The effect of foetal acidosis on bupivacaine levels in utero.

Amide local anaesthetics are weak bases with pK's of 7.80 or greater. Therefore, tissue acidosis may result in ionization and "trapping" of the basic local anaesthetics The following study was done in the pregnant ewe to determine if the highly protein-bound local anaesthetic bupivacaine demonstrates ion-trapping in the acidotic foetus. Six pregnant ewes of 135 days gestation were prepared surgically with catheters placed in maternal and foetal femoral arteries an veins. Bupivacaine was infused into the maternal femoral vein to maintain a constant concentration. After two hours of bupivacaine infusion the foetus was made acidotic by an infusion of lactic acid for 45 minutes. Then the acidosis was corrected by an infusion of bicarbonate for an additional 45 minutes. Maternal and foetal bupivacaine levels were measured at 15 minute intervals throughout the experiment. The bupivacaine levels in the foetus and the foetus-maternal ratio increased significantly during the period of foetal acidosis and declined to the control levels when the acidosis was corrected with bicarbonate. We conclude from this study that foetal acidosis results in ion-trapping of bupivacaine to a significant extent, despite protein binding in the maternal blood.