BACKGROUND

Intrathecal (IT) morphine is considered the "gold standard" for analgesia after cesarean delivery under spinal anesthesia, most commonly administered at a dose of 100 to 200 μg. There is less experience with IT hydromorphone for postcesarean analgesia and limited information on its optimal analgesic dose. We conducted this study to determine the effective analgesic dose for 90% patients (ED90) of IT hydromorphone that provides effective analgesia for women undergoing elective cesarean delivery and its potency ratio to IT morphine.

METHODS

In this dose-finding trial, 80 patients received spinal anesthesia for cesarean delivery. Participants were randomized to receive IT morphine or IT hydromorphone at a dose determined using up-down sequential allocation with a biased-coin design to determine ED90. All patients received standardized multimodal analgesia postoperatively in addition to IT opioid. An effective dose was defined as a numeric response score for pain of ≤3 (scale 0-10) 12 hours after spinal injection.

RESULTS

The ED90 was 75 μg (95% confidence interval [CI], 46-93 μg) for IT hydromorphone and 150 μg (95% CI, 145-185 μg) for IT morphine. At these doses, the 95% CI for the percentage of patients with effective analgesia (numeric rating scale ≤3) was 64% to 100% for hydromorphone and 68% to 100% for morphine. Exploratory findings showed that the incidence of nausea and pruritus was not different among the most commonly used doses of IT hydromorphone (P = 0.44 and P = 0.74) or IT morphine (P = 0.67 and P = 0.38, respectively). When administering IT opioids at ED90 doses or higher, 100% (21/21) of IT hydromorphone and 95% (37/39) of IT morphine patients were satisfied with their analgesia.

CONCLUSIONS

The ratio of IT morphine to IT hydromorphone for effective postcesarean analgesia is 2:1. Patient satisfaction was high with both medications.