- enrolled in another clinical trial that is unapproved for co-enrolment - heparin allergy or heparin-induced thrombocytopaenia (hit) - aptt > 120 seconds and this is not due to anticoagulant therapy - platelet count < 20 x 109 per l - pulmonary bleeding, which is frank bleeding in the trachea, bronchi or lungs with repeated haemoptysis or requiring repeated suctioning - uncontrolled bleeding - pregnant or might be pregnant. females aged 18-49 years are excluded unless there is documented menopause or hysterectomy or a pregnancy test was performed and is negative. - receiving or about to commence extracorporeal membrane oxygenation (ecmo) or high frequency oscillatory ventilation (hfov) - myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome - acute brain injury that may result in long-term disability - usually receives home oxygen - dependent on others for personal care due to physical or cognitive decline - death is imminent or inevitable within 24 hours - the clinical team would not be able to set up the study nebuliser and ventilator circuit as required including with active humidification - clinician objection - refusal of participant (person responsible) consent.

- enrolled in another clinical trial that is unapproved for co-enrolment - heparin allergy or heparin-induced thrombocytopaenia (hit) - aptt > 120 seconds and this is not due to anticoagulant therapy - platelet count < 20 x 109 per l - pulmonary bleeding, which is frank bleeding in the trachea, bronchi or lungs with repeated haemoptysis or requiring repeated suctioning - uncontrolled bleeding - pregnant or might be pregnant. females aged 18-49 years are excluded unless there is documented menopause or hysterectomy or a pregnancy test was performed and is negative. - receiving or about to commence extracorporeal membrane oxygenation (ecmo) or high frequency oscillatory ventilation (hfov) - myopathy, spinal cord injury, or nerve injury or disease with a likely prolonged incapacity to breathe independently e.g. guillain-barre syndrome - acute brain injury that may result in long-term disability - usually receives home oxygen - dependent on others for personal care due to physical or cognitive decline - death is imminent or inevitable within 24 hours - the clinical team would not be able to set up the study nebuliser and ventilator circuit as required including with active humidification - clinician objection - refusal of participant (person responsible) consent.