Proposed classification system for obesity hypoventilation syndrome
| Mild | Moderate | Severe | |
| PaCO2 (mmHg) | 46–60 | 60–80 | >80 |
| PaO2 (mmHg) | >70 | 60–70 | <60 |
| BMI (kg·m−2) | 30–40 | 40–50 | >50 |
| Apnoea/hypopnoea index (events·h−1) | <5 | 5–15 | >15 |
| Comorbidities | No | No | Yes |
Reproduced from [21] with permission.