
• Antifibrinolytic Agents: Such as aminocaproic acid, used to control excessive bleeding.
• Heparin or Low-Molecular-Weight Heparin (LMWH): Employed in thromboembolic conditions.

Conclusion #

Hemostasis and coagulation in dogs and cats are complex processes that require precise regulation. A variety of diagnostic tests are available to monitor coagulation status, and understanding normal and abnormal values is essential for diagnosing and treating coagulopathies effectively.

References #

1. Brooks, M. B., Catalfamo, J. L., & Raymond, S. L. (2014). “Advances in veterinary coagulation testing.” Veterinary Clinics of North America: Small Animal Practice, 44(6), 1261-1278.
2. de Laforcade, A. M., Freeman, K. P., Shaw, S. P., & Rozanski, E. A. (2016). “Coagulation disorders in small animals.” Journal of Veterinary Emergency and Critical Care, 26(1), 19-31.
3. Mischke, R., & Pohling, J. (2017). “Hemostatic disorders in dogs and cats.” Tierarztliche Praxis Ausgabe K, 45(5), 305-317.
4. Scherk, M. A., & Laflamme, D. P. (2015). “Feline coagulation and hemostatic disorders.” Journal of Feline Medicine and Surgery, 17(1), 10-26.
5. Weiss, D. J., & Wardrop, K. J. (2010). “Schalm’s Veterinary Hematology.” Wiley-Blackwell.