Anaesthetists routinely use hyperbaric local anaesthetic solutions to control the level of spinal anaesthesia. This practice is based on the assumption that when a local anaesthetic is mixed with dextrose and injected into cerebrospinal fluid, the local anaesthetic molecules will follow the heavy dextrose molecules. As a result the anaesthetic level can be controlled by controlling the position of the patient. To our knowledge, the veracity of this assumption has not been documented scientifically. Hyperbaric anaesthetic solutions in spinal anaesthesia were introduced in 1907 by Barker' who developed a glass replica of the vertebral column with several injection ports. When a hyperbaric local anaesthetic solution mixed with methyl violet dye was injected into this glass column, which was filled with "mock spinal fluid," the methyl violet always spread to the lowest point or points in the tube. Thus, when the tube was placed in the upright position (simulating the spinal canal in a sitting patient), the injected dye gravilated to the caudal end of the tube. On the other hand, when the tube was placed in a horizontal position (simulating the spinal canal in a supine patient) and the dye was injected at the level of the lumbar lordosis, it moved both cephalad and caudad as far as the thoracic and sacral curves. Ever since Barker's demonstration of this phenomenon, the baricity of a local anaesthetic and the position of the patient have been considered to represent the two major factors that determine the distribution of local anaesthetics in the subarachnoid space. 2 This concept is consistent with many clinical studies, all of which have shown the differences in CAN J ANAESTH 1991 / 38:4 / pp522-6
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