With the growing use of ultrasound (US) in the practice of regional anesthesia, peripheral nerve stimulation (PNS) continues to be useful to monitor needle-nerve relationship to decrease the risk of nerve trauma. This chapter reviews electrical nerve stimulation and its role in the practice of peripheral nerve blocks (PNBs).
BASICS OF ELECTRICAL PERIPHERAL NERVE STIMULATION
Voltage, Current, and Resistance
Voltage (U) is the difference in electrical potential between two points carrying different amounts of positive and negative charges, measured in volts (V) or millivolts (mV). Voltage can be compared to the fill level of a water tank, which determines the pressure at the bottom outlet (Figure 4-1A).
Ohm’s law and the functional principle of a constant-current source. (A) Low-resistance R1 requires voltage U1 to achieve desired current I1. (B) High-resistance R2 = 2 × R1 causes current I to decrease to I2 = I1/2 if voltage U remains constant (U2 = U1). (C) Constant-current source automatically increases output voltage to U3 = 2 × U1 to compensate for higher-resistance R2; therefore, current I increases to the desired level of I3 = I1.
Current (I) is the measure of the flow of a positive or negative charge, expressed in amperes (A) or milliamperes (mA). The current can be compared to the amount of flow of water.
The electrical resistance (R) is the obstacle to the flow of electric current, measured in ohms (Ω) or kilo-ohms (kΩ). In other words, resistance limits the flow of current at a given voltage (see Ohm’s law).
Ohm’s law describes the relationship between voltage, current, and resistance according to the following equation:
This means that, at a given voltage, the intensity of the electrical current is dependent on the resistance between the two electrodes (in patients, the resistance of the skin and tissues between the grounding electrode and needle). Figure 4-1 (Ohm’s law) illustrates Ohm’s law and the functional principle of a constant-current source.
Impedance and Constant-Current Source
During PNS, the electrical circuit consists of the nerve stimulator, nerve block needle, needle tip design, patient’s tissue characteristics, skin, skin-electrode (grounding electrode), and cables. This circuit has a complex resistance (impedance) in living tissue because of the capacitance of the tissue, intravascular fluids, electrode-to-skin interface, and needle tip. The needle design and electrode-to-skin connection contribute a great deal to the overall impedance. The first largely depends on the geometry and insulation (conductive area), while the latter varies considerably among individuals (e.g., ...