Analgesic Effect of Morphine Added to Bupivacaine in Serratus Anterior Plane Block Following Modified Radical Mastectomy. Only a Local Effect? Randomized Clinical Trial

BACKGROUND: Serratus anterior plane (SAP) block, a novel regional anesthetic procedure, involves the anterolateral chest wall. Opioid receptors have been found on peripheral nerve terminals, so morphine may have a local action. OBJECTIVE: This work aimed at exploring the analgesic efficacy of morphine added to bupivacaine in SAPB in patients for whom modified radical mastectomy was conducted and whether it is a mere local effect. METHODS: Forty female patients were planned to have modified radical mastectomy participated in the study. Patients were randomly divided into two groups; Control group (C): received ultrasound-guided serratus anterior plane block with 20 mL of bupivacaine hydrochloride 0.25%; Morphine group (M): received the same in addition to 10 mg morphine sulfate. Intra- and post-operative blood samples were taken for the assessment of morphine serum levels. All patients were assessed for VAS scores during rest and movement (VAS-R and VAS-M). Time to the first request and the total amount of the rescue analgesia were recorded. RESULTS: In group M, Morphine was not detected in the plasma of all patients. Both VAS-R and VAS-M were significantly higher in group C than in group M (P<0.001) and (P≤0.003), respectively. Time to the first request of rescue analgesia was 8.5 h in group C compared to 20 h in group M (P=0.005) with a median dose of acetaminophen consumption of 2 g in group C compared to 1 g in group M (P=0.006). CONCLUSION: Ten mg of morphine, when added to bupivacaine in SAPB, improved postoperative analgesia in patients to whom modified radical mastectomy was conducted. This effect seems to be attributed merely to local mechanisms. REGISTRATION: The registration number of this study is NCT02962024 at www.clinicaltrial.gov.