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Hematology and Coagulopathy

Platelets and Coagulation Components

  • Platelet
  • Thrombin
  • Antithrombin
  • Heparin Active Sequence
    • 2b
    • 2a
    • 2c
  • End Point
  • Covalent Bond
  • Attached Heparin Molecule
  • Prime Coat
  • Device Surface
  • Neutral AT-Thrombin Complex

Objectives

  • Describe the hemostatic system.
  • Discuss how artificial surfaces disrupt balance.
  • Describe how artificial surfaces can cause bleeding.
  • Discuss methods to blunt hemostatic and inflammatory response.
  • References: Gravlee, Ghosh, Hensley, Internet

Introduction

  • This unit describes how artificial surfaces disrupt the balance associated with bleeding and hemostasis.
  • Relation to inflammatory response is significant in understanding these disruptions.

Hemostasis in Different Systems

Arterial System:
  • High pressure operation
  • Rapid blood loss typically occurs
  • Platelets dominate the hemostatic processes (where nitric oxide inhibits platelets)
Venous System:
  • Low pressure operation
  • Diminished blood flow rate
  • Thrombin dominates (where t-PA inhibits thrombin)
Impact of CPB (Cardiopulmonary Bypass)
  • Alters hemostatic balance leading to arterial and venous bleeding

Platelet Defects During CPB

  • Decreased Platelet Counts:
    • Platelet counts typically decrease by 30% to 50% during CPB.
    • Transfusion threshold is significant if platelet counts dip below 50,000/µl.
  • Hemodilution with Crystalloid Solutions:
    • Results in decreased platelet counts.
  • Adhesion to CPB Circuit:
    • Fibrinogen (I) & von Willebrand factor (vWF) adhere to circuit surfaces, activating platelets.
    • Platelets degranulate under the influence of shear stress and hypothermia.
    • The heparin used binds to platelet surfaces and vWF, causing alpha granule degranulation (which includes proteins like GPIIb/IIIa, fibrinogen, vWF).
  • Microscopic Examination:
    • Observations show a heterogeneous mixture of discoid and shape-changed platelets alongside degranulated platelets.

Goals of Platelet Activation

  • Recruitment of additional platelets to the site of injury.
  • Vasoconstriction of smaller arteries to minimize blood loss.
  • Localized release of ligands essential for stable platelet-platelet matrix.
  • Localization and acceleration of fibrin formation associated with platelets.
  • Protection of clot from fibrinolysis.

Coagulation Cascade and Cardiac Surgery

  • Activation Process:
    • Extrinsic pathway is activated first during cardiac surgery and CPB, followed by the intrinsic pathway.
    • The common pathway activation by Factor Xa initiates the amplification phase of coagulation.

Effects of Bypass/Surgery on Coagulation

  • CPB activates both intrinsic and extrinsic pathways depending on the surface characteristics (negatively charged surfaces activate intrinsic pathway).
  • Influencing Factors:
    • Coronary suction introduces tissue factor from damaged cells activating extrinsic pathways.
    • Platelet activation occurs due to contact with surfaces, and intrinsic and extrinsic pathways are both stimulated.

Hemostatic Factors

  • Factors affecting hemostasis during CPB include:
    • Hemodilution (leads to loss of clotting factors and platelets).
    • Hypothermia (impairs platelet function).
    • Blood management from pericardial sources.
    • Broken balance in coagulation processes and potential for fibrinolysis.
    • Trauma and resultant blood loss can contribute significantly to transfusion requirements.

Pathophysiology of CPB-Related Hemostatic Abnormalities

  • Despotis GJ et al, Anesthesiology 1999; 91: 1122-51
    • HEMODILUTION:
    • Result of CPB prime using crystalloids or colloids.
    • Use of extensive cell salvage systems is relevant for platelet and factor loss.
    • **ACTIVATION: **
    • Contact activation leads to intrinsic activation (e.g., XIIa, Kallikrein).
    • Tissue factor activated via injury and monocyte interaction.
    • Fibrinolysis activated via increased tPA (from endothelial cells or pericardial cavity) and intrinsic pathways.
    • CONSUMPTION:
    • Mediated by thrombin, plasmin, and inflammatory agents (e.g., elastase, complement, leukocyte-platelet complexes).
    • Mechanical disruptions due to ECC (e.g., oxygenator, cardiotomy suction) also contribute to consumption.

Causes of Bleeding Disorders

  1. Liver Diseases: (e.g., Cirrhosis, hepatitis, cancer)
  2. Vitamin K Deficiencies:
    • Affects coagulation factors II, VII, IX, X (all dependent on Vitamin K).
  3. Genetic Disorders:
    • Hemophilia A: deficiency of factor VIII.
    • Hemophilia B: deficiency of factor IX.
    • Von Willebrand Disease: deficiency of VWF.
    • Platelet Disorders: involve defects in platelet receptors.
  4. Treatment Options:
    • Replacement therapy using purified factors, recombinant technology, FFP, or platelets.

Artificial Surfaces

  • Recognized as foreign by blood components, leading to:
    • Activation of hemostatic elements for clotting.
    • Inflammatory responses attempting to reject the artificial surface.
  • Ideal approach involves managing coagulation and inflammation systems during exposure to foreign surfaces.

Heparin Characteristics

  • Mechanism of Action:
    • Potentiates Anti-Thrombin III (AT-III) activity by approximately 1000-fold.
    • AT-III inhibits thrombin (IIa) and Factor Xa (also IXa, XIa, XIIa to a lesser degree).
  • Goal of Anticoagulation:
    • Primarily focused on thrombin inhibition.

Interaction with Artificial Surfaces

  • Blood elements interaction activates:
    • Plasma proteolytic systems, coagulation system, fibrinolysis, complement cascade, and contact system (intrinsic pathway).
  • The continuous interaction leads to damage of blood cells/proteins and promotes inflammatory cytokine release.
  • Advances in surface coatings aim at reducing these adverse effects while maintaining biocompatibility and protein conservation.

New Coating Techniques

  • Polymer Strain Development:
    • Used in medical tubing through polymerization of monomers.
    • Design encourages reduced protein contact through hydrophobic backbone and mild hydrophilic pendant groups.
  • Biocompatibility Advantages:
    • Coatings exhibit clear performance benefits through in vitro and ex vivo testing:
    • Blocks platelet adhesion and clot activation factors.
    • Reduces protein denaturation and cell attachment in oxygenators and circuits.

Biological Response to Implanted Materials in Blood

  • Material surface impacts include:
    • Protein adsorption
    • Platelet adhesion
    • Fibrin and thrombus formation
    • Interaction of red blood cells and fibrin with platelets.

Coating Technology Historical Overview

  • 1250 A.D.: Recognized properties of surface preparations for bonding.
  • 18th Century Contributions: Surface-induced catalysis.
  • 1922: Theory pertaining to protein and colloidal behavior.
  • 1946: Development of molecular films and biological implications.
  • 1963: Introduction of heparin surface coating.

Non-Heparin Based Coatings

  • Smart Coatings: (e.g., Phosphorylcholine - SMA)
  • Softline Coatings: (e.g., Jostra)
  • Xcoating: (e.g., PMEA - Terumo)

Polymethoxyethylacrylate (PMEA)

  • Functionality:
    • Forms a hydro layer impacting blood exposure to device surfaces.
    • Minimizes platelet adhesion and enhances overall compatibility.

Heparin-Immobilized Surfaces

  • Characteristics:
    • Biocompatible and non-leaching with FDA clearance.
    • Bi-polymer coating facilitates heparin bonding to surfaces.

Current Coating Technologies

  • Heparin Polymer Coatings:
    • e.g., Hyaluronan-based, human albumin-based, polyethylene oxide-based bonding from different biomedical companies.

Trillium Biosurface Features

  • Hydrophilic, negatively charged surface with heparin:
    • Enhancements include:
    • Non-leaching heparin molecules
    • Incorporation of sulfate/sulfonate groups to mimic vascular endothelial charge.
    • Hydrophilicity via Polyethylene oxide (PEO) creating a significant water layer.

Benefits of Heparin-Coated Circuits

  • Improved biocompatibility with decreased inflammatory response.
  • Thrombogenesis concerns, requiring maintained systemic heparinization to avoid complications.

Hyaluronan Overview

  • 1-10% of cartilage glycosaminoglycans is hyaluronan.
  • Present in multiple body tissues, known for unique lubricious physical/chemical properties.
  • Medical applications include improving joint function and potential drug delivery mechanisms.

Rheological Properties of Hyaluronan

  • Features unique behavior in solution due to its expansive polymer structure, enhancing lubricity and preventing adhesion formation post-surgery.

Medical Applications of Hyaluronan

  • Used in ophthalmic surgery, as an injectable for arthritis, and innovative drug release applications.
  • Cross-linked gels acting as adhesion prevention post-surgery and enhancing device coatings.

Hyaluronan-Based Heparin Bonded Circuits (GBS® Coating)

  • Characterized by covalent bonding and preservation of biological elements.
  • Promotes reduced inflammation and enhances cell health around implants.

Evaluation Parameters in CPB

  • Biochemical Analysis: Integration of standard blood tests, interleukin levels, and assessments of biocompatibility.
  • Hemodynamic Monitoring: Key indicators including heart rate (HR), mean arterial pressure (MAP), cardiac output (CO), various blood gas analyses.

Results of Studies and Observations

  • Microscopy:
    • Demonstrated less platelet adhesion and improved surface structure in coated circuits versus uncoated.
    • Evidence shows that uncoated surfaces lead to protein adsorption indicating less favorable biocompatibility.

Hematologic Data from Clinical Trials

  • Results showcase variations in white blood cell count, platelet counts, fibrinogen levels, and C3a levels based on CPB conditions across coated and uncoated conditions throughout the trials.

Conclusions and Considerations

  • Solution to hemolytic and inflammatory responses associated with artificial surfaces is multi-faceted and includes:
    • Pharmacological interventions
    • Circuit modifications & reductions in surface exposure
    • Enhanced surgical techniques fostering better outcomes.

Closing Remarks

  • The necessity for collaboration among specialists to reconcile the complexities of artificial surface interactions with the hemostatic and inflammatory responses in clinical settings.
  • Final Note: Happy Hunting!