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July 1997
Hyperbaric dye solution distribution characteristics at the pencil-point needle injection in the spinal cord model.
Holman SJ, Robinson RA, Beardsley D, et al.
Anesthesiology 1997; 86:966-73.
[ see abstract below ]
Neurologic injury has been reported after continuous spinal anesthesia with hyperbaric local anesthetics, usually 5% lidocaine in 7.5% dextrose. The flow-rate limiting and directional characteristics of caudally directed microcatheters may lead to an intrathecal maldistribution of local anesthetic, which may be responsible for the findings. Similar transient ridicular irritation has been reported after the use of pencil-point needles.
These authors investigated the distribution characteristics of hyperbaric dye solutions given through caudally directed sideport needles in order to determine the potential for maldistribution, using a digital imaging processing technique to visualize the dye path.
They injected a hyperbaric solution of blue dye through the needles into a supinely oriented transparent spinal model filled with simulated cerebrospinal fluid. The authors tested the 24 and 25 gauge Sprotte and the 25 and 27 gauge Whitacre needles, and they recorded peak dye concentrations resulting after 5 different injection rates ranging from 2-16 ml/min.
For all needles tested the injection rate had a significant effect on the peak dye concentration. Injection rates 6 ml/min (2ml/20sec) resulted in a peak dye concentration equivalent to 1% lidocaine. Injection through the 24 ga Sprotte needle, which has a larger orifice area and internal diameter, resulted in a significantly lower peak dye concentration than through the smaller Whitacre needles.
When very slow injection rates were used (2 ml/min), peak dye concentrations inversely with needle internal diameter and orifice area. These authors suggest that using an injection rate > 6ml/min may be advantageous to avoid local anesthetic pooling and maldistribution. They acknowledge however, this may increase the spread of anesthesia from the injection, and provide a more widespread block.
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ABSTRACT
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 > or = 6 ml/min (2 ml/20 s) resulted in peak dye concentrations of less than 168 mg/1 (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 > or = 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.
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