Why might a patient in ECMO require escalating doses of heparin over time?

In patients requiring Extracorporeal Membrane Oxygenation (ECMO), escalating doses of heparin over time can be attributed to acquired antithrombin deficiency due to critical illness. During severe illness or critical conditions, such as those experienced by patients on ECMO, there is a substantial change in the body's hemostatic balance. Acquired deficiencies in coagulation inhibitors such as antithrombin can occur, leading to increased clotting tendency.

Antithrombin is a key protein that inhibits several enzymes in the coagulation pathway, particularly thrombin and factor Xa. When antithrombin levels drop, the effectiveness of heparin is reduced because heparin relies on antithrombin to exert its anticoagulant effect. Therefore, as the patient becomes more critically ill, the need for higher doses of heparin arises to achieve adequate anticoagulation and prevent thrombus formation, which is crucial in the ECMO circuit.

This physiological change underscores the importance of monitoring and adjusting anticoagulation therapy in patients on ECMO, as standard dosing based on weight or initial responses may become inadequate as the patient's condition evolves.