Nerve stimulators

Nerve stimulators are designed to administer electrical stimuli to peripheral nerves. The responses may be assessed in different ways. The strength of an electrical stimulus applied to a nerve is defined by its charge (coulombs, Q). This is equivalent to the product of the current passed (amperes, A) and the duration (seconds, s) of that pulse of current.

Most nerve stimulators employ constant current circuitry, in which the difference between the current set by the user and the actual current delivered is detected and the applied voltage is automatically adjusted to minimize any disparity. This situation may arise when a dried-out skin electrode causes high impedance and consequently a low initial current flow.

Assessment of neuromuscular blockade

During assessment of neuromuscular blockade a supramaximal stimulus (i.e. one in which all axons in the nerve are made to discharge) is administered to a motor nerve through a pair of ordinary adhesive silver/silver chloride electrodes, applied to the overlying skin. Typically, the stimulus pulse duration might be 0.2 ms, the current 60 mA and the resulting charge, 12 µC. With pulses shorter than 0.2 ms, the amplitude required to generate a supramaximal stimulus may exceed the capacity of the stimulator, while pulses longer than 0.3 ms run the risk of repetitive nerve or muscle stimulation. Higher charges than are classically used may be required to guarantee supramaximal stimulation in nerves with prolonged conduction times (e.g. in diabetic neuropathy).¹ The charge required to deliver a supramaximal stimulus is less when the negative electrode is placed distally.²

Basic hand-held devices provide a stimulus with a fixed current and pulse width and permit a limited range of stimulation patterns (Fig. 17.1). Other models have liquid crystal screens and user-variable current, enabling both neuromuscular monitoring and nerve localization.

Figure 17.1 RS6 neuromuscular monitor (G. Rutter Ltd). Current (60 mA) and pulse width (0.25 ms) are fixed. A train-of-four, double-burst or tetanic stimulus may be selected. The leads may be removed to reveal probes (as shown) for direct application to moistened skin.

The motor response evoked by peripheral nerve stimulation may be measured in different ways, but is typically evaluated visually. Usually, contraction of adductor pollicis is observed following ulnar nerve stimulation at the wrist. The facial nerve in the cheek is also used, but direct muscle stimulation may contribute to the resulting contraction. More advanced devices attempt to quantify the (force of) muscular contraction elicited by peripheral nerve stimulation.

Currently available monitors allowing objective assessment of the motor response to neural stimulation use accelerometric or kinemyographic technology.

Accelerometry/acceleromyography

The basis of acceleromyography is that force is the product of mass and acceleration.³ During the assessment of neuromuscular blockade, mass is relatively unchanged so acceleration is proportional to force. An accelerometer comprises a small mass suspended on a strain gauge within a ‘box’ attached to the accelerating object to be studied, the acceleration then being derived from the force exerted on the strain gauge.

A number of accelerometric devices are available, such as the TOF-Watch (Organon Teknika, Fig. 17.2).

Figure 17.2 The TOF-Watch monitor (Organon Teknika). Stimulating current from 0–60 mA is set by the user. Accelerometric assessment of neuromuscular blockade or nerve localization may be performed. For the former, the device is calibrated after induction and before administration of muscle relaxant. (1) Stimulating electrodes applied over ulnar nerve; (2) accelerometric transducer taped to thumb; (3) display screen indicating train-of-four ratio: when less than four responses are detected, the number is indicated instead.

Piezoelectric methods (‘Kinemyography’)

When compressed or distorted, piezoelectric materials produce a charge proportional to the degree of alteration in shape. The M-NMT module for the S/5 anaesthesia monitor (GE Healthcare) uses a piezoelectric sensor incorporated into a clip placed on the patient’s thumb and index finger (Fig. 17.3). Thumb movement on stimulation is converted to an electrical signal and displayed graphically as a proportion of the maximal response. Monitoring, therefore, needs to be started before neuromuscular blockade but after anaesthetic induction. The module initially determines the current required for a supramaximal stimulus and sets a reference response level. By using different patient interface attachments the module can also be used for electromyographic recording or for nerve localization.

Figure 17.3 The mechanosensor for the Datex–Ohmeda M-NMT neuromuscular transmission module. (1) Stimulating electrodes applied over ulnar nerve; (2) piezoelectric mechanosensor.

Other methods

Mechanomyography, formerly the ‘gold standard’ for the assessment of neuromuscular blockade, has long disappeared from the clinical arena, as has electromyography. They were described in previous editions of this textbook. The general view is that, the above less-cumbersome methods are adequate for clinical purposes.⁴

Comparative studies suggest that the methods above do not give interchangeable results in assessing response to a train of four (TOF) stimulus.⁵ TOF measurements indicate residual blockade more consistently if compared with baseline readings, which should be obtained before paralysis.^6,⁷ Some authors recommend the use of ‘preload’ on the thumb to improve the accuracy of the measurement.⁴

Irrespective of the technology for assessing muscular response to nerve stimulation, numerous differing patterns of stimulation (e.g. train of four ratio, double burst stimulation, post-tetanic count, etc.) can be used in order to garner greater information regarding the extent and character of neuromuscular blockade. The details of these are in the remit of textbooks of physiology and of general anaesthesia.

Nerve stimulators for regional anaesthesia

Despite the growing use of ultrasound (see Chapter 13), there remains a need for peripheral nerve stimulators designed specifically for localization of nerves during regional anaesthesia (see Fig. 13.7). A positive surface electrode is applied to the skin and the negative electrode cable is plugged into a unipolar stimulating needle.

User-adjustable constant current is necessary and the ability to adjust stimulus frequency and duration is desirable. Usually an initial current of 1–2 mA (compare with currents needed for transcuteaneous stimulation of ∼60 mA) with a frequency of 2 Hz is appropriate as the needle is advanced toward the nerve. Lower frequencies increase the risk of going past the nerve, but may be desirable to reduce discomfort secondary to movement in injured patients.

Once contractions of the relevant muscle are seen, the current can be reduced. The stimulator-delivered charge required to generate a nerve impulse is proportional to the square of the distance from needle tip to nerve (by Coulomb’s law). Advantage can be taken of this phenomenon during neuraxial blockade in children, where a nerve stimulator attached to an insulated epidural needle may help determine whether the tip is epidural or intrathecal.⁸ For peripheral nerve blocks, traditional teaching is that injection of local anaesthetic is likely to be effective at the point where a response is still seen at a stimulating current of ≤0.3 mA.⁹ Recent work, however, suggests a similar probability of successful nerve block in paediatric patients whether the stimulation threshold is below or above 0.5 mA.¹⁰

Stimulus duration is adjustable on many devices. This should generally be set at less than 300 µs to avoid repetitive stimulation, though up to 1.0 ms may be required in neuropathic nerves, even with high currents.¹¹ Because chronaxie (the minimum stimulus duration required to elicit a response) is longer in sensory than in motor nerves, pulses longer than 0.15 ms are selected when attempting to locate a purely sensory nerve by inducing paraesthesia.

The accuracy with which peripheral nerve stimulators deliver the selected current tends to deteriorate as the current decreases. Manufacturers often quote accuracy of typically 1–5%, for a current setting of 1 mA. Amplitudes used for peripheral nerve localization, however, are less than 0.5 mA, where variability may be more than 80%.¹² A stimulator which delivers less than the set current may increase the risk of nerve damage during peripheral nerve block – with one that delivers more, a failed block may be more likely.

Nerve blocks carry a small risk of intraneural local anaesthetic injection with concomitant damage. Some nerve stimulators currently under evaluation calculate the electrical impedance (EI) of the circuit through the patient. This might be used to reduce the risk of intraneural injection, EI being higher when the stimulating needle is intraneural.¹³ The exact mechanism for this finding is yet to be elucidated and is controversial.¹⁴

Transient pacemaker inhibition by peripheral nerve stimulators has been reported,¹⁵ as has interference with the display of pacemaker impulses on the ECG monitor.¹⁶

Assessment of neural integrity

Peripheral/cranial nerves

Nerve stimulators may be used intraoperatively to help assess the integrity of peripheral nerves. In many instances the distal contractile response to a proximal stimulus, applied to an intact motor nerve, may be assessed by palpation. Stimulation of motor nerves may be useful to confirm neural function in surgery around the facial nerve, or in thyroidectomy to identify the recurrent laryngeal nerve. For the latter, electrode-embedded endotracheal tubes with attached electromyographic monitors have been employed with variable success.¹⁷

Spinal cord/nerve roots

Somatosensory evoked potentials

The incidence of neurological damage, secondary to traction and ischaemia of the spinal cord during scoliosis correction is approximately 0.5%. This may be reduced by monitoring somatosensory evoked potentials (SSEP).¹⁸ The monitors are complex and require an experienced neurophysiologist. Typically a pair of stimulating electrodes is placed over each posterior tibial nerve at the ankles for lumbar surgery, or over the median nerves for cervical surgery. Stimuli are administered at about 30 mA and 5 Hz. Recording electrodes may be applied at various points adjacent to the ascending tracts and proximal to the site of surgery. Usually two or more scalp electrodes are used (e.g. one frontal and one cervical), together with a reference and a ground electrode. The recorded responses are passed through a digital signal converter and a band-pass filter (20–1000 Hz). An average response is calculated from as many as 200 individual sweeps and the result displayed on the monitor screen. The period of interest is the first 100 ms after the stimulus, during which a characteristic W-shaped potential is seen in recordings from cortical electrodes (Fig. 17.4). Response latency also depends on the distance between the point of stimulation and the recording electrode.

Figure 17.4 Somatosensory evoked response. The stimulating electrode is over the median nerve. Recording electrodes are over A. the somatosensory cortex and B. the seventh vertebra. The sequential peaks and troughs in the evoked response are named according to their latency (ms) and direction. Particular periods of the response correspond to specific neuro-anatomical regions as shown.

(Thornton C, Sharpe RM (1998) Evoked responses in anaesthesia. British Journal of Anaesthesia 81: 771–781, © The Board of Management and Trustees of the British Journal of Anaesthesia. Reproduced by permission of Oxford University Press/British Journal of Anaesthesia.)

A baseline SSEP is recorded before induction of anaesthesia. If subsequent intraoperative spinal manipulation completely prevents conduction of the ascending impulse, the SSEP disappears altogether. When conduction is merely impaired, an increase in SSEP latency and a decrease in amplitude are observed. A 50% increase in latency compared to baseline would be regarded as a critical change and the operator would inform the surgeon accordingly. Surgical disruption apart, a number of factors affect SSEPs, including volatile anaesthetic agents, nitrous oxide, hypothermia, hypoxia and hypotension.