Hadzic's Regional Anesthesia > Part III. Clinical Practice of Regional Anesthesia > Section Five. Upper Extremity Nerve Blocks > Chapter 22. Functional Evaluation of Motor Responses for Upper Limb Blocks André P. Boezaart, MD, PhD, Steven C. Borene, MD

Functional Evaluation of Motor Responses for Upper Limb Blocks: Introduction

"A nerve stimulator delivers a charge to a motor nerve fiber, which causes a flow of ions through the nerve membrane and initiates an action potential in the nerve fiber.¹ The larger the delivered charge, the more motor fibers in the motor nerve fascicle will fire and the stronger the resulting motor response until a maximal motor response is reached. Several factors influence the total charge delivered to a nerve fiber. The duration and intensity of the current are the main variables. The impedance of the tissue between the stimulating surface (needle or catheter) and the nerve further influence the current intensity. This is reduced as the distance from the stimulating surface to the nerve is reduced. In addition, the type of tissue between the stimulating surface and the nerve, the type of electrode used, and the polarity of the electrode affect the total impedance of the system and thus affect the motor response to stimulation. Muscle mass is also important, such that a stimulus in a frail 80-year-old woman differs substantially from a motor response from a similar electrical response delivered to a young muscular patient.¹..."