Postoperative pain of a caesarean section may be of high intensity, especially in the first 48 hours after the procedure, which affects the mother / newborn relationship, in addition to having the potential to progress to chronic pain. The use of intrathecal morphine is effective in post-caesarean analgesia, but carries unwanted side effects, including nausea, vomiting, urinary retention and pruritus. Therefore, alternative techniques of analgesia become necessary.

First described in 2007, ultrasound-guided quadratus lumborum (QL) block has gained prominence due to its analgesic superiority to the TAP block. Besides providing somatic analgesia, it also seems to inhibit visceral pain because the local anesthetic reaches the paravertebral space, this was observed by magnetic resonance imaging with contrast medium injected at the QL block site.

C-section is one of the most performed surgical procedures in the world, and presents great potential for postoperative pain, especially in the first 48 hours. Pain in this period represents a risk for evolution to chronic pain, but its incidence is still very divergent when comparing the studies, ranging from 1% to 18%.

The use of intrathecal morphine (MIT) is consecrated as a first-choice method of analgesia for post-partum cesarean delivery; however, its use carries relevant side effects for the puerpera, such as pruritus, nausea, urinary retention and , more rarely, respiratory depression. In the last decade, new adjunctive forms of postoperative analgesia have become more popular, such as regional blockades, highlighting the blockage of the Transversus Abdominis plane block (TAP) and the Quadratus Lumborum block (QL), taking as benefits the prolonged analgesia they provide and the low incidence of side effects. Another factor that contributes to the popularization of regional blocks is the increasing availability of ultrasound devices in the anesthetic-surgical environment. Studies evaluating TAP block after cesarean section performed under spinal anesthesia have shown a discrete benefit in decreasing postoperative opioid consumption in addition to improving pain scores. When comparing MIT with TAP block, the superiority of MIT is due to visceral analgesia while TAP block is restricted to abdominal wall analgesia.

Ultrasound-guided Lumbar Quadrant blockade has gained prominence due to its analgesic superiority over the TAP block, because in addition to providing somatic analgesia it also seems to inhibit visceral pain since the local anesthetic reaches the paravertebral space, this was observed by magnetic resonance imaging with contrast injected at the site of the QL block.

Reviewing the literature in 2018, there are no clinical trials comparing the use of intrathecal morphine with the quadratus lumborum block.

Inclusion Criteria:

• Minimum age of 18 years
• Minimum gestational age of 37 weeks with a single fetus
• Patients scheduled for elective cesarean section through a Pfannenstiel incision under spinal anesthesia
• American Society of Anesthesiology physical status (ASA) II
• Body mass index (BMI) under 40 kg/m²
• No use of opioids for the past 4 weeks
• No history of psychotropic drug use
• No contraindication for any of the medications involved in the study
• No previously known malformations of the fetus
• No previous history of chronic pain

Exclusion Criteria:

• Difficulty in understanding how to use the patient-controlled analgesia (PCA) device
• Study protocol violation
• Patient decision to withdraw participation