Why are patients undergoing dialysis at risk of clotting?

Why Patients Undergoing Dialysis Are at Risk of Clotting

Dialysis patients are at high risk of clotting due to the complex interaction between contact activation of the coagulation system, inflammation, and platelet activation when blood passes through the extracorporeal circuit. 1

Mechanisms of Clotting During Dialysis

Contact Activation Pathway

• Blood exposure to the artificial surfaces of the dialysis circuit activates the contact pathway of coagulation
• The extracorporeal circuit components (particularly the venous air trap) trigger thrombosis more frequently than the dialyzer itself 2
• Passage through the dialyzer increases tissue factor levels, factor VIIIc, and D-dimers, indicating activation of the coagulation cascade 3

Inflammatory Response

• Dialysis triggers an inflammatory response that is closely linked to coagulation activation
• Inflammatory markers (sICAM-1 and sVCAM-1) are elevated in dialysis patients 3
• Inflammation activates both the complement system and contact pathway, contributing to thrombin generation during dialysis 1

Platelet Activation

• Dialysis causes significant platelet activation, as evidenced by increased:
  • Soluble P-selectin
  • Soluble CD40 ligand
  • Platelet microparticles 3
• Uremic toxins in CKD patients impair platelet function, paradoxically creating both bleeding and clotting risks 1

Risk Factors for Clotting During Dialysis

Patient-Related Factors

• Reduced blood flow during dialysis (average 217 ml/min vs 240 ml/min in non-clotting patients) 2
• Shortened clotting time after 2 hours of dialysis 2
• Pre-existing hypercoagulable states (Factor V Leiden, elevated Factor VIII) 4
• Comorbidities common in dialysis patients:
  • Hypertension
  • Heart failure
  • Vascular disease
  • Diabetes mellitus 1

Dialysis-Related Factors

• Increased hematocrit during high-volume online hemodiafiltration 3
• Inadequate anticoagulation during the procedure
• Dialyzer membrane type (though recent studies show minimal differences between cellulosic and polysulphone dialyzers) 3
• Venous air trap design (more prone to clotting than the dialyzer itself) 2

Anticoagulation Management During Dialysis

Standard Approach

• Unfractionated heparin is the standard anticoagulant used during dialysis 5
• Typical regimen includes:
  • Initial bolus followed by continuous infusion, or
  • Low-dose continuous administration (500-800 IU/h) without starting bolus 6

Special Considerations

• For patients at bleeding risk, heparin-free dialysis may be considered, but carries a 4% risk of system coagulation 2
• Monitoring activated partial thromboplastin time (APTT) is important, especially in patients with liver impairment or lupus anticoagulant who may have "aPTT confounding" 1
• For patients with recurrent clotting despite standard anticoagulation:
  • Consider therapeutic anticoagulation if there is evidence of clots in the extracorporeal circuits 1
  • Evaluate for underlying thrombophilia 4

Prevention Strategies

During Dialysis

• Maintain adequate blood flow (aim for >240 ml/min when possible) 2
• Regular monitoring of the extracorporeal circuit, especially the venous air trap 2
• Consider prophylactic switching of the system and dialyzer if early signs of coagulation appear 2

For High-Risk Patients

• For patients with recurrent vascular access thrombosis, evaluate for underlying thrombophilia 4
• Consider long-term oral anticoagulation for patients with confirmed thrombophilia and recurrent access thrombosis 4
• For patients on dialysis with atrial fibrillation requiring antiplatelet therapy:
  • Low-dose aspirin (75-100 mg daily) is recommended for secondary prevention
  • Clopidogrel 75 mg daily is the preferred alternative in cases of aspirin intolerance 7

Common Pitfalls and Caveats

• Dialysis patients present a paradoxical situation of both increased bleeding risk and increased clotting risk 1
• Antiplatelet agents should be used cautiously, as they may increase bleeding risk without significantly reducing cardiovascular events in primary prevention 7
• Thrombin-antithrombin complex (TAT) levels are more sensitive than prothrombin fragment 1+2 for detecting coagulation activation during dialysis 6
• System clotting typically occurs around 1.8 hours after the start of dialysis, requiring vigilant monitoring during this period 2