 Definition of apnea: absence of flow for at least 10 s. Definition de hypopnea: any reduction in the flow of at least 10 s, accompanied by a desaturation a noticeable fall in saturation with rapid recovery andor transitory waking or arousal defined according to the ASDA criteria: Sleep, 1992; 15: 173-184. Definition of hypoventilation: Prolonged periods of arterial desaturation not preceded by episodes of respiratory events. 