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Department of Anesthesiology Perioperative and Pain Medicine Brigham and Women’s Hospital Harvard Medical School Boston, MA 02115
To the Editor:
I read with interest the epidural morphine dose-response article by Palmer et al. (1). These authors are to be commended for continuing to conduct dose-response studies on the most effective doses of neuraxial opioids for labor and for postcesarean analgesia. This study supports the notion that, from an analgesic standpoint, more is not necessarily better. In addition, there is a ceiling effect for analgesia but not necessarily for side effects. Although this study didn’t demonstrate increased side effects, such as pruritus, with increasing doses, other studies have (2). I was surprised that the authors decided to put so little emphasis on respiratory depression. Although they mentioned that respiratory rate was monitored hourly, there is no data given on respiratory rate. Pruritus, nausea, and vomiting are common side effects of neuraxial opioids. Respiratory depression, although much less common, is clearly a more serious side effect. In the first report of neuraxial (intrathecal) morphine for obstetric analgesia, Baraka et al. (3) reported a high incidence of marked somnolence when high doses were used. Although respiratory depression should be rare with small doses of neuraxial morphine, it may still happen. Parturients at risk include those that have received previous parenteral opioids and those who are obese or have sleep apnea. Respiratory rate should be followed hourly for at least 18 hours after neuraxial morphine administration.
References
Department of Anesthesiology University of Arizona Health Sciences Center Tucson, AZ 85724-5114
In Response:
We thank Dr. Hepner for his comments on our article (1) and appreciate the opportunity to respond to a few of his points. The reason we "put so little emphasis on respiratory depression" is that this study lacked the power to contribute meaningful data on the complication. Respiratory depression after epidural morphine has been well characterized as a very low-incidence event, with an incidence probably less than 0.2% (2,3). In normal, healthy, nonobese parturients, our experience indicates the incidence is probably even lower. Hourly monitoring of respiratory rate is part of our routine clinical protocol for monitoring patients who have received epidural morphine, not part of the study protocol. Dr. Hepner is correct in stating that the maternal respiratory rate should be closely followed after neuraxial morphine; the protocol in our institution for more than 10 years (4) has been to monitor and record respiratory rate hourly for 24 hours after injection.
The relevance of the reference to somnolence to our study of epidural morphine is questionable. Baraka et al. (5) reported a high incidence of somnolence after very high doses of intrathecal morphine for labor analgesia (1 to 2 mg!). Based on a comparison of our study with a previous study of intrathecal morphine (6), we have estimated the analgesic potency ratio of intrathecal:epidural morphine to be approximately 40:1; using this ratio, epidural morphine doses equivalent to the intrathecal morphine doses reported by Baraka et al. (5) would be 40 to 80 mg! Although somnolence is often a marker of early respiratory depression after more lipid-soluble neuraxial opioids (7,8), it is unlikely to be often seen in the dose range of epidural morphine we studied.
Finally, while Dr. Hepner cites nausea and vomiting as common side effects of neuraxial opioids, both this study (1) and a previous one (6) failed to find any difference between control and treatment groups in the incidence of these two side effects; perhaps it is time to reevaluate the notion that neuraxial morphine at clinically relevant doses causes nausea and vomiting in this population.
References
This article has been cited by other articles:
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  A. A. Uskova, B. P. Matusic, and B. W. Brandom Desflurane, Malignant Hyperthermia, and Release of Compartment Syndrome Anesth. Analg., May 1, 2005; 100(5): 1357 - 1360. [Abstract] [Full Text] [PDF]
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P. E. Hess, A. Vasudevan, C. Snowman, and S. D. Pratt Small Dose Bupivacaine-Fentanyl Spinal Analgesia Combined with Morphine for Labor Anesth. Analg., July 1, 2003; 97(1): 247 - 252. [Abstract] [Full Text] [PDF]
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