Pain control after surgery is a crucial component of postoperative recovery. Thus, in open abdominal surgery, the use of TEA (Thoracic epidural analgesia) has become a gold standard. However, analgesia may still remain a challenge, especially in patients with large incision abdominal incisions. To address this problem, the use of the PIEB (Programmed intermittent epidural bolus) mode has been proposed, because it offers a better spread of the epidural medication. Only a few studies have evaluated the superiority of this PIEB mode in open abdominal surgery, yielding conflicting results. However, PIEB showed to be at least as effective in controlling pain as CEI, and clinical experience shows that it can be a valuable tool, especially in patients with a large laparotomy incision. However, the optimal dosage of the PIEB mode has never been determined, Furthermore, it is known whether men and women have the same dosage needs to achieve adequate analgesia since most studies have failed to account for sex-based differences in postoperative analgesia. Hence, the main objective of this study will be to determine the optimal volume dose of the PIEB delivered at a fixed interval of 60 min in patients undergoing surgery using a large midline laparotomy incision. The optimal dose for male and female patients will be determined separately by evaluating men and women in two independent groups. We hypothesize that the optimal dose of the PIEB that will provide effective analgesia in 90% of patients on the first postoperative day will range between 5 and 10 mL, for both groups. The primary outcome in both groups is successful analgesia. It is defined as a requirement of 5 or less patient-controlled epidural boluses between 8 AM and 8 PM on the first postoperative day. Secondary outcomes will be the incidence of motor block and hypotension, comparison of the level of sensory block according to group, comparison of patient satisfaction according to group and comparison of Quality of Recovery-15 score at 48h post-op according to group.