Abstract

Local anesthetic solutions of varying density and patient position are used to control the dermatomal spread of anesthesia after the intrathecal injection of local anesthetics. Density varies inversely with temperature. At room temperature (21°C), local anesthetic solutions that are believed to be isobaric with cerebrospinal fluid (CSF) are approximately 0.004 g/ml more dense than CSF at body temperature (37°C). The authors investigated the potential significance of this temperature-related difference in density in a simple spinal canal model. The model was a straight piece of plastic tubing containing normal saline. The authors injected 0.5% plain bupivacaine into the tube and aspirated samples 20 minutes after administration. Studies were conducted with the bupivacaine and the saline at the same temperature (21°C) and with the bupivacaine at 4°C when the saline was 21°C. To determine the role of gravity on the distribution of the bupivacaine in the model, the experiments were carried out with the model in horizontal and vertical positions. Bupivacaine 0.5%, which is isobaric with normal saline, was not affected by gravity when both solutions were at the same temperature (21°C). However, 0.5% bupivacaine consistently behaved as a hyperbaric solution when it was at a lower temperature (4°C) than the saline (21°C). These observations suggest that temperature-related changes in density may affect the distribution of local anesthetics in CSF during spinal anesthesia.