Maternal-fetal distribution of bupivacaine in the rabbit

Bupivacaine was infused i.v. in nine anaesthetized pregnant rabbit does near term. Pups were removed at 10-15 min intervals and bupivacaine concentrations measured in fetal plasma, brain, placenta, amniotic fluid, maternal plasma sampled synchronously and maternal brain at the end of the experiment. Mean maximum fetal:maternal (F:M) ratio was 0.31 (SD 0.16) (range 0.18-0.64). Mean fetal brain:plasma ratios ranged from 2.04 to 5.09. There was no progressive increase in fetal brain bupivacaine concentration with time. Maternal brain:plasma ratio was 1.62 (0.81). However, maximum fetal brain concentration was only 0.27-0.86 of maternal. Concentrations increased with time in amniotic fluid, but did not exceed those in maternal plasma. Although there was some accumulation of bupivacaine in rabbit fetuses, tissue uptake could not account for low F:M ratios persisting beyond 80 min.