Transfer of fentanyl across the rabbit placenta. Effect of umbilical flow and concurrent drug administration.

The transfer of fentanyl has been studied in the perfused in situ rabbit placenta. Does were infused with fentanyl and, subsequently, with fentanyl plus bupivacaine and pethidine. Antipyrine was infused throughout as an index of materno-fetal exchange. The fetal side of one placenta was perfused artificially with Krebs buffer containing dextran, and the effluent collected from the umbilical vein (uv). Drug concentrations were measured in maternal arterial blood (ma) and uv. Maternal plasma protein binding was also measured. Transfer of fentanyl was intermediate between that of pethidine and bupivacaine (Cuv:Cma for fentanyl was 0.25, for bupivacaine, 0.11 and for pethidine was 0.44), while placental clearance (Cuv X umbilical flow/Cma) increased with umbilical flow between 0.5 and 4.0 ml min-1 for all drugs. When clearances of unbound drugs were calculated, those of bupivacaine, pethidine and antipyrine were similar, but that of fentanyl was higher. The clearance of fentanyl was not affected by the presence of bupivacaine and pethidine.