VTE/PE Flashcards
Risk Factors for VTE/PE
Fractures/trauma Valvular disease or replacement Indwelling catheters Previous DVT/PE Malignancy Surgery (especially orthopedic) Immobility/bed rest/paralysis* Pregnancy Increasing age (> 40) Obesity Protein C deficiency or resistance (factor V Leiden) Protein S deficiency Antiphospholipid antibodies Antithrombin deficiency Pregnancy Estrogen therapy Malignancy
• Virchow’s triad: 3 primary factors influencing formation of a pathologic clot
o Damage to the vessel wall
o Venous stasis
o Hypercoagulable state
• Damage to the endothelium initiates two processes
o Platelet adhesion, activation and aggregation
o Activation of the coagulation cascade
Describe Blood Clot Formation
Platelet adhesion to the surface of the vessel occurs when blood is exposed to subendothelial collagen and von Willebrand factor (vWF)
Platelets become activated and release adenosine diphosphate (ADP), serotonin and thromboxane A2 (TXA2) which stimulate platelet aggregation
Platelet aggregation: glycoprotein IIb/IIIa receptor on platelet surface binds to fibrinogen (binding ligand)
Describe the Extrinsic pathway
- Tissue factor is released from subendothelial cells present in many organs extrinsic to blood
- Factor VII is activated and converted to factor VIIa when tissue factor binds
- Factor VIIa-tissue factor complex activates factor X at the beginning of the common pathway
Describe Intrinsic pathway
- Factor XII is activated when it comes into contact with the subendothelial membrane during vessel injury
- A series of reactions leads to activation of factor IXa which activates factor X at the beginning of the common pathway
- Alternatively, the factor VIIa-tissue factor complex activates factor IX in the intrinsic pathway
Describe Common pathway
• Factor Xa converts prothrombin (factor II) to thrombin (factor IIa)
Thrombin plays a central role in clot formation by
o Converting fibrinogen to fibrin monomers which begin to precipitate and polymerize to form a fibrin clot
o Activating platelets
o Enhances prothrombin activation on platelets by converting factor V to Va and factor VIII to VIIIa which accelerate the activity of factors Xa and IXa respectively
Natural inhibitors of clot formation
o Antithrombin o Protein S o Protein C o Tissue factor pathway inhibitor (TFPI) o Plasminogen o Plasmin
inhibits thrombin (IIa), factor IXa and factor Xa
o Antithrombin
cofactor for activation of protein C
o Protein S
inactivates factors Va and VIIIa
o Protein C
binds to factor VIIa-tissue factor complex and inhibits activation of factor X
o Tissue factor pathway inhibitor (TFPI)
Plasminogen is converted to plasmin via:
tissue plasminogen activator (t-PA
lyses fibrin to form fibrin degradation products
o Plasmin
Location of clot formation:
o Can form in any part of the venous circulation but the muscular veins of the calf or the valve cusp pockets of the deep calf veins are the most common locations
Deep vein thrombosis once formed can potentially do what 3 things
- Spontaneously lyse
- Extend into more proximal veins
- Embolize to the lungs causing pulmonary embolus
Clinical Presentation/symptoms of VTE
Unilateral calf or leg swelling Calf pain or tenderness Erythema Warmth Palpable cord \+Homan’s sign
Clinical Presentation/symptoms of PE
Dyspnea Pleuritic chest pain Anxiety Tachypnea (>20 breaths/min) Tachycardia (>100 beats/min) Cough Hemoptysis Diaphoresis
Diagnosis of VTE/PE include
- Clinical Presentation/symptoms
- Risk factor assessment
- D-dimer
- Diagnostic testing
Explain D-dimer testing
o Cross linked fibrin degredation fragment produced during clot dissolution
o Negative predictive value
o Negative = <0.5 mg/L
Diagnostic testing for DVT
Doppler ultrasonography and B-mode compression ultrasound Contrast venography (Gold standard)
Diagnostic testing for PE
Ventilation perfusion (V/Q) lung scan
Chest CT with contrast (most commonly used)
Pulmonary angiography (Gold standard but most invasive)
Confirmation of proximal DVT via Doppler or venography
Radiographic visualization of the involved vessels with injection of radiocontrast material
Contrast venography
Perfusion evaluated by radiolabeled albumin and ventilation evaluated by inhalation of radiolabeled particles
Ventilation perfusion (V/Q) lung scan - V/Q mismatch indicates possible PE
Injection of radiocontrast dye into the pulmonary artery
Pulmonary angiography
Ancillary tests for PE
arterial blood gas, EKG, CXR
Acute treatment for DVT/PE
Bridge therapy
o Parenteral agent: provides immediate anticoagulant activity
o Oral Therapy: Warfarin
o Bridge oral and parenteral agents for a minimum of 5 days AND until the INR is ≥ 2 for at least 24 hours
Structure of UFH
Heterogeneous mixture of glycosaminoglycan polysaccharide chains with molecular weights ranging from 3000 to 30000 kDa (mean = 1500)
MOA of UFH
Binds to antithrombin via a unique pentasaccharide sequence and accelerates its interaction and inactivation of thrombin and factor Xa by 1000-fold
Inhibition of thrombin requires
a heparin chain that is at least 18 saccharide units in length
Also inhibits Factor IXa, XIa, XIIa
UFH
can inhibit factor Xa by binding to antithrombin
Any pentasaccharide-containing heparin
Pharmacokinetics of UFH
not absorbed orally
treatment of VTE = continuous infusion, propylaxis of VTE = subcutaneous injection
Clearance: non-linear, dose-dependant
• Rapid saturable binding process
(Intravascular Binding)
• Slower, nonsaturable clearance through the kidneys
• T1/2 = 30-60 min
creates variablity and unpredictability in patient response
Intravascular binding
Monitoring for UFH
Activated partial thromboplastin time (aPTT)
Platelet counts, hemoglobin/hematocrit
Signs/symptoms of bleeding
Clotting time reported in seconds- measures activity of thrombin and factor Xa
aPTT
Dosing of UFH
- VTE treatment: IV bolus of 80 units/kg, followed by 18 units/kg/hr continous IV infusion
- VTE prophylaxis: 5000 units SC q 8 hrs
- No renal adjustment
Adverse effects
- Bleeding
- Heparin-induced thrombocytopenia
- Heparin-induced osteoporosis
How is bleeding that is caused by UFH reversed
o 1 mg of protamine will neutralize 100 units of UFH
o Short half life of heparin allows neutralization of heparin infused in the previous 60 mins
• Transfusion therapy may be required
2 types of HIT
- Type 1: transient fall in platelets due to a direct effect of heparin
- Type 2: immune-mediated reaction
MOA and onset of type II HIT
o Mechanism: IgG antibodies directed at heparin-platelet factor 4 complexes
o Patients are at an increased risk of thromboembolic events due to excessive thrombin generation, expression of tissue factor by monocytes and endothelial cells and platelet activation
o Onset: 5-14 days
o Platelet counts fall by greater than 50%
Treatment of HIT
Direct thrombin inhibitors: FDA approved
• Argatroban (see drug chart)
• Bivalirudin (see drug chart)
Fondaparinux
What types of patients are more at risk for Heparin-induced osteoporosis
• Usually only occurs with infusions > 6 months (rare)
heterogenous mixture of glycosaminoglycans derived from chemical or enzymatic degradation of UFH - Approximately 1/3 the molecular weight of UFH
• Low Molecular Weight Heparins (LMWH)
MOA of LMWHs
Binds to antithrombin via a unique pentasaccharide sequence
- Accelerates inactivation of factor Xa
- Only 25-50% of LMWH molecules inactivate thrombin
Pharmacokinetics of LMWHs
SC injection
- Intravascular binding: much less than UFH = more predictable dose-response relationship
2 LMWHs
- Enoxaparin (Lovenox)
* Dalteparin (Fragmin)
Monitoring of LMWHs
Antifactor Xa levels
Assess baseline renal function
Signs and symptoms of bleeding
Platelet counts, hemoglobin/hematocrit
Dosing (enoxaparin)
Treatment of VTE: 1 mg/kg SC q 12 hr or 1.5 mg/kg SC q 24 hr
• Renal adjustment: 1 mg/kg SC q 24 hr (CrCl < 30 mL/min)
Adverse events of LMWHs
Bleeding
Heparin-induced thrombocytopenia
Perispinal hematoma