The influence of temperature and speed of injection on the distribution of a solution containing bupivacaine and methylene blue in a spinal canal model.

Three milliliters of a solution containing 4.81 mg bupivacaine base and 0.029 mg methylene blue per milliliter (BMB) was injected in the middle of a vertically mounted spinal canal model containing 0.9% NaCl at 37 degrees C. The BMB solution injected was either equilibrated to 37 degrees C (Exp. I) or to 22 degrees C (Exp. II). Each experiment was conducted eight times, four times with a high speed of injection (+/- 0.6 ml/sec) and four times with a slow speed of injection (0.05 ml/sec). The density of the BMB solution was determined at 37 degrees C and at 22 degrees C and found to be, respectively, slightly hypobaric and slightly hyperbaric relative to the 0.9% NaCl solution of 37 degrees C. Three minutes after completion of the injection, nine 1-ml samples were drawn simultaneously from the site of injection and from eight sampling sites situated equally above and below the site of injection at 5-cm intervals, which were subsequently analyzed for methylene blue concentrations. Injection of the BMB solution equilibrated to 37 degrees C resulted in a distribution directed mainly upward, whereas injection of the BMB solution equilibrated to 22 degrees C showed distribution in a mainly downward direction. Variation in methylene blue concentrations was large, and no definite differences based on different speeds of injection were observed. It is concluded that small differences in baricity result in largely different distribution patterns that could explain the variability in sensory levels of blockade when using an isobaric solution for spinal anesthesia.