The patient will be randomized to receive isoflurane or desflurane anaesthesia prior to surgery. All the patients involved in the study will be blinded to group allocation. Screening for preoperative cognitive dysfunction will be done using the mini-cog test which includes a three word recall test and clock drawing test.The primary outcome variable will be assessment of delirium at day one and day three following surgery. Post operative delirium will be diagnosed using the 3D- CAM(Confusion Assessment Method) which tests for four features with a series of questions. The features include 1) acute onset and fluctuating course,2) inattention,3) disorganized thinking and 4) altered level of consciousness. Diagnosis of delirium is made if features 1 and 2 and either 3 or 4 are present. Similarly the severity of postoperative delirium will be assessed on day one and day three following surgery. Severity of postoperative delirium will be assessed using the CAM-S(Confusion Assessment Method-severity)long form delirium severity score.It assesses ten features including 1) acute onset and fluctuating course,2)inattention,3)disorganized thinking , 4)altered level of consciousness, 5) disorientation, 6)memory impairment ,7)perceptual disturbances ,8)psychomotor agitation, 9)psychomotor retardation and 10) altered sleep-wake cycle. The scores range from 0 to 19.