1. Patient is not expected to survive for more than 24 hours. 2. Patient is on invasive mechanical ventilation with intubation for more than 48 hours prior to Screening. 3. Severe pre-existing cardiac disease (ie, New York Heart Association Class 3 or Class 4, acute coronary syndrome or persistent ventricular tachyarrhythmias). 4. Patient has an unresolved Neisseria meningitidis infection. 5. Use of the following medications and therapies: a. Current treatment with a complement inhibitor, b. Rituximab within 3 months of Screening, c. Mitoxantrone within 3 months of Screening, and d. Intravenous immunoglobulin (IVIg) within 3 weeks prior to Screening. 6. Participation in another interventional treatment study within 30 days before initiation of ravulizumab on Day 1 in this study or within 5 half lives of that investigational product, whichever is greater. 7. Female patients who are breastfeeding or who have a positive pregnancy test result at Screening or on Day 1. 8. History of hypersensitivity to any ingredient contained in the study drug, including hypersensitivity to murine proteins.

1. Patient is not expected to survive for more than 24 hours. 2. Patient is on invasive mechanical ventilation with intubation for more than 48 hours prior to Screening. 3. Severe pre-existing cardiac disease (ie, New York Heart Association Class 3 or Class 4, acute coronary syndrome or persistent ventricular tachyarrhythmias). 4. Patient has an unresolved Neisseria meningitidis infection. 5. Use of the following medications and therapies: a. Current treatment with a complement inhibitor, b. Rituximab within 3 months of Screening, c. Mitoxantrone within 3 months of Screening, and d. Intravenous immunoglobulin (IVIg) within 3 weeks prior to Screening. 6. Participation in another interventional treatment study within 30 days before initiation of ravulizumab on Day 1 in this study or within 5 half lives of that investigational product, whichever is greater. 7. Female patients who are breastfeeding or who have a positive pregnancy test result at Screening or on Day 1. 8. History of hypersensitivity to any ingredient contained in the study drug, including hypersensitivity to murine proteins.