Serum bupivacaine concentrations in term parturients following continuous epidural analgesia for labor and delivery.

This study was undertaken to measure the blood concentrations of bupivacaine associated with a single loading dose followed by continuous epidural infusion for the management of the pain of labor and delivery with special reference to the potential for accumulation and toxicity. Four-milliliter venous blood samples were obtained every 15 min following the loading dose until delivery. If inadequate analgesia was observed just prior to delivery, an additional dose of bupivacaine was administered. Bupivacaine concentrations were measured using a double extraction technique followed by gas chromatographic analysis using a nitrogen-specific detector. Clearance, volume of distribution, and rate of absorption were estimated from the blood concentration time data and were 43.39 +/- 11.46 L/h, 67.56 +/- 17.66 L, and 8.97 +/- 3.69 h-1, respectively. Peak serum bupivacaine concentrations were 0.68 +/- 0.14 microgram/ml and occurred 0.58 +/- 0.25 h following administration of the loading dose. The duration of bupivacaine infusion was 3.42 +/- 0.80 h. Serum bupivacaine concentrations at delivery or just prior to administration of a supplemental delivery dose were significantly lower than the peak concentration in all patients (p less than 0.001). Fetal-to-maternal serum concentration ratios were found to be 0.44 +/- 0.16 for the six patients requiring a supplemental delivery dose and 0.44 +/- 0.13 for the six patients receiving bupivacaine only by infusion. The data reported here illustrate that epidural analgesia for labor and delivery achieved using a single 50-mg loading dose followed by a continuous infusion of 12.5 mg/h of bupivacaine will not result in maternal or fetal accumulation or toxicity.