ARTERIAL AND PULMONARY ARTERIAL CONCENTRATIONS OF THE ENANTIOMERS OF BUPIVACAINE AFTER EPIDURAL INJECTION IN ELDERLY PATIENTS

Bupivacaine HCl is a 50:50 racemic mixture of the levo [S(-)] and dex [R(+)] enantiomers. The R(+) enantiomer exhibits greater cardiac tissu e binding and toxicity. To determine whether the lung exhibits selecti ve uptake of one of the enantiomers of bupivacaine, we measured pulmon ary artery and radial artery blood concentrations of the two enantiome rs after a lumbar epidural injection of 20 mL of 0.75% bupivacaine in 10 elderly patients undergoing one-stage bilateral total knee arthropl asty. Significantly lower concentrations of R(+) than S(-) were noted in both pulmonary artery and arterial blood. Both enantiomers were abs orbed by the lung to a similar extent within the first 5 min after epi dural injection (extraction ratio approximately equal to 0.1 or 10%). Mean time of maximal concentration (T-max) was 6 min. In 3 of the 10 p atients, T-max occurred in 1-3 min. We conclude that the lung absorbs both the R(+) and S(-) enantiomers of bupivacaine to a similar extent after epidural injection and that this is of doubtful clinical signifi cance. This study also suggests that peak concentrations of bupivacain e may occur earlier after epidural injection in certain elderly patien ts than previously believed. Implications: In the first 5 min after ep idural injection, approximately 10% of the local anesthetic bupivacain e was absorbed by the lung. Absorption of the two enantiomers (mirror images) of bupivacaine were similar. Lung absorption of bupivacaine is unlikely to influence local anesthetic toxicity.