QLB is an injection of a local anesthetics around the quadratus lumborum muscle. It uses a fascial compartment path to extend the distribution of local anesthetics into the posterior abdominal wall and paravertebral space. This central effect can be of vital importance when managing the visceral pain after caesarean section.

Many studies have shown that the inclusion of quadratus lumborum block to a multimodal analgesic regimen would reduce pain scores, opioid consumption, and prolonging time to first rescue analgesic after cesarean delivery.

However, the dose and concentration of the local anesthetic used among the studies are varied, and literature search identified no randomized controlled trial which looked at the concentration -response of local anesthetic to optimize the concentration resulting in the best pain relief.

It is a perspective randomized controlled trial to compare the analgesic efficacy of 2 different concentrations of Bupivacaine to standardize postoperative analgesic protocol used for QLB after caesarean section.

QLB is an injection of a local anesthetics around the quadratus lumborum muscle. It uses a fascial compartment path to extend the distribution of local anesthetics into the posterior abdominal wall and paravertebral space. Acute post C-section pain is a leading anesthetic concern for women; a key determinant of maternal satisfaction; may lead to persistent postoperative pain; is a predictor of postpartum depression; and can reduce early breastfeeding success. Effective postoperative analgesia should, therefore, be prioritized to improve outcomes following caesarean delivery. This study is a Double-blinded, randomized and controlled trial.

A written, informed consent is discussed with and signed by all participants, and the participants are randomly assigned into one of two groups; (Group 1) to receive bilateral QLB with '0.125% bupivacaine 0.2 ml/ kg', (Group 2) to receive bilateral QLB with '0.25% bupivacaine 0.2 ml/kg'. For all participants, spinal anesthesia is performed with ultrasonography guidance in a standardized manner using hyperbaric bupivacaine 15mg and fentanyl 25 μg.

At the end of the procedure in the supine position, all participants receive bilateral QLBs performs under ultrasound guidance and aseptic technique. The internal oblique muscle is identified and followed laterally to the lateral interfacial triangle sitting above the quadratus lumborum muscle.

Results will be reported as

1. Total number of PCA morphine demands and the actual doses delivered at predetermined time intervals (1h, 2 h, 4h, 6 h, 12 h, 24 h and 48 h) after surgery
2. Record of supplemental and regular analgesics.
3. Visual analogue score for pain at rest (static) and with movement (dynamic) (defined as the elevation of the head and shoulders off the pillow from the supine position), (0, no pain; 10, worst pain imaginable).
4. Residual nerve block (the time to mobilization)
5. Block-related complications (i.e., hematoma, organ injury, local anesthetic systemic toxicity, and sepsis).

6- Opioids-related side effects (Nausea, Vomiting, Itching)

The investigators hypothesized that using a higher concentration (0.25%) of local anesthetic (Bupivacaine) would increase the analgesic effect without increasing the side effects and complications.

Inclusion Criteria:

• Participants who are American Society of Anesthesiologists physical status 1 or 2.
• Participants who have singleton pregnancy at a gestation of at least 37 weeks.
• Participant who are scheduled for elective caesarean section under spinal anesthesia.
• Participants who are consented to be enrolled into the study.

Exclusion Criteria:

• Patients who have contraindications to spinal or regional anesthesia (Coagulopathy or on anticoagulants).
• Who have Allergy or sensitivity to study medications.
• Who have anatomical abnormalities or localized infection.
• Who have history of chronic pain or on regular opioids use.
• Who are unable to comprehend or unable to use the verbal rating pain scoring system,
• Failed spinal anesthesia or conversion to general anesthesia after spinal anesthesia.

Participants are allocated randomly by means of computer generation sequencing. The allocation sequence with unique study number for each participant will be concealed in sequentially numbered, opaque, sealed, and stapled envelopes. This envelope will be opened by an anesthetist who was not involved in the study. This Anesthetist prepares the study medication as per the allocation and labels the syringe with the unique study number; this number will be used to identify the study medication and will be revealed only on completion of data collection at the end of the study. The investigators, participants and other healthcare providers who are involved in postoperative care, are blinded to the participant's group allocation.