The LEMON airway assessment: predicting a difficult intubation

The LEMON airway assessment can be used to predict an anatomically difficult intubation before you reach for the laryngoscope. The LEMON mnemonic was developed by the Difficult Airway Course, validated in emergency department patients, and adopted by the American College of Surgeons' Advanced Trauma Life Support course.
Each part of the assessment flags a different anatomic roadblock, from a limited mouth opening to a neck with limited range of motion. Working through this airway assessment before intubating lets you plan your approach and your backup options in advance, rather than discovering the difficulty mid-procedure.

In this lesson from our Advanced Airway Management course, you’ll learn how to:

• Predict a difficult intubation using the LEMON airway assessment
• Check mouth opening, thyromental distance, and larynx position with the 3-3-2 rule
• Correlate Mallampati score with the Cormack-Lehane grade
• Spot signs of upper airway obstruction before you intubate
• Assess neck mobility for the sniffing position
   

Why difficult intubation happens

[0:00]
The act of intubation involves placing a cuffed tube into the patient's trachea. This might sound simple enough, but remember that the trachea has evolved to be well guarded and not easily accessible. To succeed, we need to understand potential roadblocks that could make placing that tube within the trachea challenging. By predicting these obstacles, we can plan the best approach and prevent complications.

Navigating the oral and laryngeal axes during intubation

[0:24]
Imagine trying to park a car in a garage that's at a 90-degree angle to the driveway. That's essentially what we're trying to do when intubating. We approach through the oral axis, labeled here OA, but we need to get the tube into the tracheal or laryngeal axis, abbreviated LA, which is perpendicular. Along the way, we have to navigate past the lips, teeth, tongue, and hypopharynx, also known as the laryngopharynx, and each of these can pose its own challenges.

Escalating to advanced airway management techniques

[0:54]
Now that we understand the overall approach, let's review the types of anatomic challenges we can encounter when stabilizing the airway. The goal for intubation is intubation. If that fails, we'll need to ventilate the patient with a bag-valve-mask. If that fails, we will attempt to place an extraglottic device. And if all else fails, and we find ourselves in a can't intubate, can't oxygenate scenario, we will perform a surgical cricothyrotomy.

Predicting difficult intubation and other airway challenges ahead

[1:24]
In the next few Medmastery lessons, we'll discuss how to predict anatomic difficulty in each of these scenarios, and you'll find instructions for how to perform these techniques in the last chapter of the course. Our first choice is to perform orotracheal intubation. Anatomic difficulties in this scenario largely involve challenges in placing a laryngoscope or problems viewing the vocal cords. Video laryngoscope devices have made many of these challenges much easier compared to direct laryngoscopy, but difficulties can still remain. 

Introducing the LEMON mnemonic

[1:55]
The mnemonic LEMON was developed by The Difficult Airway Course, validated in emergency department patients, and adopted by the American College of Surgeons Advanced Trauma Life Support course. It's widely used to highlight common causes for difficult laryngoscopy. LEMON stands for look, evaluate 3-3-2, Mallampati score, obstruction or obesity, and neck mobility.

Looking for signs of a difficult airway

[2:22]
First, look at the patient to get an overall impression of how difficult their airway anatomy might be. Is there anything that stands out? A large tongue, obesity, a small mandible, or cervical fixation can all make laryngoscopy and visualization of the vocal cords more challenging.

The 3-3-2 rule in the LEMON airway assessment

[2:40]
Next, evaluate 3-3-2. This assessment checks if the mouth can open wide enough and if the distance from the chin to the larynx is adequate. Think of it like checking if the garage door is tall enough or if the garage is deep enough for your car to fit. The first 3 in 3-3-2 stands for the fact that the mouth should open about three fingers in order to accommodate the laryngoscope blade and permit a view past the tongue to the cords. The second 3 refers to the thyromental distance, or the distance from the chin to the voice box, which should be about three fingers to allow space for the tongue to move out of the way when placing the tube. And the last 2 reminds us that the larynx is normally two fingers from the base of the tongue. If the distance is more or less, it can make access more difficult.

Correlating Mallampati score with the Cormack-Lehane grade

[3:28]
Next, the Mallampati score should be assessed by looking in the patient's mouth. These views roughly correlate with the Cormack–Lehane laryngeal view grades, as shown, and progress from the easiest to the most difficult to intubate. In a class I view, all structures, including the hard and soft palate, uvula, and tonsils, are visible, like having a clear view of your parking space. In a class II view, all of these structures are visible, but only the upper portion of the tonsils can be seen. These first two views are not associated with difficulty intubating. 

Mallampati score: recognizing class III and IV views

[4:03]
In a class III view, the hard and soft palate are visible, and only the base of the uvula can be seen. This view is associated with increased intubation difficulty. In a class IV view, only the hard palate is visible. This is associated with greater difficulty with intubation, almost like trying to park blindfolded.

Assessing upper airway obstruction and obesity before intubation 

[4:22]
Next, we assess for obstruction, looking for structures that might be blocking access to the oropharynx, displacing the tongue, or otherwise getting in the way of the tube visualization or placement. Pooling secretions or the presence of a high-pitched wheezing sound caused by a blockage or narrowing in the upper airway, known as stridor, can also signal upper airway obstruction.

Obesity also makes intubation more challenging, as it is associated with more difficult Cormack–Lehane views of the glottic opening.

LEMON airway assessment: assessing neck mobility

[4:51]
Finally, we must assess the patient's neck mobility. In order to optimize their position for intubation and align the oral, laryngeal, and pharyngeal axes, the patient needs to be placed in a sniffing position, which is similar to the way you naturally position your head when you're trying to smell something faint. If their neck cannot accommodate that position due to injury, immobilization, an unstable cervical spine, or degenerative disease, it will make intubation more difficult. 

By using the LEMON mnemonic, you can predict and prepare for the challenges that might arise during intubation, ensuring that you're ready with the right tools and techniques to succeed.