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Peripheral Nerve Stimulators for Regional Anesthesia Can Generate Excessive Voltage Output with Poor Ground Connection

2000; Lippincott Williams & Wilkins; Volume: 91; Issue: 5 Linguagem: Inglês

10.1097/00000539-200011000-00049

1526-7598

Admir Hadžić,

Neurological disorders and treatments

To the Editor: Compared to paresthesia technique, peripheral nerve stimulators (PNS) have been associated with less patient discomfort during placement of peripheral nerve blocks. Short bursts (e.g., 100 μsec–200 μsec) of stimulating current of low intensity (e.g., 0.2 mA to 5.0 mA) more readily stimulate large A/D (motor) fibers rather than C (pain) fibers (1). Nevertheless, when performing peripheral nerve stimulator-assisted nerve blocks, some patients still complain of burning pain during block placement. Since on a few occasions, we have noticed that grounding electrodes (EKG pads) were excessively dry, we postulated that the burning sensation could be related to an excessive compensatory voltage generated by the nerve stimulator presented with an increased impedance load. To determine the voltage output with poor grounding lead connection, we simulated an abnormally increased tissue impedance by measuring the output of a commonly used constant current nerve stimulator (Stimuplex-DIG, B. Braun Medical, Inc., Bethlehem, PA). The nerve stimulator was set to deliver 1 mA, 2 mA and 5 mA into 1 kΩ, 5 kΩ, 10 kΩ, 13 kΩ and 15 kΩ loads (Resistance Substitution Set, Model 236A, Phipps & Bird, Inc., Richmond, VA). The measurements were performed in our laboratory using a factory calibrated oscilloscope (Fluke DigiMeter 123, Everett, WA). The impedance of several grounding (EKG) electrodes which appeared dry on inspection ranged from 5 kΩ to ∞ Ω. Despite the increase in impedance, the nerve stimulator delivered the target currents by generating higher voltage output. Of note, with impedances of 10 kΩ and 15 kΩ, the voltage output reached 50 Volts and platoed at 70 volts, respectively (Figure 1).Figure 1: When set at 5.0 mA, a nerve stimulator can deliver nearly 70 volts to the impedance load of 15 kΩ.Modern peripheral nerve stimulators used in regional anesthesia are typically of a constant current generator type, such as the tested unit. While the current output used for clinical purposes is relatively small (≤ 5.0 mA), the compensatory voltage output generated by the constant current circuitry in case of abnormally high impedance can reach 70 Volts or higher. The output voltage increases with increasing impedance between the leads, resulting in high current density at the point of high impedance in the circuit (e.g., dry EKG electrode, poor skin-electrode contact, or the tip of the needle). This in turn, may result in local heat production and discomfort or pain. Thus, the condition of the grounding electrode and its attachment to the skin should be carefully inspected when performing nerve stimulator-assisted peripheral nerve blocks. Incorporating an indicator of abnormally high impedance in the future circuit designs of nerve stimulators could add additional useful clinical information and help explain or prevent patient discomfort. Admir Had˘zić MD, PhD Jerry D. Vloka, MD, PhD