[Spread of hyperbaric local anesthetics in a spinal canal model. The influence of Trendelenburg position and spinal configuration]

Abstract

Background: The influence of Trendelenburg positions and variations in spinal canal configuration on the spread of hyperbaric spinal anesthetics was examined in two models of the subarachnoid space.

Methods: Both models included simulations of the spinal cord, filum terminale and cerebrospinal fluid. Model I had a straight shape, thus omitting replications of lumbar lordosis and thoracic kyphosis. It allowed the evaluation of fluid dynamics and the spread of 0.5% hyperbaric bupivacaine and 4% hyperbaric mepivacaine in 0° (supine position), 5° and 10° head-down tilt positions. Model II included reconstructions of average adult spinal curvatures for closer analysis of the intrathecal spread of 0.5% hyperbaric bupivacaine in 0°, 5°, 10° and 15° head-down tilt positions. Concentration gradients within the artificial cerebrospinal fluid were calculated using a digital image processing technique. Data from both model investigations were compared to elaborate the effect of varying lumbar lordosis angles.

Results: Model I: Only the 5° head-down tilt caused a significant difference in maximum spread of both local anesthetics. Model II: A 15° head-down tilt resulted in the local anesthetic solution spilling over lumbar lordosis and effusing into the thoracic areas. With increasing degree of head-down tilt, the local anesthetic solution was also detectable in ventral parts of the spinal canal cross-section.

Conclusions: Diffusion processes represent the decisive factor for distribution patterns of hyperbaric anesthetics in the supine position. Only the 5° head-down tilt demonstrated an influence of specific gravity. When tilted 10° head-down gravitation prevailed over differences in density. A 15° head-down tilt is a precondition for the mobilization of sacrally pooled local anesthetic. Data comparison of both model investigations showed that the extent of spread depends more on initial bidirectional distribution of the local anesthetic than on increasing flow rate due to the slope of lumbar lordosis.