Density, specific gravity, and baricity of spinal anesthetic solutions at body temperature

One of the most important physical properties affecting the level of analgesia achieved after intrathecal administration of a local anesthetic is its density relative to the density of the cerebrospinal fluid (CSF) at 37 degrees C. In this study, density, specific gravity, and baricity of local anesthetic solutions at body temperature were determined volumetrically to five significant figures. Standard solutions of 2% lidocaine, 0.5% and 0.75% bupivacaine, and 0.9% sodium chloride were tested. Bupivacaine, 0.75%-water dilutions and tetracaine, 0.2%, in water were also studied. The densities of all commercially prepared solutions were less than that of the normal range of CSF at 37 degrees C. The density of 0.2% tetracaine was the same as water. Continued dilution of 0.75% bupivacaine with water resulted in increasingly hypobaric solutions. However, only the 0.075% bupivacaine (1:9 dilution) solution had a density comparable to water. Knowledge of relative baricities aids in the selection of an appropriate local anesthetic for intrathecal use.