- the total morphine consumption by means of the patient-controlled analgesia pump over the first 24-h after surgery, which can be measured precisely through the record of the dose delivered by the PCA pump. With an alpha error of 0.05, a power of 80% and a standard deviation of 7 mg of morphine consumption per 24 h, 20 patients (10 in each group) will be required for the study to detect a mean difference of 10 mg of morphine between the two groups