PRINCIPLES OF PERIPHERAL NERVE STIMULATION

The reaction of a single muscle fiber to a stimulus follows an all-or-none pattern. In contrast, the response of the whole muscle depends on the number of muscle fibers activated. If a nerve is stimulated with sufficient intensity, all fibers supplied by the nerve will react, and the maximum response will be triggered. After administration of a neuromuscular blocking drug, the response of the muscle decreases in parallel with the number of fibers blocked. The reduction in response during constant stimulation, reflects the degree of neuromuscular blockade.

For the preceding principles to be in effect, the stimulus must be truly maximal throughout the period of monitoring; therefore, the electrical stimulus applied is usually at least 20% to 25% above that necessary for a maximal response. For this reason the stimulus is said to be supramaximal; however, supramaximal electrical stimulation hurts, which is not a concern during anesthesia, but during recovery the patient may be awake enough to experience the discomfort of nerve stimulation. Therefore, some researchers advocate stimulation with submaximal current during recovery. Although several investigations indicate that testing of neuromuscular function can be reliably performed postoperatively using submaximal stimulation, ^{[9] [10] [11]} the accuracy of the monitoring is unacceptable at low current.^[12]