Relationship between the transplacental gradients of bupivacaine and alpha 1-acid glycoprotein.

1 The binding of bupivacaine (400 ng/ml) to isolated alpha 1-acid glycoprotein was studied at two protein concentrations. At 20 mg/100 ml the extent of bupivacaine binding was 31.0 +/- 1.8% (mean +/- s.d., n = 4), and at a protein concentration of 60 mg/100 ml binding of bupivacaine was 85.8 +/- 1.5% (n = 4). 2 Bupivacaine and alpha 1-acid glycoprotein concentrations were measured in plasma samples collected from a maternal peripheral vein and the umbilical vein at delivery (n = 23). The ratio of the foetal:maternal bupivacaine concentrations ranged from 0.17 to 0.52, while the foetal:maternal ration for alpha 1-acid glycoprotein concentrations ranged from 0.20 to 0.96. A positive relationship emerged between the two ratios (P less than 0.01). 3 The alpha 1-acid glycoprotein concentration gradient across the placenta, and interindividual variability in the gradient appear to contribute to the low and variable transplacental bupivacaine concentration ratio observed.