HYPERBARIC DYE SOLUTION DISTRIBUTION CHARACTERISTICS AFTER PENCIL-POINT NEEDLE INJECTION IN A SPINAL-CORD MODEL

Background: The flow-rate limiting and directional characteristics of caudally directed microcatheters, which lead to intrathecal maldistrib ution of hyperbaric 5% lidocaine, are believed to have contributed to at least 11 cases of cauda equina syndrome, The authors investigated t he distribution characteristics of hyperbaric dye solutions via caudal ly directed side-port needles at various rates of injection in a spina l cord model to determine the potential for maldistribution. Methods: Using a digital video image processing technique, we injected a hyperb aric solution of phthalocyanine blue dye through caudally directed sid e-port needles into a supinely oriented transparent spinal canal model filled with simulated cerebrospinal fluid. Injections via commonly us ed spinal needles (24-gauge and 25-gauge Sprotte, and 25-gauge and 27- gauge Whitacre) were recorded using five injection rates (2, 4, 6, 8, and 16 ml/min), Results: For all needles tested, injection rate had a significant effect on the peak dye concentration (P < 0.0001). Injecti on rates greater than or equal to 6 ml/min (2 ml/20 s) resulted in pea k dye concentrations of less than 168 mg/l (extrapolated concentration of 1% lidocaine). Injection via the 24-gauge Sprotte needle, which ha s a larger orifice area and internal diameter, resulted in significant ly lower peak dye concentrations than via the smaller Whitacre needles tested (P < 0.05). Conclusions: Sacral maldistribution could be minim ized by using injection rates greater than or equal to 6 ml/min (2 ml/ 20 s), for all of the side-port spinal needles used in this model stud y, When very slow injection rates (2 ml/min) are used, peak dye concen trations varied inversely and significantly with needle internal diame ter and orifice area.