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Chapter 21: Monitoring Neuromuscular Function

Steven W. Price, MD; Sudha Ved, MD

BASIC CONCEPTS

Neuromuscular blocking drugs (NMBDs) interfere with neural transmission at the neuromuscular junction (NMJ). This effectively produces paralysis, which is advantageous to facilitate conditions for intubation by decreasing the tone of supralaryngeal muscles, inhibiting spontaneous ventilation, improving lung dynamics for mechanical ventilation, and providing proper skeletal muscle relaxation to optimize surgical conditions.

Proper monitoring of the degree and adequacy of neuromuscular blockade is vital in clinical practice. Providing too little neuromuscular blockade can lead to substandard conditions for the surgeon and anesthesiologist alike. Meanwhile, overzealous or inappropriate use of NMBDs could result in delayed extubation or the need for reintubation in the post-anesthesia care unit (PACU) (Table 21-1). In addition to the interference with pulmonary mechanics, residual blockade also depresses the ventilatory response to hypoxia. As NMBDs possess no analgesic or anesthetic properties, the use of NMBDs could also lead to increased intraoperative awareness during general anesthesia. It is therefore important to use anesthetics concurrently with the administration of NMBDs.

TABLE 21-1Clinical Signs and Symptoms of Residual Paralysis in Awake Volunteers after Mivacurium-Induced Neuromuscular Blockade
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TABLE 21-1 Clinical Signs and Symptoms of Residual Paralysis in Awake Volunteers after Mivacurium-Induced Neuromuscular Blockade
Train-of-Four Ratio Signs and Symptoms
0.70-0.75

Diplopia and visual disturbances

Decreased handgrip strength

Inability to maintain apposition of the incisor teeth

“Tongue depressor test” negative

Inability to sit up without assistance

Severe facial weakness

Speaking a major effort

Overall weakness and tiredness
0.85-0.90

Diplopia and visual disturbances

Generalized fatigue

(Reproduced with permission from Kopman AF, Yee PS, Neuman GG. Relationship of the train-of-four fade ratio to clinical signs and symptoms of residual paralysis in awake volunteers. Anesthesiology. 1997;86:765.)

Several methods exist to measure the status of neuromuscular blockade (Table 21-2). Clinical signs such as 5-second head lift and the ability to hold a tongue depressor between the teeth represent reliable indication of neuromuscular function to tolerate extubation. However, these clinical signs cannot be elected during the course of anesthesia. The use of peripheral nerve stimulators to produce mechanically evoked responses to electrical stimulation, therefore, remains the best means to accurately determine neuromuscular status. Additionally, it aids in the determination of the adequacy of reversal with acteylcholinesterase inhibitors.

TABLE 21-2Clinical Tests of Postoperative Neuromuscular Recovery
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TABLE 21-2 Clinical Tests of Postoperative Neuromuscular Recovery
Unreliable
Sustained eye opening
Protrusion of the tongue
Arm lift to the opposite shoulder
Normal tidal volume
Normal or nearly normal vital capacity
Maximum inspiratory pressure <40-50 cm H2O
Most Reliable
Sustained head lift for 5 seconds
Sustained leg lift for 5 seconds
Sustained handgrip for 5 seconds
Sustained “tongue depressor test”
Maximum inspiratory pressure ≥40-50 cm H2O

(Reproduced with permission from Miller RD, Miller’s Anesthesia, 7th ed. Philadelphia, PA: Churchill Livingstone/Elsevier; 2010.)

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