Intrathecal (IT) opioids have been established as a safe and efficacious modality to treat postoperative pain. In the setting of colorectal surgery, studies have shown that intrathecal opioids together with multimodal analgesic regimens provide pain relief superior to multimodal analgesia alone. Furthermore, in the setting of multimodal analgesia, IT opioids also appear to be equianalgesic to epidural analgesia while conferring an improved safety profile. As a result, many institutions have incorporated intrathecal opioids into their Enhanced Recovery after Surgery (ERAS) pathways.

While morphine has traditionally been considered the "gold standard" in IT opioid therapy for postsurgical pain, hydromorphone has been gaining popularity as an alternative. The doses ranging between 0.005 mg to 0.25 mg for hydromorphone12-15 and 0.05 mg to 0.625 mg (with doses as high as 10 mcg/kg in the setting of cardiac surgery) for morphine has been found to be efficacious in this patient population. However, increasing opioid doses are associated with increased incidence of adverse effects. A meta-analysis reviewing twenty-eight studies which investigated intrathecal morphine versus placebo demonstrated moderate increases in the incidences of pruritus, nausea and vomiting. In fact, the incidence of nausea with IT morphine has been reported to be 33%. While hydromorphone is similar chemically to morphine, it is metabolized differently. Differences in pharmacokinetics may allow for differences in side effect profiles. Hydromorphone is more lipid soluble than morphine. This decreases its spread within the intrathecal space and enhances its penetration into the dorsal horn of the spinal cord where interactions with opioid receptors occur. Some studies (performed in the women undergoing cesarean delivery) have also found that hydromorphone causes less nausea and pruritus than morphine, while others have not.

Despite the widespread use of IT hydromorphone and morphine for pain after colorectal surgery, the optimal dose for neither drug has been established in prospective trials. The investigators have previously performed a dose-finding study of IT hydromorphone and morphine in women undergoing cesarean delivery. Briefly, 80 parturients scheduled for elective cesarean delivery 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, which was found to be 75 mcg for IT hydromorphone and 150 mcg for IT morphine. The follow-up study performed by the investigators also found no differences in adverse effects or efficacy between the drugs. The results from the obstetric population, however, cannot be directly translated to the colorectal surgery population due to pharmacodynamic and pharmacokinetics differences related to the pregnancy, age, presence of comorbidities, differences in surgical techniques, and co-administration of IT local anesthetic.

This study applies the methodology the investigators have previously used in the obstetric population to the patients undergoing colorectal resection and aims to identify the optimal dose of IT hydromorphone and morphine that provides good pain relief without causing significant side effects. Secondarily, the investigators will compare each drug at its optimal dose in terms of opioid consumption and side effects. Based on their prior findings, the investigators hypothesize that the optimal dose of intrathecal hydromorphone will be 75 mcg and the optimal dose of intrathecal morphine will be 150 mcg. Additionally, the investigators hypothesize that exploratory findings comparing the two drugs at their optimal doses will show no difference in the incidence of adverse effects.