Plasma concentrations of lidocaine and bupivacaine after subarachnoid administration.

Plasma concentrations were measured after subarachnoid administration of 75 mg lidocaine in a hyperbaric solution and 15 mg bupivacaine in either a hyperbaric or an isobaric solution. Peak concentration times (tmax), peak concentrations (Cmax), apparent elimination half-lives (t1/2el), and areas under the curves (AUC) were determined. The values of tmax, Cmax, t1/2el and AUC were 66 +/- 32 min, 444 +/- 203 ng/ml, 2.0 +/- 0.3 h, and 87.0 +/- 9.8 mg . 1(-1) . min, respectively, for lidocaine. Those obtained after administration of a hyperbaric bupivacaine solution were 54 +/- 11 min, 63 +/- 22 ng/ml, 3.6 +/- 1.4 h, and 19.1 +/- 5.4 mg . 1(-1) . min. Results, obtained with isobaric bupivacaine solutions, were: 86 +/- 24 min, 63 +/- 15 ng/ml, 3.2 +/- 0.4 h, and 18.1 +/- 5.4 mg . 1(-1) . min. The results indicate roughly similar absorption rates from the subarachnoid space into the general circulation after administration of hyperbaric solutions of lidocaine and bupivacaine. Bupivacaine tended to be absorbed slower when it is administered in an isobaric solution.