Abstract ID: A6

Abstract Title: Experimental Neural Stimulation and Abolition of Motor Response after Extra and Intraneural Local Anesthetic Injection

Authors: Carlos B¹, William U², Luis V³, Fernando C⁴
Instituto Argentino de Diagnostico y Tratamiento Capital Federal Buenos Aires Argentina¹, Hospital for Special Surgery New York New York USA², University of Kentucky, medical Center Lexington Kentucky USA³, Hospital Universitario Austral Pilar Buenos Aires Argentina⁴
Poster Type: Either

ABSTRACT BODY

Introduction: Our objective was to experimentally test the hypothesis that a paresthesia might be obtained without a MR (1) by needle contact with or penetration of the nerve. In addition we examined the mechanism responsable for abolition of the MR after LA injection (2, 3)
Methods: After ethics approval, three dogs (mean 35 kg) were anesthestized with IM/IV ketamine, while spontaneous breathing was maintained with 1-2% isofluorane. A skin incision was made at a point between the great trochanter and the ischium. Different planes were carefully dissected until the sciatic nerve was exposed. The dissected area was approximately 5 cm long, 2 cm wide and 1 cm deep.
Two PNS were employed, a Stimuplex HNS 11 (BBraun, Germany) and Multistim Vario (Pajunk, Germany) using a 24G Stimuplex (BBraun, Melsungen, Germany) insulated needle. Different steps were performed:
a) Point of the needle was positioned in contact with the nerve with PNS off. PNS set at 0.5 mA and 0.1 msec. MR obtained; 10 ml lidocaine 2% injected, immersing the nerve completely under the clear solution.
b) Dry nerve. Puncturing the nerve with the PNS off, and increasing the current output until the first MR was produced or 1 mA reached.
c) Local anesthetic solution injected into the nerve.
Results:
a) Twitch disappeared during the injection of the first 2 ml of LA solution “Raj's Sign”. No displacement of the nerve from the tip of the needle was observed. The nerve was contacted, pushed and displaced with the point of the needle under the clear solution. A new MR was obtained in contact with the nerve into the solution, increasing to 2.0 mA and 0.3 msec. After 5 minutes, the solution was dried, and a MR was obtained again and with the same intensity at 0.5 mA and 0.1 msec.
b) There was no motor response with the current output between 0 to 0.2 mA. MR appeared with current output greater than 0.2-0.3 mA.
c) MR disappeared after 2 ml and no displacement of the nerve was observed.
Discussion:
We demonstrated that after injecting a LA solution with an immobile technique, the point of the needle remained positioned in contact, or very close to the nerve. Twitch disappearance is better explained due to the presence of the solution itself. This changes the conditions for the propagation of the electric impulse (3).
When a nerve is contacted or punctured, paresthesias are elicited. In this study we assumed that paresthesia would have been elicited when the tip of the insulated needle was located in the dog’s sciatic nerve. Based upon these observations, it appears to be difficult to position a needle intraneurally in a mixed nerve without eliciting a MR. Disappearance of MR after intraneural LA injection added new evidence against using PNS during other forms of anesthesia.

References
1 Reg Anesth Pain Med. 2003 Sep-Oct; 28(5): 384-8.
2 Reg Anesth Pain Med 1984; 9:42; 43.
3 Reg Anesth Pain Med 2004; 29: 189-193.

ATTACHED FILES

Reg Anesth Pain Med 2005; 30(3):A6