Abstract ID: A2

Abstract Title: INTENSITY OF THE STIMULATING CURRENT IS NOT A RELIABLE INDICATOR OF INTRANEURAL NEEDLE PLACEMENT

Authors: Tsai T¹, VUCKOVIC I², ELDAN K³, KUCUK-ALIJA D⁴
St. Luke's-Roosevelt Hospital Center New York NY ¹, Medical University of Sarajevo Sarajevo Bosnia-Herzegovina², Medical University of Sarajevo Sarajevo Bosnia-Herzegovina³, Medical University of Sarajevo Sarajevo Bosnia-Herzegovina⁴
Poster Type: Discussion

ABSTRACT BODY

Introduction
The optimal intensity of the stimulating current during nerve localization is controversial. Nerve stimulation with currents of < 0.2 milliamperes (mA) at 0.1 milliseconds (msec) has been suggested to be associated with intraneural injection and possible neurologic injury. On the other hand, motor response to nerve stimulation has been reported to be absent 70% of the time when paresthesia (presumed needle to nerve contact) has been elicited with currents ≤ 1.0 mA. In this study, we hypothesized that intraneural placement of the needle does not result in consistent motor response to nerve stimulation with currents < 0.2 mA.

Materials and Methods
The study was conducted in accordance with the principles of laboratory animal care and was approved by the Laboratory Animal Care and Use Committee. Twenty pigs of both sexes and weighing between 20-25 kilograms were studied. After induction of general anesthesia the sciatic nerve (SN) was exposed bilaterally. Electrical current (Life-Tech Model NL-3) was applied using an insulated 25G needle (LifeTech, PB-25SCS) starting 2 cm, 1 cm, 0.5 cm, 0.2 cm and 0.1 cm away from the SN. The needle was also placed transepineurally and epineurally. Stimulation was started with a current intensity of 2.0 mA and decreased to the minimal current at which visible motor response was obtained. Blinded observers agreed on the strength of the response and classified the response as either a non-specific (local muscle or leg response) or specific distal response emanating from either division of the SN.

Results
At the distance of 2 cm away from the SN, there was no motor response stimulation of the SN elicited in any of the nerves. However, strong non-specific local response mimicking SN twitch was obtained in 50% of the nerves. Specific SN response with stimulation was obtained with the needle positioned at 0.1cm and on but not within the epineural sheath with a current intensity of 0.90 mA ¡À 0.36 (range 0.24-1.48 mA) and 0.4 mA ¡À 0.34 (range 0.15-1.4 mA), respectively. With needles placed intraneurally, motor response was obtained with a current of 0.64 mA ¡À 0.63 (range 0.08-1.8 mA).

Discussion
If our results in an experimental open SN stimulation model in pigs are applicable to clinical practice, nerve localization with currents > 1.0 mA can result in false motor response. While an intraneural placement of the needle can result in stimulation with as little as 0.08 mA, motor response may absent even with a current intensity of up to 1.7 mA. These findings may explain the reported lack of consistent motor response after eliciting paresthesia. In conclusion, the absence of motor response and current of low intensity does not rule out intraneural needle placement.

References
1. Voelckel, WG, Klima, G, Krismer AC, et al. Signs of Inflammation After Sciatic Nerve Block in Pigs. Anesth Analg 2005;101:1844-6.
2. Urmey WF, Stanton J. Inability to Consistently Elicit A Motor Response Following Sensory Paresthesia during Interscalene Block Administration. Anesthesiology 2002;96:552-4.
3. Mulroy MF, Mitchell B. Unsolicited paresthesias with nerve stimulator: case reports of four patients. Anesth Analg 2002;95:762-3.

ATTACHED FILES

Reg Anesth Pain Med 2005; 30(3):A2