Introduction:
Unfractionated heparin (UFH) is the traditional anticoagulant of choice in critically ill COVID-19 patients requiring extracorporeal membrane oxygenation (ECMO). Nadroparin, a low molecular weight heparin, potentially offers advantages such as predictable pharmacokinetics and reduced bleeding risks compared to UFH, with complex pharmacokinetics, influencing activated partial thromboplastin and causing substantial haemorrhagic risks. Bleeding, the most common adverse event during ECMO, is associated by many with increased activated partial thromboplastin time.

Material and methods:
This retrospective, bicentric analysis involved 38 consecutive ECMO-supported COVID-19 patients from two Polish hospitals. The study compared 27 patients receiving UFH and 11 patients treated with 5700 IU of nadroparin administered subcutaneously twice daily. Thrombotic and haemorrhagic complications were assessed to determine the safety and feasibility of each anticoagulant.

Results:
Resistance to flow throughout the therapy in the ECMO membrane oxygenator was significantly lower in the group anticoagulated with UFH (1.74 mmHg × minute × L^–1 [1.38–2.6] vs. 6.13 mmHg × minute × L^–1 [5.93–14.81]; P < 0.001). However, the number of transfused red blood cell packs in the aforementioned group was significantly greater (10 units [5–17] vs. 4 units [2–8]; P = 0.027), and the haemoglobin level after ECMO therapy was significantly lower (7.8 g dL^–1 [6.9–8.8] vs. 10.2 g dL^–1 [8.5–12.2]; P = 0.003). Moreover, there was a higher number of life-threatening events in the UFH group.

Conclusions:
UFH anticoagulation may provide better flow optimization in the oxygenator, but the risk of life-threatening bleeding may increase. The present findings need to be fully elucidated in prospective studies on a larger critically ill population supported with respiratory ECMO.