[In electric nerve stimulation can the distance from the nerves be inferred from the intensity of muscle contraction? Possible parameters and sources of error]

P März
Regional-Anaesthesie 1990, 13 (7): 179-85
Electrical nerve stimulation is a useful tool in regional anesthesia; it assists in locating the nerves. This investigation deals with electrical resistances at adhesive electrodes attached to the skin. The influence of external electrical resistance on the stimulating impulse is demonstrated. A new type of nerve stimulator is described; which integrates a measuring device that indicates the electrical impulse actually flowing in the patient. With this device, relationships between stimulating impulse, intensity of muscle contraction, and distance of the puncture cannula from the nerve can be assessed. MATERIALS AND METHODS. The electrical curve I = f (R) was determined using 3 different nerve stimulators (R = 1 - 15k omega). On each of five test persons, five adhesive electrodes were attached to the skin 10 cm apart. The resistance was assessed between these electrodes and a subcutaneously inserted puncture needle. The right and left brachial plexuses of the 5 subjects were punctured, using the axillary approach. A synchronized video camera simultaneously recorded the following values: (1) electrical impulse; (2) corresponding muscle contraction; and (3) the position of the puncture cannula. A scale ranging from 0-5 was applied to define the strength of the muscle contractions. RESULTS. The external resistance as measured under clinical conditions may limit the output impulse of nerve stimulators. In this case, the exerting impulse is lower than that indicated by the appliance. This error can only be identified using an impulse-measuring device. Nerve stimulators not equipped with a measuring device increase the risk of malpuncture, potentially resulting in nerve lesions. Skin resistance at adhesive electrodes varied from patient to patient within a range of 1.1 to 8.2 K omega. Preparing the skin appropriately (wiping with sandpaper) decreased the resistance by only 6% during the first 30 min. Within a distance of 50 cm to the puncture needle, the position of the adhesive electrode did not play a significant role. Axillary puncture of the brachial plexus resulted in the following values: (1) A stimulating impulse (cross-wave) of 1 mA and 1 ms exerted at a distance of 4 mm to the nerve induced a contraction of strength 3. (2) Reducing the impulse at this site by one-half (0.55 mA) resulted in just-visible contractions (strength 1). (3) Advancing the cannula at a stimulating impulse of 0.55 mA inside the neurovascular sheath again produced contractions of strength 3. (4) Performing the puncture with a blunt needle and tracing a distinct resistance, the impulse of 0.6 mA elicited contractions of strength 2. The needle tip was still outside the vascular nerve sheath. If this resistance was overcome and the needle tip lay inside the neurovascular sheath, the impulse could be reduced by one-half (0.32 mA) to produce contractions of strength 2.

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