Prolongation of spinal anesthesia. Differential action of a lipid drug carrier on tetracaine, lidocaine, and procaine.

This study evaluates prolongation of spinal anesthesia by incorporating local anesthetics in lipid formulation. The duration and intensity of local anesthetic effect produced by different concentrations of procaine (1%, 2%, 4%), lidocaine (1%, 2%, 4%), or tetracaine (0.5%, 1%, 2%) dissolved in normal saline were compared to those produced by the same concentration of drugs in lipid (iophendylate) solution. Fifty rabbits with chronic indwelling subarachnoid catheters were divided into ten equal groups. Three days after the operation the catheters were injected with aqueous solutions of the anesthetics, and 24 h later each animal received an equivalent dose of the corresponding drug in free-base form dissolved in iophendylate. The duration and intensity of motor blockade were assessed using a modified Bromage scale. A separate group of animals received plain normal saline and, 24 h later, iophendylate alone. The Kruskal-Wallis test followed by the Tukey-type test for nonparametric multiple comparisons and the Mann-Whitney and Friedman tests were used for statistical analysis at P less than 0.05. Normal saline or iophendylate alone did not produce any motor blockade. Our data show that iophendylate preparations of local anesthetics produce prolonged but less intense motor blockade than the aqueous solutions, except for tetracaine 0.5% in iophendylate, which produced shorter duration of motor blockade. The reduced intensity of motor blockade may be explained by decreased availability of local anesthetic at the nerve tissue due to storage of drug in the lipid depot. The increased duration of blockade signifies a sustained release of drug from the depot.