DOES INTRATHECAL FENTANYL PRODUCE ACUTE CROSS-TOLERANCE TO IV MORPHINE

We have examined the hypothesis that intrathecal fentanyl at operation can increase postoperative i.v. morphine requirements. We studied 60 patients undergoing Caesarean section. All received intrathecal 0.5% p lain bupivacaine 2 ml combined with either fentanyl 0.5 ml (25 mu g) ( group F) (n=30) or normal saline 0.5 ml (group S) (n=30). In addition, 10 ml of an extradural solution (fentanyl 1 ml (50 mu g) combined wit h 0.5% bupivacaine 9 ml) was administered after delivery. Extradural s olution was only given before delivery if the intrathecal injection fa iled to produce a block above T6 or the patient required further analg esia. Postoperative analgesia was provided with i.v. morphine patient- controlled analgesia. At operation, intrathecal fentanyl reduced the n eed to administer extradural solution before delivery, increased the a naesthetist's satisfaction with analgesia and reduced nausea, but incr eased pruritus. Up to 6 h after delivery there was no difference in po stoperative morphine requirements or pain scores. Between 6 h and 23 h there was a 63% increase in morphine requirements in group F. We cons ider the most likely explanation for this finding to be that intrathec al fentanyl induced acute spinal opioid tolerance.