Different Doses of Intrathecal Morphine on Postoperative Analgesia and Pruritus after Cesarean Section: a Prospective Randomized Triple-Blinded Trial

Abstract

Objective: Intrathecal morphine (ITM) is an effective postoperative analgesia provided after Cesarean section. However, pruritus is an undesirable side effect that disrupts maternal breast feeding. This study aimed to evaluate the effects of three different doses of ITM added to spinal bupivacaine on postoperative analgesia and opioid-related side effects in Cesarean section. 
Material and Methods: This prospective randomized, triple-blinded trial was conducted in 321 patients undergoing Cesarean section. They were allocated to receive either; one intervention group; intrathecal morphine 50 mcg (IT50 group, N=98), 100 mcg (IT100 group, N=100), or 200 mcg (IT200 group, N=101), added to spinal bupivacaine for Cesarean section. The primary outcome was the incidence of pruritus in the recovery room, and then every 4 hours for 24 hours. The secondary outcomes were the pain scores at rest and during activities, time to the first analgesia, 24 hours of morphine consumption and the incidence of postoperative nausea and vomiting (PONV). 
Results: Patient characteristics between the three groups were comparable. Both of the IT50 and IT100 groups had a lower incidence of pruritus, when compared with the IT200 group at all-time points, with the exception of at 24 hours; wherein, there were no differences between the three groups. Pain scores during activities were not different between the three groups. However, at rest the IT50 group had a higher pain score than those in the IT100 and IT200 groups; at 4, 8, and 16 hours. Time to the first analgesic requirement was shorter in the IT50 group (2 hours) than in the IT100 (2.4 hours) and IT200 (2.6 hours) groups (p-value=0.03). Moreover, the median [IQR] of morphine consumption in 24 hours was higher in the IT50 group [17 mg (9, 29.8)] than those in the IT100 [13 mg (5, 23.2)] and IT200 [12 mg (4, 17)] groups (p-value<0.001). However, the incidence of PONV was lower in the IT50 group compared to other groups. 
Conclusion: This study demonstrates that reducing the dose of ITM to 100 mcg added to spinal bupivacaine is effective to maintain postoperative analgesic effects. Additionally, it reduces the incidence of postoperative pruritus compared to ITM at 200 mcg.