Evaluation of efficacy of erector spinae plane block performed with two different bupivacaine concentrations on postoperative analgesia after mastectomy surgery

In the operating room (OR), all of the patients will receive a standard monitoring including electrocardiography, non-invasive blood pressure, peripheral oxygen saturation and bi-spectral index monitoring. After the placement of a 22-gauge intraveous line, 15 ml kg-1 isotonic saline infusion will be started. All patients will receive intravenous 0.05 midazolam for sedation. Then the patients will randomly divided into two groups based on a computerized randomization table created by a researcher who will not involve in the study. There will be two anesthesiologists in the OR. For each randomized patient, the first anesthesiologist will take the corresponding sealed envelope from a folder, which indicates the treatment assigned to the patient, while the second anesthesiologist will be blind to the group allocations. The first anesthesiologist will prepare the drug solutions for use in the study (0.25% bupivacaine or 0.375% bupivacaine solution) in two identical 20 ml syringes and a 10 ml syringe of isotonic saline for hydrodissection and pass the labeled syringes to the blinded anesthesiologist. And the patients will be placed in sitting position. In the first group (Group High ESP), the second anesthesiologist will locate the ultrasound probe in longitudinal orientation at the level of T4 spinous process and then place 3 cm laterally from the midline. The ultrasound landmarks, T4 transverse process and the overlying trapezius, rhomboideus and erector spinae muscle, will be identified. Under aseptic conditions, a 80 mm 21-gauge block needle will be inserted in-plane at an angle of 30-40 degrees in cranial-to-caudal direction until the tip contacts the T4 transverse process. After the hydrodissection with 3 mL of isotonic saline solution confirmes correct needle tip position, the anesthesiologist will inject 20 mL of 0.375% bupivacaine deep into the erector spinae muscle. The block procedure will take approximately 5-10 minutes and then the anesthesiologists will start induction of anesthesia with intravenous propofol, fentanyl and rocuronium bromide.