Nerve localization techniques

A 62-year-old female presents for an open reduction and external fixation of a radius fracture. The surgeon is requesting that the procedure be performed with a peripheral nerve block. The hospital currently has both an ultrasound (US) machine and a nerve stimulator available for use. 1. Describe how nerve stimulation assists with nerve localization. List advantages and limitations to nerve stimulation. Discuss the use of ultrasound for nerve localization. Analyze the benefits and drawbacks to ultrasound guidance. Discuss benefits and drawbacks to combining nerve stimulation with ultrasound guidance. Present the evidence about the incidence and complications secondary to intraneural injections with both peripheral nerve block methods. Describe the development of new needle-guidance systems to complement ultrasound imaging. Describe how nerve stimulation assists with nerve localization Most peripheral nerves contain a mixture of sensory and motor fibers. Neural transmission occurs as a result of depolarization of cell membranes secondary to transmembrane ionic flux. The classical nerve action potential describes the electrophysiological changes secondary to rapid influx of positive sodium ions and repolarization secondary to slower potassium ion extrusion. Peripheral nerve stimulators are used to locate peripheral nerves. The physiological principle of peripheral nerve stimulation (PNS) is that the closer the needle tip to the nerve, the lower the current needed to stimulate a muscle contraction [1]. The peripheral nerve stimulator is battery powered, and configured so that current flows from the anode to the cathode, reducing the ionic gradient across the cell membrane [2]. Once an electrical threshold is reached, an action potential is generated. For clinical use, the EKG skin electrode is the grounded positive anode (colored red) and the tip of the needle is chosen as the negative cathode (colored black). Reversal of anode and cathode positions inhibits nerve depolarization because the intraneural electrical potential becomes more negative and resistant to firing. The skin electrode position does not influence block performance.