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Remifentanil for Labor Pain: Is the Drug or the Method the Problem?

Lacassie, Hector J., MD; Olufolabi, Adeyemi J., MBBS, DCH, FRCA

doi: 10.1213/01.ANE.0000173680.92044.55
Letters to the Editor: Letters & Announcements
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Division of Women’s Anesthesia; Department of Anesthesiology; Duke University Hospital; Durham, NC; lacas001@mc.duke.edu; Pontifica Universidad Catolica deChile; Santiago, Chile

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To the Editor:

We read with interest the study by Evron et al.(1), but must take exception to the study’s methodology and conclusions.

The study was intended to evaluate and compare systemic remifentanil versus meperidine for analgesia during labor. Remifentanil has a short T1/2 KeO of 1.3 min (2). However, to be able to benefit from this pharmacokinetic profile, the delivery time of the drug must be brief. In their study, they did not state the drug delivery rate—they stated that the top infusion was 1500 μg/h; however, this was the top possible rate rather than the speed of the bolus actually delivered. We estimate that the time from initiation of bolus to effect would approximate 2 min, ensuring the full effect occurring towards the end of contraction. This is based on their quoted rate (100 s for a 40-μg bolus) and the arm-brain circulation time of 30 to 40 s (3). This makes the drug less efficient, not from a pharmacodynamic standpoint, but from a methodological one.

Although their pain visual analog scale (VAS) and satisfaction scores were impressive, previous reports on the use of remifentanil have obtained poor VAS scores during contractions (1,3), raising doubts about the benefit of the drug. Furthermore, the retrospective evaluation of satisfaction at 24 h is not a reliable assessment. A near-perfect score (3.9) seems questionable in laboring women receiving IV opioids. Besides, their method for obtaining the average value of their pain assessment (VAS) to make a comparison is not clear. Was it the best three values per hour or a single random value throughout labor? Conversely, we would suggest that the high satisfaction numbers are more a result of the highly motivated population they studied than a true drug effect.

The power analysis is based on repeated measures analysis of variance; however, on their analysis, they only describe a Student’s t-test with no mention of multiple comparisons or post hoc testing. Statistical analysis of baseline VAS scores was stated to be significant (P < 0.001). This is very unlikely and is rather just another oversight in result reporting because this is a baseline value, with no drug administered yet. The results will not vary much, but these oversights in analysis make the conclusions questionable. Although we commend the authors for their attempt to study this drug for labor, we continue to seek improved methodology that takes advantage of remifentanil’s pharmacokinetics to treat labor pain.

Hector J. Lacassie, MD

Adeyemi J. Olufolabi, MBBS, DCH, FRCA

Division of Women’s Anesthesia

Department of Anesthesiology

Duke University Hospital

Durham, NC

lacas001@mc.duke.edu

Pontifica Universidad Catolica deChile

Santiago, Chile

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References

1. Evron S, Glezerman M, Sadan O et al. Remifentanil: a novel systemic analgesic for labor pain. Anesth Analg 2005;100:233–8.
2. Glass PS, Hardman D, Kamiyama Y, et al. Preliminary pharmacokinetics and pharmacodynamics of an ultra-short-acting opioid: remifentanil (GI87084B). Anesth Analg 1993;77:1031–40.
3. Olufolabi AJ, Booth JV, Wakeling HG, et al. A preliminary investigation of remifentanil as a labor analgesic. Anesth Analg 2000;91:606–8.
© 2005 International Anesthesia Research Society