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Department of Anesthesiology; Duke University Medical Center; Durham, NC; habib001{at}mc.duke.edu
In Response:
We appreciate Dr. Balestrieri’s interest in our article (1). We address his comments about the limitations of our study:
1. A power of 80% is standard in the medical literature. As we were able to demonstrate a reduction in pain scores of significance both clinically and statistically, both at rest and on movement in patients receiving intrathecal morphine, we believe that our study was adequately powered for its primary endpoint.
2. The 30-mg dose of ketorolac that we used preemptively is currently recommended by the manufacturer and is in keeping with our standard practice. We started the patients on oral naprosyn 500 mg twice a day in the postanesthesia care unit. We believe this is a good nonsteroidal antiinflammatory drug coverage. Although 60 mg was the dose formerly recommended for IV ketorolac, this recommendation has been changed, as Dr. Balestrieri (2) reports: "Because of concerns brought to the attention of the FDA (after this study was completed), new guidelines for ketorolac’s use were released. These guidelines include decreasing the dose to 30 mg." Furthermore, this referenced study did not provide a comparison between the higher dose (60 mg) and the 30-mg dose, as only the 60 mg dose was used (2).
3. We agree that there was a trend towards a higher incidence of vomiting and pruritus in patients receiving intrathecal morphine in our study. However, despite this higher incidence of opioid-induced adverse effects, the patients in this group were significantly more satisfied overall than the patients who did not receive intrathecal morphine.
4. As we reported, none of the patients in our study developed respiratory depression during the 24-h duration of the study. In one study, respiratory depression occurred in 0.9% patients after 0.2 mg spinal morphine for Cesarean delivery (3). The rate is almost certainly less with a lower dose of morphine, such as the 0.05 mg used in our study. A study powered to look at respiratory depression with this dose of intrathecal morphine would therefore need thousands of patients and is not practical.
5. Dr. Balestrieri suggests that improved analgesia can be achieved by infiltration with ropivacaine at skin closure. In our study, the surgeons provided local anesthetic infiltration before skin incision with 0.5% bupivacaine for a possible preemptive analgesic effect. Injecting the local anesthetic at closure would have eliminated any potentially helpful preemptive analgesic effect and would have been given 
40 min later, prolonging the local anesthetic effect by only 40 min. We do not believe this would be a meaningful difference.
Overall, we found that adding a very small dose of intrathecal morphine (0.05 mg) to spinal bupivacaine and fentanyl, together with local anesthetic infiltration and regular nonsteroidal antiinflammatory drugs, produced improved postoperative analgesia and higher patient satisfaction despite a higher incidence of opioid-related side effects. We continue to recommend that clinicians consider using a small dose of neuraxial morphine for analgesia after postpartum tubal ligation in women who will be inpatients for 24 h after the operation.
References
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