Percutaneous Nerve Stimulation as a Means to Assess Nerve Block Effectiveness

Background and rationale Peripheral nerve blocks (PNBs) are highly effective methods of anesthetizing regions of the body for surgery. Throughout its history, the field of regional anesthesiology has lacked an objective, quantitative tool for assessing the effectiveness of a given block. Current methods for assessing blocks are mainly subjective and rely on patient feedback to determine how well a nerve is blocked; these methods include 1) sensory tests, such as ability to detect a pinprick or cold sensation, 2) motor tests, for example, ability to flex or extend the body part that has been blocked, and 3) verbal feedback by the patient. Although these methods usually provide a satisfactory assessment of the block, an objective tool that could decisively and consistently determine how well a nerve is blocked would avoid the ambiguity inherent to the subjective tests and may allow anesthesiologists to more precisely control pain during and after surgery.

Nerve stimulation technology represents one possible method for objectively assessing PNBs by eliciting paresthesia in the region of the body undergoing blockade. Nerve stimulation devices operate by sending an electrical current to a nerve, depolarizing it, and eliciting muscle contraction or paresthesia, depending on the type of nerve stimulated (motor or sensory, respectively). In theory, patients may exhibit a higher current perception threshold in regions of the body that have been anesthetized. Several nerve stimulation devices are commercially available, although they vary in cost, setup, and features. The Neurometer diagnostic stimulator is a percutaneous nerve stimulation device that records sensory nerve conduction thresholds and is commonly used to diagnose neuropathies associated with conditions such as carpal tunnel syndrome, diabetes, and spinal cord lesions. The Neurometer generates constant alternating current sinusoid waveform stimuli at three different calibrated frequencies: 2000 Hz, 250 Hz, and 5 Hz with a range of stimulus output intensities from 0.01 to 9.99 milliAmperes (mA). The current is delivered to the skin surface via a pair of 1 cm diameter gold electrodes separated by 1.7 cm.

Other devices, such as peripheral nerve stimulators, are commonly connected to insulated needles or stimulating catheters to locate nerves to be blocked. When connected to a surface electrode probe, a peripheral nerve stimulator delivers an electric current that elicits motor or sensory activity as the current stimulates the appropriate target nerve. The device generates square pulses with selectable stimulus duration and continuously adjustable stimulation current. The pulse current can be adjusted to appropriate levels, allowing extremely precise adjustments of the stimulation current. The stimulation frequency and stimulus duration can be varied for different applications. The nerve stimulator features the option of selecting a frequency of 1 Hz or 2 Hz for sequential electrical nerve stimulation and stimulus duration of 0.05 milliseconds (ms) to 1 ms.

Although the Neurometer represents the gold standard in percutaneous nerve stimulation, it is expensive, making it impractical for use in everyday clinical applications. We hypothesize that a common peripheral nerve stimulator device can be used as a functional substitute for the Neurometer in generating a sensory stimulus to assess PNB effectiveness. The aim of this study is to compare the Neurometer and a common nerve stimulator to assess anesthesia following PNB. We predict that the nerve stimulator can be used successfully as a substitute for the Neurometer in assessing PNBs in patients receiving regional anesthesia of the extremities.

Study objective To compare the Neurometer and a common peripheral nerve stimulator in measuring current perception threshold in patients receiving PNB.