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Online ISSN 1827-1596
SMART 2006 - Milan, May 10-12, 2006
Urmey W. F.
Hospital for Special Surgery Weill Medical College of Cornell University New York, NY, USA
Conventional methodology for nerve location utilizes anatomical landmarks followed by invasive exploration with a needle to a suitable endpoint. An appropriate endpoint can be either anatomical in nature (e.g. transaterial technique) or functional (paresthesia or motor response to electrical stimulation). Ability to electrically stimulate a peripheral nerve or plexus depends upon many variables, including; 1) conductive area at the electrode, 2) electrical impedance, 3) electrode-to-nerve distance, 4) current flow (amperage), and 5) pulse duration. Electrode conductive area follows the equation R=ρL/A, where R=electrical resistance, p= tissue resistivity, L=electrode-to-nerve distance, and A=electrode conductive area. Therefore resistance varies to the inverse of the electrode’s conductive area. Tissue electrical impedance varies as a function of the tissue composition. In general, tissues with higher lipid content have higher impedances. Modern electrical nerve stimulators are designed to keep current constant, in spite of varying impedance. The electrode-to-nerve distance has the most influence on the ability to elicit a motor response to electrical stimulation. This is governed by Coulomb’s law: E=K(Q/r2) where E=required stimulating charge, K=constant, Q=minimal required stimulating current, and r=electrode-to-nerve distance. Therefore, ability to stimulate the nerve at low amperage (e.g. <0.5 mA), indicates an extremely close position to the nerve. Similarly, increasing current flow (amperage) increases the ability to stimulate the nerve at a distance. Increasing pulse duration increases the flow of electrons during a current pulse at any given amperage. Therefore, reducing pulse duration to very short times (e.g. 0.1 or 0.05 ms) diminishes current dispersion, requiring the needle tip to be extremely close to the nerve to elicit a motor response. The above parameters can be varied optimally to enhance successful nerve location and subsequent blockade. Unlike imaging modalities such as ultrasonography, electrical nerve stimulation depends upon nerve conduction. Similarly, percutaneous electrode guidance (PEG) makes use of the above variables to allow prelocation of the nerve by transcutaneous stimulation.