| Age <55 years |
| ABO compatibility |
| Clear chest radiograph |
| PaO2 >40 kPa (FiO2 100% and 5 cmH2O PEEP) |
| Smoking history <20 pack years |
| Absence of chest trauma |
| No evidence of aspiration or sepsis |
| No prior cardiopulmonary surgery |
| Absence of organisms on sputum Gram stain |
| Absence of purulent secretions at bronchoscopy |