• Kenneth E Nelson, Traci Rauch, Victor Terebuh, and Robert D'Angelo.
• Department of Anesthesiology, Wake Forest University School of Medicine, Winston-Salem, NC 27157-1009, USA. kenelson@wfubmc.edu
• Anesthesiology. 2002 May 1;96(5):1070-3.

BackgroundThe use of intrathecal opioids for labor analgesia continues to gain popularity, but there are limited data to guide this use. Previously, the authors established the ED50 for 60 min of labor analgesia from intrathecal sufentanil using an up-down sequential allocation study design. The current study first establishes an ED50 for intrathecal fentanyl using this same study design to establish an intrathecal potency ratio for fentanyl and sufentanil and then uses this ratio to compare the efficacy, duration of analgesia, and side effects from comparable doses of intrathecal fentanyl and sufentanil.MethodsSeventy-five healthy nulliparous women requesting labor analgesia were enrolled in this two-part study. In phase I, 20 women received varying doses of fentanyl to establish an ED50 for 60 min of labor analgesia. In phase II, 55 women were randomized to receive either 36 microg intrathecal fentanyl or 8 microg sufentanil (2 times the ED50s) via a combined spinal-epidural technique and by double-blinded design. Pain relief, side effects, block height, maternal hemodynamics, and fetal heart rate were assessed throughout the study. The duration of spinal analgesia was considered to be the time from injection of study drug to the time of the patient's first request for additional analgesia.ResultsThe ED50 of intrathecal fentanyl for 60 min of labor analgesia was found to be 18.2 microg, and therefore, the potency ratio of intrathecal sufentanil to intrathecal fentanyl at the ED50 level is 4.4:1. The duration of spinal analgesia was significantly longer from 8 microg intrathecal sufentanil than from 36 microg intrathecal fentanyl (104 +/- 34 vs. 79 +/- 34 min, P = 0.009). Otherwise, patient demographics, maternal hemodynamics, duration of labor, mode of delivery, motor block, subjective leg weakness, pruritus, nausea, pinprick sensory levels, visual analog scale pain scores, fetal bradycardia, and Apgar scores were similar between groups.ConclusionThe relative potency of intrathecal sufentanil to fentanyl for labor analgesia is 4.4:1. When using intrathecal opioids alone for early labor analgesia, 8 microg sufentanil produces labor analgesia lasting approximately 25 min longer than from 36 microg fentanyl, without a statistically significant increase in side effects. However, when making a choice between fentanyl and sufentanil, one must consider other important factors, such as the higher cost of sufentanil and the greater risk of dosing error due to the higher potency of sufentanil compared with fentanyl.

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