Heparin Resistance in Sepsis

Primary Mechanisms

Hyperfibrinogenemia is the key factor causing heparin resistance in septic patients, as elevated fibrinogen levels directly bind to heparin molecules and reduce their bioavailability 1
C-reactive protein and other heparin-binding acute phase reactants are markedly elevated during the inflammatory response in sepsis, creating additional binding sites that sequester heparin away from its anticoagulant targets 1
These proteins compete with antithrombin III for heparin binding, reducing the formation of the heparin-antithrombin complex necessary for anticoagulation 1
Activated macrophages and endothelial cells—both prominently activated in sepsis—bind and internalize heparin, further reducing circulating bioavailable drug 1
The systemic inflammatory response characteristic of sepsis causes widespread endothelial activation, creating numerous additional binding sites for heparin on vascular surfaces 1
Antithrombin levels decrease in sepsis through multiple mechanisms: increased vascular permeability causing extravasation, consumption by pathologically activated coagulation, cleavage by proteases, and decreased hepatic synthesis during acute hepatic dysfunction 2
Since heparin requires antithrombin to exert its anticoagulant effect, even modest reductions in antithrombin levels can contribute to apparent heparin resistance 1, 2

Excessive thrombopoietin production by the liver during sepsis can cause thrombocytosis, which has been suggested as an additional mechanism for heparin resistance 1
Elevated platelet counts provide more surface area for thrombin generation, potentially overwhelming heparin's anticoagulant capacity 1
Sepsis-induced coagulopathy involves systemic activation of coagulation with suppression of fibrinolysis, creating a prothrombotic state that requires higher heparin doses to overcome 2, 3

Heparin resistance is defined as requiring daily UFH doses exceeding 35,000 units to achieve therapeutic aPTT or the inability to achieve therapeutic anticoagulation despite high doses 1

Measure both aPTT and concomitant anti-factor Xa heparin levels when heparin resistance is suspected, as aPTT may be unreliable in inflammatory states 1
Point-of-care aPTT testing may be influenced by elevated acute phase reactants, making anti-Xa monitoring more reliable in septic patients 1
If monitoring is impractical or resistance persists, consider longer-acting, non-aPTT adjusted subcutaneous fondaparinux or danaparoid if renal function is normal 1
Prophylactic-dose LMWH (dalteparin) has been studied in critically ill patients with markedly impaired renal function without adverse effects 1

While antithrombin supplementation is theoretically attractive for overcoming heparin resistance, large trials have not demonstrated mortality benefit, and concomitant heparin administration may impair antithrombin's beneficial effects 2, 4, 5
Subgroup analyses suggest antithrombin may be effective in septic patients with coagulopathy who are not receiving concurrent heparin 2