Neuromuscular blockade

Neuromuscular blockade

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Pharmacodynamics
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Neuromuscular blockade involves the use of neuromuscular blocking agents (NMBAs) (i.e. paralytics) to facilitate endotracheal intubation and/or surgical procedures.

Drugs used

Non-depolarizing agents bind to and INHIBIT acetylcholine receptors at the end plate of the neuromuscular junction. Common examples below include:

Steroidal non-depolarizing agents (hepatic and/or renal excreted):

• Rocuronium
• Vecuronium

Benzylisoquinolinium non-depolarizing agents (broken down by Hoffman Elimination):

• Cisatrocurium (the cis-enantiomer of atrocurium)

Depolarizing agents on the other hand bind and ACTIVATE acetylcholine receptors causing depolarization of the neuromuscular junction (i.e. muscle contraction) followed by paralysis. The prototypical example being:

• Succinylcholine

Depolarizing agents have different implications for neuromuscular blockade monitoring as discussed below.

Monitoring

Train of Four

Equipment (Quantitative Monitoring)

In order to assess the level of paralysis during an anesthesia event, train of four monitoring (either quantitative or qualitative) is typically used throughout the duration of the procedure (e.g. during routine assessments) and prior to extubation. Quantitative train of four monitoring is the most accurate assessment of neuromuscular blockade and is preferred as it mitigates the risk of residual paralysis and subsequent post-op pulmonary complications. Equipment that can be used includes:

• Electromyography (EMG)
• Acceleromyography (AMG)
• Kinemyography (KMG)

All of the above involve the stimulation of peripheral nerves, the differences being what is actually measured. EMG measures evoked muscle responses (a.k.a. action potentials). AMG measures the acceleration of thumb contraction using Newton's Second Law of Motion (Force = Mass x Acceleration). KMG uses mechanosensors that general measurable electrical signals. All are valid for use in quantitative train of four monitoring. AMG and KMG require unrestricted motion of the hand or muscle interrogated while EMG does not. AMG and KMG are also subject to the phenomenon of "reverse fade." EMG on the other hand can be affected by electrical interference in the operating room (cautery) but nonetheless is usually the monitoring device of choice.

Qualitative Monitoring

In lieu of EMG, provider may also use a nerve stimulator and palpate for muscle contraction in order to assess depth of paralysis. This is less accurate and generally overestimates recovery from paralysis. The risk of this is residual paralysis which contributes to higher rates of post-op pulmonary complications.

Fade

Fade is the progressive decrease in amplitude in response to nerve stimulation as a result of increased levels of paralysis. This is important Of note, fade is generally not observed when depolarizing agents such as succinylcholine are used (instead, there is a uniform decrease in amplitude). Fade may still be observed with depolarizing agents if patients are in phase II of paralysis where there is desensitization of the neuromuscular junction after prolonged acetylcholine channel opening.