If you've read an EMS-related journal or attended an EMS conference in the last couple of years, you're probably aware that prehospital airway management in general, and prehospital intubation specifically, is under close scrutiny.
Multiple studies have indicated that paramedics may not have high intubation success rates, that prehospital intubation may worsen patient outcomes, and that airway management may receive less teaching time than it needs in EMS education programs.1 This body of evidence has led to changes in how EMS providers manage their patients' airways in emergency situations. At the same time, airway assessments aimed at predicting "difficult airways" have gained popularity; providers are now being encouraged to assess airways and make procedure choices based on their likelihood of success.
However, many of the airway assessment techniques currently recommended have been adapted from other specialties, have practical limitations to their use in emergency airway management and have questionable validity from a statistical point of view. This article will look at the history of airway assessments, the popular mnemonic LEMON for predicting difficult laryngoscopy, and some of the limitations to its use in EMS, both practical and statistical.
History of Airway Assessment
Airway assessment grew up in the OR as anesthesiologists sought ways to predict patients whom they would be unable to effectively ventilate. The development of alternative airway devices, such as the Combitube, offered choices in how to manage the operative patient's airway, but how do you choose which option is best for a particular patient? In the mid-1980s, Vijayalakshmi Patil, MD, proposed that the areas surrounding the head and superior neck had predictive value for difficult intubations, and Seshagiri Rao Mallampati, MD, published two articles describing a scale for judging ease of intubation based on pharyngeal structures visible when the patient opens his mouth and extends his tongue.2 Various combinations of these parameters, combined with other factors such as receding mandibles, buck teeth and x-ray findings, were also proposed.2 In 1993, the American Society of Anesthesiologists published a recommended Difficult Airway Algorithm, attempting to prevent airway-related deaths in the OR by standardizing an approach to airway management.
In the meantime, rapid sequence intubation techniques were emerging in emergency rooms as a viable option for emergency airway management. This development raised the stakes for emergency providers. Previous methods of emergency airway management held the safety net that if the procedure was unsuccessful, the patient was still, for the most part, spontaneously breathing. The prospect of pharmacologically inducing respiratory arrest necessitated a careful risk/benefit analysis for every patient considered for RSI. Hence, airway assessment techniques were transplanted from the OR to the ED to help provide some guidance for practitioners. In much the same way, as RSI spread from the ED to the helicopter or ambulance, so did traditional airway assessments.
Current Airway Assessments: LEMON
Recently, mnemonics for airway assessment that address all aspects of emergency airway management (laryngoscopy, BVM ventilation, extraglottic devices and surgical airway techniques) have been disseminated.3 The most commonly known mnemonic seems to be the one for laryngoscopy--LEMON:
- Look externally
- Evaluate 3-3-2
- Mallampati scoring
- Neck mobility.
For those not familiar with the mnemonic, "Evaluate 3-3-2" refers to the geometry of the airway and oral access; the provider should be able to fit three of the patient's fingers into his mouth, fit three fingers underneath the chin, and fit two fingers from the top of the thyroid cartilage to the top of the neck. The 3-3-2 refers directly back to Patil's proposal of the importance of airway geometry and the thyromental distance (distance from the thyroid cartilage to the mentum of the chin).