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The clinical effects of spinally (subarachnoid) administered, preservative-free fentanyl were assessed in 120 healthy women who underwent cesarean section with spinal anesthesia using 0.5% hyperbaric bupivacaine. Subjects were divided at random into four groups (n = 30) the first of which received 2 mL of saline containing no fentanyl (group 0); the second, 0.25 μg/kg (group 25); the third, 0.5 μg/kg (group 50); and the fourth, 0.75, μg/kg (group 75) of fentanyl in a blinded manner. Surgical anesthesia was excellent in 100% of treated patients and in 87% of group 0. Respiratory rate decreased significantly in groups 50 and 75 and was recorded as early as 4 min after the administration of the drug. Nevertheless, respiratory depression did not develop in any patient, and 40 min later all groups had a similar respiratory rate. Regression of anesthesia to the T-12 dermatome took a longer time as the dose of fentanyl increased, but all patients had recovered by 240 min after the injection. Effective postoperative analgesia lasted longer and significantly increased with the dose of fentanyl ad-ministered: group 0, 197 ± 77 min; group 25, 305 ± 89 min; group 50, 640 ± 142 min; and group 75, 787 ± 161 min (data expressed as mean ± sD; P < 0.001 between groups). Neonatal status was the same in all groups. Sedation and pruritus were the main side effects. The combination of bupivacaine and a low dose of fentanyl (0.25 μg/kg) provides excellent surgical anesthesia with short-lasting postoperative analgesia and very few negative side effects. As the dose of fentanyl increases to 0.5 or 0.75 μg/kg, postoperative pain relief lasts longer, but respiratory changes occur and the incidence of adverse effects also increases.