Analgesia, pruritus, and ventilation exhibit a dose-response relationship in parturients receiving intrathecal fentanyl during labor

Several studies have characterized the 50% and 95% effective doses (ED50 an d ED95, respectively) of intrathecal sufentanil for labor analgesia. Few ha ve investigated these same criteria for the less expensive alternative, fen tanyl. In addition, the ventilatory effects of intrathecal fentanyl at clin ically relevant doses are unclear. We performed this study to establish the dose-response relationship of intrathecal fentanyl for both analgesia and ventilatory depression. Ninety parturients in active early labor (less than or equal to 5 cm dilation) received intrathecal fentanyl 5, 7.5, 10, 15, 2 0, or 25 pg in a double-blinded, randomized fashion (n = 15 patients in eac h group). Parturients were monitored for degree of pain (measured using a 1 00-mm visual analog pain scale), blood pressure, arterial oxygen saturation (Sao(2)) respiratory rate, ETCO2, and fetal heart rate 0, 1, 5, 10, 15, 20 , 25, and 30 min after the administration of intrathecal fentanyl. An absol ute visual analog pain scale score less than or equal to 25 mm was defined a priori as analgesic success. The percentage of parturients who achieved a nalgesic success was used to construct quantal dose-response curves, from w hich the ED50 and ED95 values were derived for the total population (mixed parity) and the nulliparous and multiparous subpopulations separately. Over all ED50 and ED50 values (95% CI) were 5.5 (3.4-7.2) and 17.4 (13.8-27.1) m u g, respectively. Nulliparous values were lower (5.3 and 15.9 mu g, respec tively) than multivarous values (6.9 and 26.0 mu g, respectively) but were within the 95% CIs of the total population. Pruritus incidence in parturien ts with analgesic success displayed a dose-response relationship identical to that seen for analgesia. ETCO2 displayed a dose-related increase, partic ularly at doses greater than or equal to 15 mu g, without concomitant chang es in respiratory rate or Sao, which suggests a decrease in tidal volume. E ven in the absence of overt signs or symptoms of somnolence, intrathecal fe ntanyl at doses within the effective analgesic range induced a change in ve ntilation that may last longer than the 30-min period we studied. Implicati ons: Intrathecal fentanyl induces rapid and satisfying dose-dependent analg esia in early labor; however, it also produces dose-related decreases in ve ntilation in the absence of overt somnolence.