IN-VITRO MODELING OF SPINAL-ANESTHESIA - A DIGITAL VIDEO IMAGE-PROCESSING TECHNIQUE AND ITS APPLICATION TO CATHETER CHARACTERIZATION

Background: Maldistribution of intrathecal local anesthetic has recent ly been implicated as a contributor to neurotoxic injury. In vitro mod eling can be used to understand the distribution of anesthetic agents within the subarachnoid space. We describe an in vitro modeling techni que that uses digital video image processing and its application to ca theter injection of local anesthetic. Methods: A clear plastic model o f the subarachnoid space, including a simulated spinal cord and cauda equina, was filled with lactated Ringer's solution. Phthalocyanine blu e dye of known concentration was injected into the model through small -bore (28-G) and large-bore (18-G) catheters. Injections were performe d at a variety of controlled rates and sacral catheter positions, and the propagation of dye throughout the model was recorded on videotape, digitized by computer, and converted to a two-dimensional image of dy e concentration. A subset of data was compared with results obtained f rom spectrophotometric analysis.Results: There was a strong correlatio n (r = 0.98) between data obtained with analysis by digital video imag e processing and those obtained spectrophotometrically. Catheter size, catheter angle, and injection rate significantly influenced the distr ibution and peak concentration of simulated anesthetic. No major diffe rences in distribution or peak concentration were observed with the tw o types of 28-G catheters. Conclusions: The digital video image proces sing technique can be used to quantify anesthetic distribution rapidly within a model of the subarachnoid space without disturbing the distr ibution. The current results demonstrate a strong dependence of anesth etic distribution on catheter angle, catheter size, and injection rate . Comparisons between 28-G catheters suggest that the difference in re ported incidence of cauda equina syndrome associated with different 28 -G catheters cannot be explained on the basis of differences in anesth etic distribution.