To relate two different predictive values of sleep apnea: the STOP score (snoring while sleeping, daytime tiredness, observed breathing stoppages, and high blood pressure) and the estimated respiratory disturbance index (eRDI) with the occurrence of apnea-hypopnea during propofol sedation,thirty four middle-aged male patients who underwent urologic surgery under spinal anaesthesia will be enrolled. Before surgery, patients will be asked to complete a STOP-questionnaire. The eRDI will be calculated using the modified Mallampati's grade, tonsil grade, and the body mass index. After performing spinal anaesthesia, propofol will be infused and adjusted to BIS 70-75. An ApneaLinkTM, which measures airflow through a nasal cannula, will be then applied to estimate the level of apnea-hypopnea.