| Grade I | 10–15 cm H2O (7.5–11 mm Hg); |
| Grade II | 15–25 cm H2O (11–18 mm Hg); |
| Grade III | 25–35 cm H2O (18–25 mm Hg); and |
| Grade IV | >35 cm H2O (>25 mm Hg). |
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