Sampling end tidal gas via nasal prong has been shown to be a noninvasive and reliable method of monitoring end-tidal CO2 in spontaneously breathing patients. It is used to assess the adequacy of ventilation and enhance patient safety. When using a nasal cannula to sample expired gases by a patient, air from the room may dilute the sample and affect the accuracy of ETCO2. It is necessary to have accurate values of ETCO2 during intravenous sedation to detect respiratory depression including apnea. Therefore, the aim of our study is to assess the correlation of end tidal carbon dioxide to arterial partial pressure of carbon dioxide by using Sentri nasal cannula among sedated and non-sedated patient undergoing cataract surgery under local anaesthesia. Patients who are classified as ASA physical status 1 to 3 with age more than 18 years' old, and are scheduled for elective eye surgery in which arterial blood pressure monitoring is appropriate will be eligible. This study will be conducted at University Malaya Medical Centre.