Maternal and neonatal effects of 1% and 2% mepivacaine for lumbar extradural analgesia.

Continuous lumbar extradural analgesia with mepivacaine was administered to two groups of patients in normal labour. One group (26 patients: 1% mepivacaine) received a mean total dose of 342 mg (4.93 mg/kg) per patient, and developed a mean blood concentration of mepivacaine at delivery of 1.82 mug/ml. The neonatal umbilical vein concentration was 0.84 mug/ml. The other group (30 patients: 2% mepivacaine) received a mean total dose of 776 mg (11.65 mg/kg) per patient, and developed a mean blood concentration of mepivacaine at delivery of 3.47 mug/ml. The neonatal umbilical vein concentration was 2.61 mug/ml. Four of the infants of mothers who received 1% mepivacaine were depressed (1-min Apgar score 6 or less), and six of the other group were depressed also. Usually, depression appeared to be related to obstetric factors, rather than to analgesia. Eleven of the 56 infants had umbilical vein mepivacaine concentrations of 3 mug/ml or greater; of these, three were depressed. This does not agree with the concept that the toxic threshold for mepivacaine is 3 mug/ml. In both groups a significant linear correlation was obtained between umbilical vein concentration and total dose of mepivacaine. A maximal dose of 12 mg/kg maternal weight in the non-obese or 12 mg/kg lean body mass in the obese is suggested for continuous extradural analgesia with mepivacaine, although healthy mothers and infants may tolerate much more.