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Background and Aims Intrathecal opioids confer superior postoperative analgesia to systemic opioids as they deliver analgesia directly to their site of action while curtailing the undesired side effects. The present study aimed to determine the optimal dose of intrathecal morphine and hydromorphone in patients undergoing minimally invasive colorectal surgery.

Methods The study was a double-blinded prospective trial and was approved by the ethics committee (IRB 20-009205). Patient were randomized into intrathecal morphine or hydromorphone group and dosing was determined by a sequential up-down method using a biased coin design. The primary outcome was ED90 of both agents to achieve numeric rating pain scores (NRS) ≤4 with postoperative opioid requirements ≤15 oral morphine equivalent at 12 hours.

Results Eighty patients completed the study. The ED90 for intrathecal morphine and hydromorphone could not be determined for the examined dose ranges (25-400 mcg and 10-200 mcg, respectively). The ED50 for morphine was 100 mcg (95% CI was below the lowest dose) and for hydromorphone was 75 mcg (95% CI 50–200 mcg). Morphine median (interquartile range, IQR) NRS scores was 3.0 (1.5-4.5) at 12 hours and 3.0 (2.0-4.0) at 24 hours, whereas hydromorphone median (IQR) NRS were 3.0 (1.5-5.0) at 12 hours and 3.0 (2.0-5.0) at 24 hours.

Conclusions The study could not establish an ED90 for IT dosing for minimally invasive colorectal surgical patients. However, we did determine the ED50 for both agents. Patients reported to be very satisfied in their postoperative analgesic regimen in all dosing categories and no serious adverse events were observed throughout the study duration.