Figure 4: 5-min epoch of polysomnography with central events induced by noninvasive ventilation. Arterial oxygen saturation measured by pulse oximetry (SpO2) oscillations related to central events occurring in stage-1 sleep. Note the decrease in respiratory effort during events, identified by pulse transit time (PTT). Tho: thoracic movements; Abd: Abdominal movements; Flow: Flow signal measured using a pneumotachograph; Pre: Pressure. Reproduced with permission of the European Respiratory Society©. Eur Respir Mon November 2008 41:350-366.