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. 1981 The British Pharmacological Society