Continuous mini-infusion of bupivacaine into the epidural space during labor. Part II: Blood concentration of bupivacaine.

Mini-infusion of 0.25 per cent bupivacaine plain into the epidural space was used in 16 healthy women for pain relief during labor. After a loading dose, continuous administration was ensured by using an automatic pump at an infusion rate of 5 or 8 ml per hour. Maternal venous blood concentrations of bupivacaine were determined 20, 40, and 60 minutes after the loading dose. Further samples were taken hourly until delivery. Fetal blood concentrations were determined from fetal scalp blood during mini-infusion, and from umbilical venous blood at delivery. Maternal blood levels averaged 206 ng/ml. The maximum concentration (604 ng/ml) did not exceed 30 per cent of the toxic level. The blood concentration of bupivacaine did not tend to increase during the course of continuous infusion. Tachyphylaxis could not be detected statistically. The fetal blood concentration during blockade averaged 58 ng/ml, giving a feto-maternal ratio of 0.28. The neonatal-maternal ratio, calculated from the umbilical venous blood, was 0.32. With a shift of the acid-base status towards acidosis in the neonate, the ration of bupivacaine slightly increased. Absence of maternal hypotension as well as absence of fetal heart irregularities during infusion may be attributed to the low and stable blood concentration of bupivacaine. The mean Apgar score was 9.5 at one minute and 9.9 at five minutes. The maternal arterial acid-base balance at delivery was normal. The fetal acid-base status at delivery showed wide variation, without any relation to the Apgar score or the total dose of bupivacaine. It is concluded that continuous mini-infusion of bupivacaine by means of an automatic pump has advantages over intermittent administration.