After due approval from ethical review board and written consent, 40 patients who is scheduled for an elective eye procedure in which direct arterial blood pressure monitoring is appropriate will be recruited into this study. End tidal carbon dioxide will be sampled via SentriTM ETCO2 adult nasal cannula (Intersurgical ® code 1144002) during preoperative and intraoperative period. ETCO2 is measured by infrared spectroscopy (Datex-Ohmeda GE Healthcare S5, Compact monitor, Switzerland) with a side stream aspirator which has a flow rate of 150mL/min.