PONV after intrathecal morphine, occurs up to 30-40 percent. The patients having TKR normally are female, obese and non-smoker which are risk factors for PONV. Recently, a multimodal approach, combining several means to minimize PONV, has found wide acceptance as a standard of care. In our hospital, most of the patients are supposed to fast after midnight The oral rehydration therapy reduce patients thirst and increase his satisfaction, but whether or not this method can reduce the incidence of PONV after low dose (0.2 mg) intrathecal morphine is not investigated yet. The aim of this study is to investigate the effect of preoperative oral rehydration therapy on the incidence of PONV.

We enroll patients who are undergoing TKR under spinal anesthesia with 0.5% heavy bupivacaine 10-15 mg (2.0-3.0 ml)+ intrathecal morphine 0.2 mg+ femoral nerve block with 0.25% bupivacaine 20 ml.

The enrolled patients will fast after midnight and in the morning they are allowed to drink 10% carbohydrate drink or not to drink (according to their randomization). 400 ml of 10% carbohydrate drink is drunk between evening and midnight and extra water if needed in the study group. In the control group, the patients are allowed to drink until midnight. Total of water consumption between 18.00-24.00 will be recorded in both groups. In the morning, the study group will drink another 400 ml of 10% carbohydrate drink between 6 - 7 am. The premedications should not include sedation or GI mobility drugs. Before starting anesthesia, the patients will be asked some questions about thirsty, hungry, anxiety and nausea and weigh their feeling from 0-10.

The surgery and pain therapy will be tha same in both groups and the PONV (incidence, severity and rescue therapy) at recovery room and 24 hours postoperative period will be recorded.

Inclusion Criteria:

• Patients are under going unilateral TKR, with ASA I-III who scheduled started in the morning list (before noon)

Exclusion Criteria:

• Diabetes
• History of motion sickness
• Chronic kidney disease (CKD, creatinine > 2 mg/dl)
• Hiatus hernia or gastro esophageal reflux
• Patients receiving drugs that might affect GI motility such as opioids