* intubated and on mechanical ventilation, or requiring immediate intubation as per the treating clinician's assessment * heparin allergy or heparin-induced thrombocytopaenia * aptt \> 120 seconds, not due to anticoagulant therapy and does not correct with administration of fresh frozen plasma * platelet count \< 20 x 109 per l * pulmonary bleeding or uncontrolled bleeding * pregnant or might be pregnant * acute brain injury that may result in long-term disability * myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome * treatment limitations in place, i.e. not for intubation, not for icu admission * death is imminent or inevitable within 24 hours * clinician objection * refusal of participant (person responsible) consent

* intubated and on mechanical ventilation, or requiring immediate intubation as per the treating clinician's assessment * heparin allergy or heparin-induced thrombocytopaenia * aptt \> 120 seconds, not due to anticoagulant therapy and does not correct with administration of fresh frozen plasma * platelet count \< 20 x 109 per l * pulmonary bleeding or uncontrolled bleeding * pregnant or might be pregnant * acute brain injury that may result in long-term disability * myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome * treatment limitations in place, i.e. not for intubation, not for icu admission * death is imminent or inevitable within 24 hours * clinician objection * refusal of participant (person responsible) consent

- intubated and on mechanical ventilation, or requiring immediate intubation as per the treating clinician's assessment - heparin allergy or heparin-induced thrombocytopaenia - aptt > 120 seconds, not due to anticoagulant therapy and does not correct with administration of fresh frozen plasma - platelet count < 20 x 109 per l - pulmonary bleeding or uncontrolled bleeding - pregnant or might be pregnant - acute brain injury that may result in long-term disability - myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome - treatment limitations in place, i.e. not for intubation, not for icu admission - death is imminent or inevitable within 24 hours - clinician objection - refusal of participant (person responsible) consent

- intubated and on mechanical ventilation, or requiring immediate intubation as per the treating clinician's assessment - heparin allergy or heparin-induced thrombocytopaenia - aptt > 120 seconds, not due to anticoagulant therapy and does not correct with administration of fresh frozen plasma - platelet count < 20 x 109 per l - pulmonary bleeding or uncontrolled bleeding - pregnant or might be pregnant - acute brain injury that may result in long-term disability - myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome - treatment limitations in place, i.e. not for intubation, not for icu admission - death is imminent or inevitable within 24 hours - clinician objection - refusal of participant (person responsible) consent