I vividly recall the first time I administered a neuromuscular blockade agent (NMBA) to a patient in the surgical intensive care unit. My patient was hypoxemic and had been “bucking the vent,” meaning they were fighting the ventilator and experiencing patient-ventilator dyssynchrony. The patient needed to be emergently paralyzed and sedated in order to establish adequate oxygenation. It’s unnerving knowing that you are intentionally paralyzing an individual for their safety and clinical benefit. There’s a constant underlying fear that they might break through their sedation, frightened to find they are completely immobilized. As clinicians working in the operating room or intensive care unit (ICU), it’s essential that we understand NMBAs as these drugs are commonly administered in these clinical environments. How exactly do these drugs work?
Mechanism of Action of NMBAs
To understand how neuromuscular blockade agents work, let’s first look at acetylcholine. In our previous blog on
cholinergic drugs, we reviewed acetylcholine, a major neurotransmitter of the peripheral nervous system. Examining the nervous system a bit closer, we find that the motor nerve axon divides to form branching terminals called motor end plates. These are enfolded in muscle fibers, but separated from the fibers by the synaptic cleft. A stimulus to the nerve causes an impulse to travel down the neuron. When a nerve impulse reaches the end of a motor neuron, acetylcholine is released into the neuromuscular junction or synaptic cleft where it occupies receptor sites on the muscle cell membrane, depolarizing the membrane and resulting in muscle contraction.
Neuromuscular blocking agents act at the motor end plate by blocking depolarization of the membrane or by competing with acetylcholine for the receptor sites (nondepolarizing).
- Depolarizing NMBAs bind to cholinergic receptors on the motor endplate, causing initial depolarization on the endplate membrane but then preventing neuromuscular transmission (Bittner, 2021).
- Nondepolarizing NMBAs competitively inhibit the acetylcholine receptor on the motor endplate either by preventing a change in the receptor or by obstructing the ion channels so that an endplate action potential is not created (Bittner, 2021).
NMBA Agents and Indications
Essentially, NMBAs are commonly administered in the operating room to facilitate endotracheal intubation and in critical units to assist in mechanical ventilation in patients who have decreased lung compliance (Cook & Simons, 2022). The onset of paralysis or muscle flaccidity for most NMBA’s is within a minute or two and many last for about 10 minutes.
Approved indications include:
- Neuromuscular blockade for endotracheal intubation, surgery, or mechanical ventilation.
- Adjunct to general anesthesia to facilitate endotracheal intubation and to relax skeletal muscles during surgery or mechanical ventilation in adequately sedated ICU patients.
- Severe, refractory, or life-threatening hypoxemia, particularly those with severe ventilator dyssynchrony.
It’s important to note that NMBA’s are not the first line option for managing movement, agitation, or ventilator asynchrony as they do not possess sedative, amnestic, or analgesic properties (Bittner, 2021).
Off-label uses include:
- Shivering due to therapeutic hypothermia following cardiac arrest.
- Decreasing undesireable movement or muscle tone in patients with:
- refractory status asthmaticus
- increased intracranial pressure (ICP) with ventilator dyssynchrony
- massive hemoptysis (to prevent coughing and clot dislodgement)
- conditions that increase muscle activity (i.e., tetanus, malignant neuroleptic syndrome)
- increased intra-abdominal pressure (IAP)
- need for temporary paralysis to facilitate short procedures (i.e., bronchoscopy, endoscopy, tracheostomy, or radiologic interventions)
- Acute respiratory failure requiring emergent intubation.
Neuromuscular Blocking Agents
(Bittner, 2021; Renew, 2022) |
| NMBAu |
Action |
Onset |
Class |
| Succinylcholine |
Depolarizing |
Rapid-acting |
None |
| Rocuronium |
Nondepolarizing |
Intermediate-acting |
Steroidal |
| Vecuronium |
Nondepolarizing |
Intermediate-acting |
Steroidal |
| Pancuronium |
Nondepolarizing |
Long-acting |
Steroidal |
| Atracurium |
Nondepolarizing |
Intermediate-acting |
Benzylisoquinolinium |
| Cisatracurium |
Nondepolarizing |
Intermediate-acting |
Benzylisoquinolinium |
Nursing Considerations with NMBAs (Bittner, 2021; Cook & Simons, 2022; Renew, 2022)
- Patients should be adequately sedated prior to, during and following discontinuation of paralysis.
- NMBAs can be administered by intermittent intravenous injection or continuous infusion. Avoid intramuscular injection as absorption is inconsistent.
- Train-of-four (TOF) stimulation is the most common method used to monitor neuromuscular blockade based on the number of twitches evoked from an electrical stimulus, also known as train-of-four count (TOFC).
- TOFC of 1 = > 95% of receptors blocked
- TOFC of 2 = 85-90% of receptors blocked
- TOFC of 3 = 80-85% of receptors blocked
- TOFC of 4 = 70-75% of receptors blocked
- Depth of block – monitor the patient’s depth of block with regular assessment (i.e., triggering ventilator, degree of shivering, TOFC) every 2 to 3 hours until stable, then every 8 to 12 hours.
- Deep neuromuscular block (i.e., 0 to 1 twitch with neuromuscular monitoring) is used for endotracheal intubation and some surgical indications.
- Moderate neuromuscular block (i.e., 2 to 3 twitches) is used for most surgical procedures and in the intensive care unit setting.
- Daily interruptions are recommended to assess the ongoing need for paralysis.
- Lubricate the patient’s eyes with eye drops or gels every 2 to 4 hours and eyelids should be taped shut to prevent corneal drying, ulceration, infection, injury, and scarring.
- Ventilator management:
- Closely monitor mechanical ventilation, ensuring no disconnection.
- Perform endotracheal suctioning frequently and as needed to clear secretions.
- Full recovery of neuromuscular function must be established before extubation.
- Turn patient frequently and ensure bedding is dry and wrinkle-free to prevent skin breakdown.
- Administer venous thromboembolism prophylaxis.
- Monitor pupillary reflexes to assess neurologic status.
- Reversal of block can occur by spontaneous recovery, or by administration of reversal agents such as neostigmine or sugammadex.
If you work in a clinical setting where NMBAs are utilized, take some time to review these drugs in depth until you are confident in administering and managing each one. For complete information, please consult the drug’s specific package insert or the
Nursing2022 Drug Handbook® + Drug Updates.
References
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