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LETTER TO THE EDITOR

Remifentanil Patient-Controlled Analgesia in Labor

Department of Anesthesia, Stanford University School of Medicine, Stanford, CA, bcarvalho{at}stanford.edu

To the Editor:

We have several concerns regarding the study design and results of the Evron et al. (1) article. Our primary concern is the use of an IV infusion of meperidine as the active control. Meperidine infusions are uncommon practice, and the authors' failure to use a patient-controlled IV analgesic (PCIA) modality for the meperidine group, as used by two previous labor remifentanil versus meperidine studies (2,3), limits direct group comparisons. Does the observed difference in analgesic efficacy between the two study groups instead reflect the different modes of administration and non-equianalgesic doses used?

Second, we were surprised that the authors used escalating doses of remifentanil up to a maximum bolus dose of 70 µg (0.93 µg/kg). Previous studies (4,5) have shown that PCIA doses of 0.4–0.5 µg/kg are effective in reducing labor pain scores. Indeed, the authors in this study reported that most parturients had satisfactory analgesia with boluses of 25–40 µg (equivalent to a PCIA bolus dose of 0.3–0.5 µg/kg based on the study's reported mean weight). Standardizing PCIA boluses would adjust for differences in body weight and allow more rigorous comparisons to be made between study groups. Moreover, failure to calculate doses according to body weight may have resulted in dangerously large doses (1.2–1.6 µg/kg) in the subjects 1–2 sd (16 kg) from the study group's mean weight (75 kg) when receiving the 70 µg dose. Transient respiratory depression is commonly reported in remifentanil PCIA labor studies, even with doses <0.5 µg/kg (4–6). With the higher doses used in this study, we would have expected a higher incidence of respiratory depression. We would be interested to know if any respiratory depression occurred among the three specific time points reported.

Third, we are unsure why the time to epidural analgesia cross-over was so much longer in the meperidine group (by almost 2 h) even though the remifentanil group had better pain control and a lower overall cross-over rate.

Lastly, it is unclear why the authors found a statistically significant difference in baseline VAS pain scores (P value <0.001). Is this an error in Table 2? If this is a true difference, we feel that the difference in pain scores before analgesia between the groups would invalidate further comparative data interpretation.

References

1. Evron S, Glezerman M, Sadan O, et al. Remifentanil: a novel systemic analgesic for labor pain. Anesth Analg 2005;100:233–8.[Abstract/Free Full Text]
2. Blair JM, Hill DA, Fee JPH. Patient-controlled analgesia for labor using remifentanil: a feasibility study. Br J Anaesth 2001;87:415–20.[Abstract/Free Full Text]
3. Volmanen P, Akural EI, Raudaskoski T, Alahuhta S. Remifentanil in obstetric analgesia: a dose finding study. Anesth Analg 2002;94:913–7.[Abstract/Free Full Text]
4. Volikas I, Male D. A comparison of pethidine and remifentanil patient-controlled analgesia in labor. Int J Obstet Anesth 2001;10:86–90.[Web of Science][Medline]
5. Babenco DH, Conard PF, Gross JB. The pharmacodynamic effect of a remifentanil bolus on ventilatory control. Anesthesiology 2000;92:393.[Web of Science][Medline]
6. Blair JM, Dobson GT, Hill DH, et al. Patient controlled analgesia in labour: a comparison of remifentanil with pethidine. Anaesthesia 2005;60:22–7.[Medline]

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