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 maldistribution of hyperbaric 5% lidocaine, are believed to have contributed to at least 11 cases of cauda equina syndrome, The authors investigated the distribution characteristics of hyperbaric dye solutions via caudally directed side-port needles at various rates of injection in a spinal cord model to determine the potential for maldistribution. Methods: Using a digital video image processing technique, we injected a hyperbaric solution of phthalocyanine blue dye through caudally directed side-port needles into a supinely oriented transparent spinal canal model filled with simulated cerebrospinal fluid. Injections via commonly used 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). Injection rates greater than or equal to 6 ml/min (2 ml/20 s) resulted in peak dye concentrations of less than 168 mg/l (extrapolated concentration of 1% lidocaine). Injection via the 24-gauge Sprotte needle, which has a larger orifice area and internal diameter, resulted in significantly lower peak dye concentrations than via the smaller Whitacre needles tested (P < 0.05). Conclusions: Sacral maldistribution could be minimized 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 study, When very slow injection rates (2 ml/min) are used, peak dye concentrations varied inversely and significantly with needle internal diameter and orifice area.