Hypopnœa is a medical term for abnormally shallow breathing or slow respiratory rate. As far as sleep medicine occurs, it is not a true hypopnea unless it is a 50% (or greater) reduction in flow and associated with a 3% (or greater) desaturation in the persons O2 levels, or resulting in fragmentation of sleep or arousal. This is distinct from a true apnea because there remains some flow of air.