INTRANEURAL LIDOCAINE UPTAKE COMPARED WITH ANALGESIC DIFFERENCES BETWEEN PREGNANT AND NONPREGNANT RATS

Background and objectives. Pregnant patients need less local anestheti c in order to obtain the same quality of functional block as nonpregna nt patients. Our goal was to demonstrate a similarly increased functio nal susceptibility to local anesthetics in the awake pregnant rat duri ng peripheral nerve block and to investigate the pharmacokinetic and/o r pharmacodynamic mechanisms responsible for this phenomenon. Methods. Radiolabeled lidocaine uptake was determined in vivo during sci- atic nerve block with 0.1ml of 1% lidocaine in the nerves of nine pregnant and five nonpregnant female rats and six male rats at the return of d eep pain sensation, assessed by withdrawal of the hindlimb from a brie f squeeze of a digit with serrated forceps. During recovery from compl ete functional block, the time at which deep pain returned and the amo unt of lidocaine in the nerve at that time were compared among the thr ee groups of rats. Lidocaine content was also determined in vitro afte r exposure of ensheathed sciatic nerves from pregnant and nonpregnant rats to a 0.2% lidocaine bath for specified times. Results. Full block of function developed in all groups within 6 minutes of the lidocaine injection and lasted significantly longer in pregnant rats than in no npregnant and male rats (49.0 +/- 3.3 vs 34.0 +/- 3.1 and 32.0 +/- 1.3 minutes mean +/- SEMI, respectively. At the time of deep pain return, the intraneural lidocaine content of pregnant rats was significantly lower than that of nonpregnant and male rats (2.2 +/- 0.25 vs 3.9 +/- 0.7 and 3.7 +/- 0.6 nmoles/mg of wet nerve, respectively). No differen ce in lidocaine uptake kinetics between P and NP nerves was observed i n vitro. Conclusions. Block of peripheral neural function is prolonged in pregnant rats, and lidocaine content in the nerve is lower at a sp ecific stage of neural block. These results are consistent with a phar macodynamic mechanism for increased susceptibility to lidocaine neural block during pregnancy.