What is the LEMON law for airway assessment prior to rapid sequence intubation (RSI)?

Updated: Apr 07, 2020
  • Author: Keith A Lafferty, MD; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
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Several methods exist to quickly assess the probability of success during tracheal intubation. [1] One tool for rapid assessment is the LEMON law, as described below. A patient in extremis may not be able to cooperate with all the sections of the LEMON assessment.

L: Look externally

Assessing the difficulty of an airway based on external physical features is not sensitive (not all patients who have a difficult airway appear to have a difficult airway prior to intubation) but is quite specific (most patients who appear to have a difficult airway do indeed have a difficult airway). Physical features such as a small mandible, large tongue, and short bull neck are all red flags for a difficult airway.

E: Evaluate the 3-3-2 rule

The chance for success is increased if the patient is able to insert 3 of his or her own fingers between the teeth, can accommodate 3 finger breadths between the hyoid bone and the mentum (see hyomental distance in the first image below), and is able to fit 2 finger breadths between the hyoid bone and the thyroid cartilage (see thyrohyoid distance in the second image below).

Hyomental distance (3 finger breadths). Hyomental distance (3 finger breadths).
Thyrohyoid distance (2 finger breadths). Thyrohyoid distance (2 finger breadths).

M: Mallampati classification

The Mallampati assessment is ideally performed when the patient is seated with the mouth open and the tongue protruding without phonating. In many patients intubated for emergent indications, this type of assessment is not possible. A crude assessment can be performed with the patient in the supine position to gain an appreciation of the size of the mouth opening and the likelihood that the tongue and oropharynx may be factors in successful intubation (see image below).

Mallampati classification. Mallampati classification.

O: Obstruction

Obstruction of the upper airway is a marker for a difficult airway. Three signs of upper airway obstruction are difficulty swallowing secretions (secondary to pain or obstruction), stridor (an ominous sign which occurs when < 10% of normal caliber of airway circumference is clear), and a muffled (hot-potato) voice.

N: Neck mobility

The inability to move the neck affects optimal visualization of the glottis during direct laryngoscopy. Cervical spine immobilization in trauma (with a C-collar) can compromise normal mobility, as can intrinsic cervical spine immobility due to medical conditions such as ankylosing spondylitis or rheumatoid arthritis.

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