* no agreement to the terms of this study; * moderate or severe respiratory failure requiring admission to the icu and the need for invasive mechanical ventilation or non-invasive ventilation (niv) with positive pressure; * pregnancy or puerperium; * patients with hematological diseases, coagulation disorders, use of anticoagulants, previous heparin-induced allergy or thrombocytopenia, thrombocytopenia with a count of fewer than 50,000 platelets / mm3; * covid-19 not confirmed by rt-pcr within 72 hours of inclusion in the study.

* no agreement to the terms of this study; * moderate or severe respiratory failure requiring admission to the icu and the need for invasive mechanical ventilation or non-invasive ventilation (niv) with positive pressure; * pregnancy or puerperium; * patients with hematological diseases, coagulation disorders, use of anticoagulants, previous heparin-induced allergy or thrombocytopenia, thrombocytopenia with a count of fewer than 50,000 platelets / mm3; * covid-19 not confirmed by rt-pcr within 72 hours of inclusion in the study.

- no agreement to the terms of this study; - moderate or severe respiratory failure requiring admission to the icu and the need for invasive mechanical ventilation or non-invasive ventilation (niv) with positive pressure; - pregnancy or puerperium; - patients with hematological diseases, coagulation disorders, use of anticoagulants, previous heparin-induced allergy or thrombocytopenia, thrombocytopenia with a count of fewer than 50,000 platelets / mm3; - covid-19 not confirmed by rt-pcr within 72 hours of inclusion in the study.

- no agreement to the terms of this study; - moderate or severe respiratory failure requiring admission to the icu and the need for invasive mechanical ventilation or non-invasive ventilation (niv) with positive pressure; - pregnancy or puerperium; - patients with hematological diseases, coagulation disorders, use of anticoagulants, previous heparin-induced allergy or thrombocytopenia, thrombocytopenia with a count of fewer than 50,000 platelets / mm3; - covid-19 not confirmed by rt-pcr within 72 hours of inclusion in the study.