Advancing Airway Management: Special Consideration for Non-Paralytic RSI Using Etomidate Only
Rapid sequence intubation, or more commonly RSI, is defined as “the administration, after preoxygenation, of a potent induction agent followed immediately by a rapidly acting neuromuscular blocking agent (NMBA) to induce unconsciousness and motor paralysis for tracheal intubation.”1 The benefits of RSI are multifaceted and include, but are not limited to, increasing the likelihood of successful intubation, reduction of aspiration, and control of adverse effects to the patient.
While RSI is the hallmark of emergency department airway management, many EMS systems have not fully embraced RSI with its requisite use of paralytics. Some EMS medical command physicians believe induction-only intubation is a safer alternative to RSI since the patient does not receive a dose of a paralytic medication. To avoid confusion, two terms have emerged to describe these two approaches: paralytic RSI and non-paralytic RSI. The definition above is considered paralytic RSI because it includes the use of a neuromuscular blocking agent, most often succinylcholine. In non-paralytic RSI, the neuromuscular blocking agent is omitted and only an induction agent is used. The most commonly used EMS induction agents are etomidate (Amidate) and midazolam (Versed). Midazolam is generally a poor drug choice for airway management because of its long onset of action and need for high doses (0.3mg/kg–0.4mg/kg). Generally, etomidate is the preferred single agent used for non-paralytic RSI.
Etomidate has been described in many articles and texts as being rapidly acting and hemodynamically neutral in doses of 0.3mg/kg to as high as 0.6mg/kg. Although generally considered safe, there is the possibility of fatal side effects including severe hypotension and the inability to ventilate and/or intubate the patient due to masseter muscle spasm. This column will focus on etomidate-induced masseter muscle spasm.
Masseter muscle spasm is defined as spastic contraction of the masseter muscles resulting in forceful jaw closure. This condition is also known as trismus, leading to the inability to fully open the mouth, which may result in difficulty and/or failure in ventilation and intubation. Masseter muscle spasm is a muscular condition. It is not related to tempero-mandibular joint dysfunction which is a mechanical condition. Masseter muscle spasm is a material concern when using etomidate in a non-paralytic RSI. It is usually not of concern when used in paralytic RSI since the rapid administration of Succinylcholine quickly paralyzes the masseter muscles.
Etomidate, when used in paralytic RSI, is pushed as rapidly as possible and is immediately followed by the neuromuscular blocking agent. This medication sequence renders the patient almost instantly unconscious and paralyzed. Because of the rapid onset of both etomidate and succinylcholine, the patient usually becomes unconscious, flaccid, and apneic at nearly the same time. The neuromuscular blocking agent prevents the occurrence of masseter spasm. Administration of etomidate during paralytic RSI can be likened to the rapidity of onset of adenosine (Adenocard) for the conversion of narrow QRS complex tachycardia. Etomidate’s onset of action occurs within one “arm-heart-brain circulation” time or the time it takes for the medication to travel from the injection site to the brain.
When used alone for non-paralytic RSI, etomidate must be injected slowly, perhaps over 30 seconds to 45 seconds, to reduce the risk of etomidate-induced masseter muscle spasm. Although the exact mechanism of etomidate-induced masseter muscle spasm is unknown, it is thought to be related to myoclonus. Myoclonus, or abnormal contraction of muscles, is a well-documented side effect of etomidate that is often confused with seizure activity. It is thought that the masseter muscles may tonic spasm due to a release of lactic acid during myoclonic activity or an overdose of the medication. This condition make makes ventilation and intubation difficult or impossible.
Treatment of the patient with etomidate-induced masseter muscle spasm focuses on oxygenation and ventilation of the patient. If the patient is spontaneously breathing it should be possible to maintain oxygenation with a non-rebreather mask, high-flow nasal cannula, or bag mask device with supplemental oxygen until the effects of the medication are no longer seen. If minute volume is inadequate or the patient becomes apneic, active ventilations employing bag mask ventilation is attempted. The use of a nasal airway or two should be utilized if available to assist in upper airway patency, especially if the masseter spasm prevents the insertion of an oral airway.
Patients receiving etomidate generally return to baseline cognitive and neuromuscular status within five minutes.
If the patient is unable to be ventilated and succinylcholine is available, it should be administered immediately at the RSI dose of 1.5mg/kg. Generally succinylcholine administration will relieve the masseter within thirty to forty-five seconds. If succinylcholine is not available, surgical, percutaneous, or needle cricothyrotomy may be indicated if ventilation via other methods is not successful.
In summary, etomidate is an effective adjunct in airway management that has many desirable attributes. However, it is not without side effects. Providers who utilize etomidate in non-paralytic RSI need to understand the risk of masseter muscle spasm, how to minimize the risk and how to respond if it occurs.
Reference
1. Walls RM, Murphy MF. Manual of Emergency Airway Management, 4th edition. Philadelphia, PA: Lippincott, Williams and Wilkins, 2012.
Fred Ellinger, Jr. is a Nationally Registered EMT-Paramedic who has over 20 years of EMS operations, EMS management, instruction, firefighting and rescue experience. He is the owner/operator of SafeTec Training Services, a Pennsylvania Bureau of EMS approved continuing education sponsor and trainer for many healthcare agencies and corporations in the tri-state Pennsylvania area. He serves as a flight paramedic for MidAtlantic Medevac, Hahnemann University Hospital, Philadelphia, Pennsylvania and is an original/active medical specialist with Pennsylvania Urban Search and Rescue Response System's PA Company 2 and the Bucks County Technical Rescue Task Force. Fred is the Mid-Atlantic Regional Course Director for The Difficult Airway Course: EMS™ and serves on the content committee of Airway World (https://www.airwayworld.com/), a unique single-source website for all information related to airway management. Fred can be reached at fred.ellinger@difficultairway.net.
