| Parameter | Value |
| Clinical assessment | |
| Apnea | |
| Stridor | |
| Severely depressed mental status | |
| Flail chest | |
| Inability to clear respiratory secretions (eg, excessive secretions, loss of protective reflexes, neuromuscular failure) | |
| Trauma to mandible, larynx, trachea | |
| Loss of ventilatory reserve | |
| Respiratory rate | >35 breaths/min |
| Tidal volume | <5 mL/kg |
| Vital capacity | <10 mL/kg |
| Negative inspiratory force | Weaker than –25 cm H2O (2.44 kPa) |
| Minute ventilation | <10 L/min |
| Rise in PaCO2 | >10 mmHg (1.33 kPa) |
| Refractory hypoxemia | |
| Alveolar-arterial gradient (FiO2 = 1) | >450 |
| PaO2/PAO2 | <0.15 |
| PaO2 with supplemental O2 | <55 mmHg (7.32 kPa) |
