3.6 Neuromuscular Blocking Agents
Key Takeaways
- Succinylcholine is a depolarizing neuromuscular blocker that causes transient muscle fasciculations and can raise serum potassium by 0.5-1.0 mEq/L.
- Succinylcholine is strictly contraindicated in hyperkalemia, burns/crush injuries >24 hours, denervating neuromuscular diseases, and history of malignant hyperthermia.
- Rocuronium is a nondepolarizing agent dosed at 1.0-1.2 mg/kg IV to achieve rapid onset, but its long duration (45-70 min) requires a proactive sedation plan.
- Cisatracurium undergoes Hoffmann elimination, a spontaneous chemical degradation independent of hepatic or renal function, making it ideal for organ failure.
Neuromuscular Blocking Agents
Neuromuscular blocking agents (NMBAs) are administered during Rapid Sequence Intubation (RSI) to induce skeletal muscle paralysis. This paralysis facilitates laryngoscopy, optimizes glottic visualization, and mitigates the risk of airway trauma. In the transport environment, where rescue airways are difficult to perform, selection must be based on onset, duration, and contraindications. NMBAs are classified into depolarizing and nondepolarizing agents based on their action at the nicotinic acetylcholine (ACh) receptor on the motor endplate of the neuromuscular junction.
| Medication | Class | RSI Dose | Onset | Duration | Primary Contraindications | Clinical Notes |
|---|---|---|---|---|---|---|
| Succinylcholine | Depolarizing | 1.5-2.0 mg/kg IV/IO | 45-60 sec | 5-10 min | Hyperkalemia, burns/crush >24h, neuromuscular diseases, malignant hyperthermia history | Fasciculations occur; transiently increases potassium by 0.5-1.0 mEq/L |
| Rocuronium | Nondepolarizing | 1.0-1.2 mg/kg IV/IO | 60-90 sec | 45-70 min | None (relative: difficult airway where long duration is unsafe) | Primary alternative to succinylcholine; requires immediate sedation plan |
| Vecuronium | Nondepolarizing | 0.1 mg/kg IV/IO | 2.5-3 min | 30-45 min | None | Slow onset limits RSI utility; must be reconstituted |
| Cisatracurium | Nondepolarizing | 0.15-0.2 mg/kg IV/IO | 2-3 min | 40-60 min | None | Undergoes Hoffmann elimination; ideal for renal/hepatic failure |
Depolarizing NMBAs: Succinylcholine (Anectine)
Succinylcholine is the only depolarizing NMBA used clinically. It consists of two joined acetylcholine molecules and binds to nicotinic receptors at the motor endplate, causing initial depolarization of the muscle membrane. This depolarization causes transient, disorganized muscle contractions visible as fasciculations. Succinylcholine remains bound to the receptor, preventing repolarization and rendering the muscle refractory to further stimulation, leading to flaccid paralysis.
- RSI Dose: 1.5 to 2.0 mg/kg IV or IO. In pediatric patients, the dose is higher (2.0 to 3.0 mg/kg IV/IO) due to their larger volume of distribution. For intramuscular administration (in cases of lost IV access), the dose is 4.0 mg/kg IM.
- Onset and Duration: Onset is 45 to 60 seconds; duration of action is 5 to 10 minutes.
- Hyperkalemia Risk: Depolarization causes the release of potassium from intracellular stores into the extracellular space. In healthy individuals, this increases serum potassium by approximately 0.5 to 1.0 mEq/L, which is clinically insignificant. However, in patients with up-regulated nicotinic receptors (extrajunctional receptors), succinylcholine can trigger massive, life-threatening hyperkalemia. Contraindications include:
- Pre-existing hyperkalemia (e.g., missed dialysis with ECG changes).
- Subacute burns (>24 hours post-injury; safe within the first 24 hours).
- Crush injuries or major trauma (>24 hours post-injury).
- Denervating neuromuscular disorders (e.g., Amyotrophic Lateral Sclerosis [ALS], Multiple Sclerosis [MS], Guillain-Barré Syndrome, stroke with residual hemiplegia).
- Muscular dystrophies (e.g., Duchenne muscular dystrophy, where succinylcholine can cause rhabdomyolysis, hyperkalemia, and cardiac arrest).
- Severe intra-abdominal sepsis (>72 hours).
- Malignant Hyperthermia (MH): Succinylcholine can trigger malignant hyperthermia in genetically susceptible individuals (mutations in the RYR1 ryanodine receptor). MH is characterized by uncontrolled calcium release from the sarcoplasmic reticulum, causing sustained muscle rigidity, masseter spasm, severe metabolic and respiratory acidosis, hypercarbia (abrupt rise in $EtCO_2$), tachycardia, and hyperthermia.
- Treatment: Immediate cessation of succinylcholine, hyperventilation with 100% oxygen, and administration of Dantrolene at 2.5 mg/kg IV (repeated as needed). Active cooling must be initiated.
Nondepolarizing NMBAs
Nondepolarizing agents act as competitive antagonists at the nicotinic ACh receptor. They bind to the receptor and prevent acetylcholine from binding, but do not cause depolarization (no fasciculations).
Rocuronium (Zemuron)
Rocuronium is an aminosteroid nondepolarizing agent and is the primary alternative to succinylcholine for RSI.
- RSI Dose: 1.0 to 1.2 mg/kg IV or IO. This elevated dose is used to achieve an onset of action comparable to succinylcholine.
- Onset and Duration: Onset is 60 to 90 seconds; duration of action is 45 to 70 minutes.
- Clinical Profile: Rocuronium is hemodynamically stable and has no contraindications related to hyperkalemia or malignant hyperthermia. It is the agent of choice when succinylcholine is contraindicated.
- Clinical Caveats: Its long duration of action means the patient will remain paralyzed for over an hour. If the flight crew is unable to intubate or ventilate the patient, they must immediately transition to a rescue airway (e.g., supraglottic device) or surgical cricothyroidotomy, as the paralysis cannot be easily waited out. The selective relaxant binding agent Sugammadex (16 mg/kg IV for emergency reversal of rocuronium) can reverse block within minutes, but is not universally available in all transport configurations. Paramedics must immediately initiate post-intubation sedation, as the patient will remain paralyzed and awake if sedatives wear off.
Vecuronium (Norcuron)
Vecuronium is another aminosteroid nondepolarizing agent, but is less suited for primary RSI due to its slower onset.
- Dose: 0.1 mg/kg IV or IO.
- Onset and Duration: Onset is 2.5 to 3.0 minutes; duration is 30 to 45 minutes.
- Clinical Profile: Like rocuronium, it is hemodynamically neutral. It must be reconstituted from a powder, which makes it less ideal for emergent RSI in the field but suitable for maintaining paralysis post-intubation.
Cisatracurium (Nimbex)
Cisatracurium is a benzylisoquinolinium nondepolarizing agent.
- Dose: 0.15 to 0.2 mg/kg IV or IO.
- Onset and Duration: Onset is 2 to 3 minutes; duration is 40 to 60 minutes.
- Metabolism: Cisatracurium undergoes Hoffmann elimination, a spontaneous, non-enzymatic degradation that is dependent on blood pH and temperature. It does not rely on renal or hepatic function for clearance. This makes cisatracurium the preferred paralytic for patients with severe liver or kidney failure.
Paralyzed Patient Management in Flight
Flight paramedics must strictly adhere to the clinical axiom: Never paralyze an awake patient. Neuromuscular blockers provide zero analgesic or sedative properties; a patient paralyzed without sedation experiences terrifying wakefulness and pain. Furthermore, when a patient is paralyzed, their respiratory drive is obliterated. The flight crew must continuously monitor mechanical ventilation, chest rise, and end-tidal $CO_2$ ($EtCO_2$). A sudden drop in $EtCO_2$ or ventilator pressure alarms in a paralyzed patient is a lethal emergency requiring immediate troubleshooting (DOPE mnemonic: Displacement, Obstruction, Pneumothorax, Equipment).
A 45-year-old female was pulled from a house fire and sustained deep partial-thickness burns to 40% of her body. The injury occurred approximately 36 hours ago. She now requires emergent intubation for respiratory failure. Which neuromuscular blocker is contraindicated?
A patient with end-stage renal disease and severe hepatic cirrhosis requires neuromuscular blockade for transport. Which agent undergoes Hoffmann elimination, making it the most appropriate choice for this patient?