Sample size calculation
The sample size calculation was based on the primary outcome of our study: the total amount of opioid use, measured on the PCA device, in the first 48 hours after surgery. Based on a mean post caesarean IV morphine consumption by PCA of 22 mg (SD 8 mg), we calculated the simple sizes. (12) Using an alpha risk of 5 and a power of 95 %, each group required 34 patients to detect a reduction in morphine use of 30 %.