Bupivacaine protein binding in the term parturient: Effects of lactic acidosis

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Abstract
The effects of lactic acidosis on bupivacaine serum protein binding were studied in a group of term parturients and a group of nonpregnant female control subjects. Groups were matched in age and health. Distribution characteristics of bupivacaine in pregnancy were determined. Bupivacaine protein binding was best characterized by the model for 2 classes of binding sites in all studies. The parturients exhibited a lower capacity for the high-affinity, low-capacity (.alpha.1-acid glycoprotein) site and higher affinity for the low-affinity, high-capacity (albumin) site. Lactic acidosis decreased the affinity constant for the high-affinity, low-capacity site in the control group but did not change binding characteristics in the parturients. Free concentration of bupivacaine (Cu) was elevated at low total bupivacaine concentrations (Ct) (< 10 .mu.g/ml). No differences in Cu were detected at concentrations in the cardiotoxic range (> 20 .mu.g/ml). The Cu values predicted by the estimated binding parameters from in vitro experiments were compared with actual Cu measured in 9 parturients at delivery; they correlated significantly (r = 0.94). Distribution changes for bupivacaine in the parturients were consistent with known physiologic changes in body composition associated with pregnancy. Alterations that occur in serum protein binding during pregnancy should not result in increased risk of CNS or cardiovascular system toxicity since these alterations do not increase free tissue concentration.

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