• Peripheral nerve stimulators (PNSs) are used in association with the administration of neuromuscular blocking drugs (NMBDs) to block skeletal muscle activity.

• NMBDs are given in the intensive care unit, along with sedatives and opioids, most commonly to coordinate contemporary modes of mechanical ventilation with breathing in patients with severe lung injury. Neuromuscular blocking agents are also used to assist with the management of increased intracranial pressure after a head injury; for severe muscle spasms associated with seizures, tetanus, and drug overdose; to reduce intraabdominal hypertension¹; in hypothermia protocols for cardiac arrest⁶; and for preservation of delicate reconstructive surgery.

• NMBDs do not affect sensation or level of consciousness. Because NMBDs lack amnesic, sedative, and analgesic properties, sedatives and analgesics should always be given concurrently to minimize the patient’s awareness of blocked muscle activity and discomfort. Sedatives and analgesics should be initiated before NMBDs because neuromuscular blockade hinders the assessment of anxiety and pain.⁵

• Numerous medications, such as aminoglycosides and other antibiotics, beta blockers, calcium channel blockers, corticosteroids, and anesthetics, and conditions, such as acidosis and various electrolyte imbalances, potentiate the effects of neuromuscular blocking agents. Thus, the level of blockade is subject to variation, which necessitates vigilant monitoring with a PNS and titration of the NMBD.⁶

• The muscle twitch response to a small electrical stimulus delivered by the PNS corresponds to an estimated number of nerve receptors blocked by the NMBDs and assists the clinician in the assessment and titration of the medication dosage. The level of blockade is estimated by observing the muscle twitch after stimulating the appropriate nerve with a small electrical current delivered by the PNS.

• The train-of-four (TOF) method of stimulation is most commonly used for ongoing monitoring of NMBD use. After delivery of four successive stimulating currents to a select peripheral nerve with the PNS, in the absence of significant neuromuscular blockade, four muscle twitches follow. The four twitches signify that 75% or fewer of the receptors are blocked. Three twitches correspond to approximately 80% blockade, and two to one twitches in response to four stimulating currents correlate with approximately 85% to 90% blockade of the neuromuscular junction receptors.⁹ One to two twitches is the recommended level of block, although the appropriate level has not yet been determined through research in the critically ill population.⁵ Absence of twitches may indicate that 100% of receptors are blocked, which exceeds the desired level of blockade (Table 38-1).

• The stimulating current is measured in milliamperes (mA). The usual range of mA required to stimulate a peripheral nerve and elicit a muscle twitch is 20 to 50 mA, although increasing the current to 70 or 80 mA may be necessary, especially in the obese patient.⁹

• Some stimulators do not indicate the mA. Instead, digital or dialed numbers ranging from 1 to 10 represent the range of mA from 20 to 80 mA. With use of these instruments, the usual setting is 2 to 5, although a setting of 10 is sometimes necessary. Other stimulators (with and without digital displays) automatically adjust the voltage output relative to resistance and deliver the current accordingly.¹⁰

• The ulnar nerve in the wrist is recommended for testing, although the facial and the posterior tibial nerves may also be used.

• Peripheral nerve monitoring is used in conjunction with the assessment of clinical goals, and clinical decisions should never be made solely on the basis of the twitch response.

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