Obesity hypoventilation syndrome classification

Two subtypes are recognized. The first is OHS in the context of obstructive sleep apnea; this is confirmed by the occurrence of 5 or more episodes of apnea, hypopnea or respiratory-related arousals per hour (high apnea-hypopnea index) during sleep. The second is OHS primarily due to “sleep hypoventilation syndrome”; this requires a rise of CO₂ levels by 10 mmHg (1.3 kPa) after sleep compared to awake measurements and overnight drops in oxygen levels without simultaneous apnea or hypopnea. In many patients, both phenomena are present.

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