5. Peripheral Nerve Stimulators

two types of peripheral nerve stimulator: those which assess the degree of neuromuscular blockade + those which are used to aid accurate needle placement in regional analgesia

It is now considered mandatory to use a peripheral nerve stimulator to assess the degree of residual muscular blockade after any neuromuscular blocking drugs have been given. Clinical signs

grip strength, the generation of a tidal volume of between 15 and 20 ml kg1, the ability to keep the head lifted from the pillow for 5 seconds and the capacity to retain a tongue depressor gripped between the teeth are cited as useful, if crude indicators of recovery from neuromuscular block.

the degree of block can be assessed using a battery-operated nerve stimulator that is capable of delivering different patterns of square wave monophasic pulses of uniform amplitude.

The threshold current, which is the current required to elicit a detectable muscle response, is around 15 mA. In order to ensure recruitment of all the muscle fibres, a supramaximal impulse is delivered, typically of around 50–60 mA From Ohm’s law any increase in resistance (secondary to cool or greasy skin, for example) requires an increase in voltage to maintain a constant current. Modern nerve stimulators change the internal voltage to maintain a constant current over a range of different resistances. The different patterns of stimulation include the following.

Single supramaximal stimulus is delivered once every 10 seconds (0.1 Hz). A decrease in twitch height will be apparent only after 75% or more receptors are blocked and will disappear at 90% occupancy, so this is of limited use in monitoring non-depolarizing block. It can be used for assessing block caused by depolarizing relaxants (which do not exhibit fade or post-tetanic facilitation).

Four identical supramaximal stimuli are delivered at 2 Hz and repeated every 10 seconds. The number of twitches observed corresponds approximately to the percentage receptor blockade (0 twitches = 100% blockade, 1 twitch = 90%, 2 twitches = 80%, 3 twitches = 75%, 4 twitches = <75%). The ratio of twitch heights can be quantified to give an objective measure of block. The T4:T1 ratio must be 90% before it can be assumed that protective airway reflexes are intact.

two tetanic bursts at 50 Hz and separated by 750 ms are applied every 20 ms. The muscle response is detectable as two twitches which show a more exaggerated fade than that of the TOF. DBS is more sensitive at detecting residual block, which makes it of particular value at the end of surgery

stimuli of 50 or 100 Hz for 5 seconds may produce fade in situations when the twitch response after TOF or DBS has returned to normal. It is therefore a more sensitive means of detecting low levels of receptor blockade. Fade is also seen with phase II block. It is followed by post-tetanic potentiation if single twitch stimulation is given within 2 minutes. Tetanic stimulation cannot be used in the conscious patient who may be aware of marked residual discomfort even if the stimulus has been applied during anaesthesia

A tetanic stimulus as described earlier is followed by single stimuli at 1-second intervals. Tetany triggers supranormal acetylcholine release (post-tetanic facilitation) which transiently overcomes the neuromuscular blockade. The twitches which result comprise the post-tetanic count. The technique is used to monitor significant degrees of block (for example, in neurosurgery during which any patient movement could be disastrous), and a PTC of less than 5 indicates profound block. A PTC of greater than 15 approximates to two twitches following TOF stimulation, at which point pharmacological reversal should be possible.

Mechanomyography, electromyography and acceleromyography: these methods allow much more accurate measurement of neuromuscular blockade during onset and offset of effect

Mechanomyography measures the isometric contraction force in the adductor pollicis following ulnar nerve stimulation.

records the electrical activity of the stimulated muscle immediately prior to contraction. It determines the amplitude of the signal, usually the sum of the compound muscle action potentials.

uses a small piezoelectric transducer to measure the isotonic acceleration of the muscle (isotonic describes a change in muscle length without any change in tension). If mass remains constant, as it clearly does, then from Newton’s second law (force = mass x acceleration) the force of contraction can be calculated. Whether or not such (relatively expensive) accuracy is necessary during routine clinical practice remains contentious, and at present these instruments are used mainly in research.

the sensitivity of muscle groups to non-depolarizing muscle blocking drugs varies considerably and so recovery in one particular set does not necessarily confirm adequate overall reversal

The motor nerves that are usually stimulated are the • ulnar nerve at the wrist – evoking a response in the adductor pollicis, • facial nerve in the region of the temporo-mandibular joint – evoking a response in the orbicularis oculi muscle around the eye. • The diaphragm is the most resistant muscle of all and requires up to twice the dose for the same effect as the adductor pollicis The muscles of facial expression and of the larynx are much less resistant. orbicularis oculi is a better indicator of laryngeal muscle block than adductor pollicis, which itself is slow to recover and better reflects the degree of any residual diaphragmatic paralysis.

constant current despite the changes in resistance that the needle will encounter as it penetrates tissues of different densities linear output which can easily be varied. monophasic so that the current flows in one direction only have the facility to alter the frequency of the stimulus delivered (usually 1–2 Hz). clear digital display across the current range from 0.1 to 5.0 mA. It should have a short pulse width of 50–100 s, which provides better discrimination of the distance between the needle and the nerve.

Electrodes: the negative electrode should be attached to the stimulator needle rather than the positive. In this situation, the current flow towards the needle produces an area of depolarization which readily triggers an action potential

Rheobase is the minimum current required to stimulate a nerve, while the chronaxie is the duration of current stimulus required to stimulate that nerve at twice the rheobase This has some clinical relevance because different nerves have different chronaxie, and this is a means of quantifying their excitability Large A motor fibres have shorter chronaxie (0.05–0.1 ms) than the fibres subserving touch and pain (A and C fibres with chronaxie of 0.15 and 0.4 ms, respectively). This means that, in an awake patient, it is possible to use a short pulse duration to stimulate motor fibres without eliciting pain.

Thresholds: techniques vary; some anaesthetists start with a relatively high current of up to 2.0 mA, whereas others stay below 1.0 mA. As the needle approaches the likely site of injection, the current should be reduced to about 0.5 mA; at this stimulus the needle tip will be 1–2 mm from the nerve. Coulomb’s (inverse-square) law in that the minimum current needed to stimulate a nerve is directly proportional to the square of the distance from that nerve.

: a small amount of local anaesthetic will abolish the twitch by physical displacement. This has been demonstrated experimentally using saline and air. If the twitch does not disappear on injection, it suggests that the needle may be intraneural and so should be withdrawn slightly.

Insulated or non-insulated: most needles are insulated (with Teflon coating) apart from the uncovered tip through which the current passes. You should be aware that non-insulated needles can also be used effectively because the current density remains greater at the tip of the needle than down the shaft Long bevel, short bevel or side-ported: the choice of needle is contentious. A long-bevelled needle is sharp, penetrates tissues readily and so reduces the appreciation of fascial planes. Should it penetrate a nerve, however, the clean cut may actually cause less damage either than a short-bevelled (30) or pencil-point needle with a proximal side hole. Increasing experience with ultrasound is showing that even when an effective twitch has been elicited these ‘atraumatic’ needles may deliver the local anaesthetic too far from the nerve. They may also produce a less clear ultrasound image. Sizes: there are numerous sizes, depending on the manufacturer, but common lengths include 30, 50, 90, 100 and 150 mm. Most are 22G. Nerve stimulators complement, but do not obviate, the need for accurate anatomical knowledge. The rationale for their use is twofold. — Efficacy: their use has been reported to double the success rate of some blocks (pre-ultrasound). Their value may in due course be reduced to that of providing confirmation that an ultrasound-guided needle is correctly placed. — Safety: their use removes the need to elicit paraesthesia. Paraesthesia occurs only when the advancing needle touches a nerve, and some chronic pain specialists believe that paraesthesia is associated almost invariably with later dysaesthesia. A low current setting of around 0.2–0.3 mA will warn the anaesthetist that the needle is too close to a nerve