Peripheral Nerve Stimulation

Peripheral Nerve Stimulation

Indications

Because of the variation in patient sensitivity to neuromuscular blocking agents, the neuromuscular function of all patients receiving intermediate- or long-acting neuromuscular blocking agents should be monitored. In addition, peripheral nerve stimulation is helpful in assessing paralysis during rapid-sequence inductions or during continuous infusions of short-acting agents. Furthermore, peripheral nerve stimulators can help locate nerves to be blocked by regional anesthesia.

Contraindications

There are no contraindications to neuromuscular monitoring, although certain sites may be precluded by the surgical procedure.

Techniques & Complications

A peripheral nerve stimulator delivers a current of variable frequency and amplitude to a pair of either ECG silver chloride pads or subcutaneous needles placed over a peripheral motor nerve. The evoked mechanical or electrical response of the innervated muscle is observed. Although electromyography provides a fast, accurate, and quantitative measure of neuromuscular transmission, visual or tactile observation of muscle contraction is usually relied upon in clinical practice. Ulnar nerve stimulation of the adductor pollicis muscle and facial nerve stimulation of the orbicularis oculi are most commonly monitored. Because it is the inhibition of the neuromuscular receptor that needs to be monitored, direct stimulation of muscle should be avoided by placing electrodes over the course of the nerve and not over the muscle itself. To deliver a supramaximal stimulation to the underlying nerve, peripheral nerve stimulators must be capable of generating at least a 50-mA current across a 1000- load. This current is uncomfortable for a conscious patient. Complications of nerve stimulation are limited to skin irritation and abrasion at the site of electrode attachment.

Clinical Considerations

The degree of neuromuscular blockade is monitored by applying various patterns of electrical stimulation. All stimuli are 200 s in duration, of square-wave pattern, and of equal current intensity. A twitch is a single pulse that is delivered from every 1 to every 10 s (1–0.1 Hz). Increasing block results in decreased evoked response to stimulation.

Train-of-four stimulation denotes four successive 200- s stimuli in 2 s (2 Hz). The twitches in a train-of-four pattern progressively fade as relaxation increases. The ratio of the responses to the first and fourth twitches is a sensitive indicator of nondepolarizing muscle paralysis. Because it is difficult to estimate the train-of-four ratio, it is more convenient to visually observe the sequential disappearance of the twitches, as this also correlates with the extent of blockade. Disappearance of the fourth twitch represents a 75% block, the third twitch an 80% block, and the second twitch a 90% block. Clinical relaxation usually requires 75–95% neuromuscular blockade.

Tetany at 50 or 100 Hz is a sensitive test of neuromuscular function. Sustained contraction for 5 s indicates adequate—but not necessarily complete—reversal from neuromuscular blockade. Double-burst stimulation (DBS) represents two variations of tetany that are less painful to the patient. The DBS_3,3 pattern of nerve stimulation consists of three short (200- s) high-frequency bursts separated by 20-ms intervals (50 Hz) followed 750 ms later by another three bursts. DBS_3,2 consists of three 200- s impulses at 50 Hz followed 750 ms later by two such impulses. DBS is more sensitive than train-of-four stimulation for the clinical (ie, visual) evaluation of fade.

Because muscle groups differ in their sensitivity to neuromuscular blocking agents, use of the peripheral nerve stimulator cannot replace direct observation of the muscles (eg, the diaphragm) that need to be relaxed for a specific surgical procedure. Furthermore, recovery of adductor pollicis function does not exactly parallel recovery of muscles required to maintain an airway. The diaphragm, rectus abdominis, laryngeal adductors, and orbicularis oculi muscles recover from neuromuscular blockade sooner than the adductor pollicis. Other indicators of adequate recovery include sustained ( 5 s) head lift, the ability to generate an inspiratory pressure of at least –25 cm H₂O, and a forceful hand grip. Twitch tension is reduced by hypothermia of the monitored muscle group (6%/°C). 

Response to Peripheral Nerve Stimulation The use of peripheral nerve stimulators to monitor neuromuscular function is discussed in Chapter 6. Four patterns of electrical stimulation with supramaximal square-wave pulses are considered: Tetany: A sustained stimulus of 50–100 Hz, usually lasting 5 s. Twitch: A single pulse 0.2 ms in duration. Train-of-four: A series of four twitches in 2 s (2-Hz frequency), each 0.2 ms long. Double-burst stimulation (DBS): Three short (0.2 ms) high-frequency stimulations separated by a 20-ms interval (50 Hz) and followed 750 ms later by two (DBS3,2) or three (DBS3,3) additional impulses (see Figure 6–32). The occurrence of fade, a gradual diminution of evoked response during prolonged or repeated nerve stimulation, is indicative of a nondepolarizing block (Table 9–2). Fade may be due to a prejunctional effect of nondepolarizing relaxants that reduces the amount of ACh in the nerve terminal available for release during stimulation (blockade of ACh mobilization). Adequate clinical recovery correlates well with the absence of fade. Because fade is more obvious during sustained tetanic stimulation or double-burst stimulation than following a train-of-four pattern or repeated twitches, the first two patterns are the preferred methods for determining adequacy of recovery from a nondepolarizing block. Table 9–2. Evoked Responses during Depolarizing (Phase I and Phase II) and Nondepolarizing Block. The ability of tetanic stimulation during a partial nondepolarizing block to increase the evoked response to a subsequent twitch is termed posttetanic potentiation. This phenomenon may relate to a transient increase in ACh mobilization following tetanic stimulation. In contrast, a phase I depolarization block does not exhibit fade during tetanus or train-of-four; neither does it demonstrate posttetanic potentiation. If enough depolarizer is administered, however, the quality of the block changes to resemble a nondepolarizing block (phase II block). Application of neuromuscular monitoring Getting Started • Check functioning of neuromuscular monitor • Choose the appropriate nerve-muscle to be monitored • Careful preparation of skin (by degreasing) • Apply positive electrode proximally to prevent direct depolarization of muscle • Monitoring should ideally be started before administration of neuromuscular blocker but after induction of anaesthesia • Select supramaximal current (turn current slowly during repetitive single twitches until maximum plateau is achieved and then increase current level by 20%) • Use of recording devices is preferable • Select appropriate method of stimulation • Observation and interpretation of evoked response Clinical Application The various levels of relaxation achieved with use of neuromuscular blocking drugs occurs within well defined limits of evoked responses. Hence neuromuscular monitoring can be applied at various phases during the use of these drugs. Onset of block and assessment of adequate intubating conditions : The latent onset time of a drug is time from injection until there is a measurable effect. The onset time is defined as the time from injection to peak effect. The measured onset time of the muscle relaxants varies according to the muscle group that has been monitored and the stimulation parameters that are being used. Until recently virtually all comparisons between drug doses and stimulation patterns have been performed at the thumb but the onset for the larynx, vocal cords and diaphragm is faster than the thumb.28 If the clinician is interested in the accurate assessment of onset time, such as during rapid sequence induction and intubation, monitoring of orbicularis oculi is perhaps more useful. When patient movement is unacceptable, trachea is intubated 30-90 seconds after response to TOF stimulation disappears. The stimulation parameters also affect the apparent onset time. Faster rates of stimulation appear to have faster onset time due to increased muscle blood flow and increased drug delivery. The onset of actual adequate intubating conditions are independent of the rate of stimulation. Maintenance of block : Ideally, the goal should be to maintain the minimum depth of block that is required for surgery. Adequate surgical relaxation for abdominal surgery can be achieved with greater than 80% twitch depression, that correlates with 1 or 2 twitches present in the train of four.37 Diaphragm is relatively resistant and requires greater plasma concentration of non-depolarising muscle relaxants. When paralysis of diaphragm is required, the administration of NMB is titrated to paralysis of orbicularis oculi or suppression of post tetanic response (PTC 0 at thumb). Such a block would take considerable time to be antagonised adequately. Detecting reversible block : Airway patency and adequate ventilation require more than an intact diaphragm, hence it is important to assess the degree of block at a muscle that does not overestimate the rate of recovery of muscles maintaining the airway. The increased sensitivity and slower time course of the adductor pollicis muscle make this site preferable for monitoring recovery. The PTC may be used when no twitch response is attainable. The time to recovery of twitch response is inversely proportional to the number of PTC. When twitch response is attained TOF at 10-20 seconds interval is used. In the absence of recording device DBS may be preferable to TOF to reliably detect a fade of less than 0.3. Atracurium, vecuronium, pancuronium induced neuromuscular block is antagonised by neostigmine in 30 minutes when single twitch height is 10% of control or more or just prior to emergence of second twitch of TOF count and in 10   minutes when all the four responses to TOF stimulation are present. When zero twitches are present in the TOF the block is considered not antagonizable. Ensuring adequate neuromuscular function :  Safe extubation of trachea can be performed only after adequate restoration of neuromuscular function. The TOF ratio greater than 0.75 at the thumb correlates with restoration of strength of the muscles of respiration and airway protection, vital capacity, maximum expiratory force, peak expiratory flow rate, hand grip and 5 second head lift.16,41 Eriksson et al have shown that moderate degrees of neuromuscular block decrease the chemoreceptor sensitivity to hypoxia, leading to insufficient response to a decrease in oxygen tension in blood.42 They also showed that residual block (TOF<0.9) is associated with functional impairment of pharynx and upper esophagus most probably predisposing to regurgitation and aspiration.43 The TOF ratio recorded mechanically or by EMG must exceed 0.8 or even 0.9 to exclude clinically important neuromuscular blockade. The response to neurostimulation should be assessed in correlation with clinical assessment of ventilatory function. Table 4 : Relationship between clinical and train-of-fourevoked stimulation Test TOF Equivalent 5-sec head or leg lift 0.6 Normal grip strength 0.7 Masseter 0.86 All subjects uncomfortable at TOF < 0.75 It is important that all patients be reassessed in the post-anaesthesia care unit as the incidence of residual neuromuscular blockade and recurarization may be as high as 30% in patients receiving long-acting NMBs. Persistent neuromuscular blockade contributes to postoperative pulmonary complications.