Ultrasound controlled nerve stimulation in the elbow region: high currents and short distances needed to obtain motor responses

Abstract

Electrical nerve stimulation is commonly used to perform peripheral nerve blocks. The purpose of this study was to investigate the relation between stimulating currents and the distance between the needle-tip and stimulated nerves. In 18 volunteers the radial and ulnar nerves were stimulated in the elbow region from opposite directions. Needle-to-nerve distances were measured by high-frequency ultrasound when motor responses to electrical stimulation were elicited with currents of 5, 2, 1 and 0.5 mA and impulse widths of 0.1 ms. Additionally, current thresholds for nerve stimulation were identified when the needle-tip was in contact with the nerve. The ulnar nerves responded to electrical stimulation from a significantly greater distance than the radial nerves. Threshold levels at needle-to-nerve contact ranged from 0.4 to 4.5 mA for the radial nerve and from 0.32 to 2.0 mA for the ulnar nerve. They were significantly lower for the ulnar nerve than for the radial nerve. Currents required to obtain neuromuscular responses often exceeded the recommended current levels for nerve stimulation. Our results show significant differences in the ease of stimulation between the radial and ulnar nerves. High current thresholds and short nerve-to-needle distances were often needed to obtain neuromuscular responses in two nerves in the elbow region.