The effect of spinal hyperbaric bupivacaine-fentanyl or hyperbaric bupivacaine on uterine tone and fetal heart rate in labouring women: a randomised controlled study.

ABSTRACT

The mechanism for fetal heart rate abnormalities following spinal opioids remains controversial. We evaluated uterine tone, using an intra-uterine pressure catheter, and fetal heart rate abnormalities in 30 women in spontaneous labour with cervical dilation of 3-5 cm having combined spinal-epidural analgesia. Women were randomly assigned to receive a spinal with 2.0 mg hyperbaric bupivacaine plus 15 µg fentanyl, or 2.5 mg hyperbaric bupivacaine. The primary outcome measure was an increase > 10 mmHg in baseline uterine tone in the 30-min period following spinal injection. Only three (20%) women who had a bupivacaine-fentanyl spinal showed a > 10 mmHg increase in baseline tone vs. none who had bupivacaine (p = 0.63). The mean (SD) baseline uterine tone after the spinal injection was 13.3 (7.0) mmHg in the bupivacaine-fentanyl group and 7.7 (2.5) mmHg in the bupivacaine group (p = 0.01). Seven (47%) in the bupivacaine-fentanyl group showed new onset fetal heart rate changes during the 30-min period after the spinal, compared with two (13%) in the bupivacaine group (p = 0.04); however, these were transient and responded to intra-uterine resuscitation. Pain scores, sensory and motor block as well as neonatal outcomes were comparable between the groups. We found that raised baseline uterine tone was not more frequent when using bupivacaine-fentanyl rather than bupivacaine in the spinal component of combined spinal-epidural, although absolute values of baseline tone were higher, and fetal heart rate changes more frequent, in the former group.