Electrical nerve localization: effects of cutaneous electrode placement and duration of the stimulus on motor response.
BACKGROUND
Recommendations regarding the technical aspects of nerve stimulator-assisted nerve localization are conflicting. The objectives of this study were to determine whether the placement of the cutaneous electrode affects nerve stimulation and to determine the duration and intensity of an electrical stimulus that allows nerve stimulation with minimal discomfort.


METHODS
Ten healthy volunteers underwent an interscalene and a femoral nerve block. After obtaining a clearly visible motor response of the biceps (interscalene) and quadriceps (femoral) muscles at the minimal current (0.1 ms, 2 Hz), the position of the cutaneous electrode was varied. Next, the duration of the stimulating current was set at 0.05, 0.1, 0.3, 0.5, or 1.0 ms, in random order. Intensity of the motor response and discomfort on stimulation were recorded.


RESULTS
The minimal current at which a visible motor response was obtained was 0.32 +/- 0.1 mA (0.23-0.38 mA) for the inter-scalene block and 0.29 +/- 0.1 mA (0.15-0.4 mA) for the femoral block. Changing the position of the return electrodes did not result in any change in the grade of the motor response or in the current required to maintain it. Currents of longer duration caused discomfort and more forceful contraction at a lower current intensity as compared with currents of shorter duration (P < 0.01). When the current was adjusted to maintain the same visible motor response, there was no significant discomfort among studied current durations.


CONCLUSIONS
Site of placement of the cutaneous electrode is not important when constant current nerve stimulators are used during nerve localization in regional anesthesia. There is an inverse relation between the current required to obtain a visible motor response and current duration. Selecting a current duration between 0.05 and 1.0 ms to specifically stimulate sensory or motor components of a mixed nerve does not seem to be important in clinical practice.
Hadzic
Admir
A
Department of Anesthesiology, St. Luke's-Roosevelt Hospital Center, College of Physicians and Surgeons, Columbia University, New York, New York 10025, USA. admir@nysora.com
Vloka
Jerry D
JD
Claudio
Richard E
RE
Hadzic
Nihad
N
Thys
Daniel M
DM
Santos
Alan C
AC
D000328:Adult; D000704:Analysis of Variance; D004566:Electrodes; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009407:Nerve Block / Q000295:instrumentation / Q000379:methods; D004561:Transcutaneous Electric Nerve Stimulation / Q000295:instrumentation* / Q000379:methods*