The airway is one of the most important components in the body to be protected regardless of why a patient is in a hospital, whether for outpatient surgery or admission to the intensive care unit (ICU) for observation and therapy. For this reason, when a physician considers intubation, they must evaluate the risk of failure to intubate and optimize variables for success. One percent to 3% of the patient population that requires endotracheal intubation has difficult airways. Recognizing these patients is crucial as it allows the clinician to prepare accordingly to minimize complications. The 3-3-2 rule is an assessment tool for the prediction of difficult intubations in the unexpected difficult airway.[1][2][3]

According to the American Society of Anesthesiologists, intubation is determined to be difficult to secure when an appropriately trained and experienced anesthesiologist requires more than three attempts or longer than 10 minutes for successful endotracheal intubation. Similarly, ventilation is determined to be difficult when a trained clinician is unable to maintain an oxygen saturation of more than 90% when using a facemask for ventilation, and 100% fraction of inspired oxygen (FIO2) is in use for oxygenation.

The airway should be managed in a very time-sensitive way as poor oxygenation or ventilation can lead to hypoxia and hypercapnic abnormalities; this can be detrimental at the cellular level. Hypoxic brain injury can lead to permanent neuronal damage and acidosis due to hypoxia and hypercapnia, which together can lead to cardiac arrest or death.

The 3-3-2 rule functions to estimate whether the anatomy of the neck will allow for appropriate opening of the throat and larynx. It serves to roughly estimate if the alignment of the openings for direct visualization of the larynx is possible given anatomical findings.[3]

3: A measurement of three fingers between the upper and lower teeth of the open mouth of a patient indicates the ease of access to the airway through the oral opening. A typical patient can open their mouth sufficiently to permit placement of three of their fingers between the incisors. Adequate mouth opening facilitates both insertions of the laryngoscope and obtaining a direct view of the glottis.

3: A measurement of 3 fingers from the anterior tip of the mandible to the anterior neck provides an estimate of the volume of the submandibular space. A typical patient can place three fingers on the floor of the mandible between the mental angle and the neck near the hyoid bone. Normally this distance should measure close to 7 cm. If this distance is less than three finger-widths, the laryngeal axis will be at a more acute angle with the pharyngeal axis, indicating that alignment of the oral opening to the pharyngeal opening will be difficult. It also indicates that there will be less space to displace the tongue within the throat. The rule has limitations as the distance can vary according to height and ethnicity. For this reason, an alternative in the form of a ratio of height to thyromental distance (RHTMD) has been suggested.

2: A measurement of 2 fingers between the floor of the mandible to the thyroid notch on the anterior neck identifies the location of the larynx relative to the base of the tongue. A typical patient can place two fingers in the superior laryngeal notch. If the larynx is too high in the neck, measuring less than two fingers, direct laryngoscopy will be difficult and potentially impossible; this is because the angle between the base of the tongue to the larynx is too acute to be negotiated for direct visualization of the larynx easily.

A likely indication of difficult intubation is present if the inter-incisor or hyoid-mental distance is less than three fingers or the hyoid-thyroid cartilage distance is less than two fingers. Depending on the patient population, reports of difficult intubation occur in 1.5% to 13% of patients. When combined with the Mallampati score in evaluating an airway, the positive predictive value for determining a difficult airway increases.

Additional estimations to be considered in preparing for intubation of a patient should include:

Assessment of atlanto-occipital extension is performed by asking the patient to look at the floor and the wall after fully flexing and fixing the neck.  Flexion movement of the cervical spine is assessed by asking the patient to touch the manubrium sterni with the chin. If successful, this indicates that the flexion and extension range of motion is sufficient to help in aligning the oral pharyngeal and laryngeal axis in a straight line, thus indicating easier intubation.

A combined assessment of the mandibular space with the 3-3-2 rule and atlanto-occipital extension will further determine how easily the laryngeal and pharyngeal axis will fall in line with the atlantoaxial joint during extension of the neck.

The Warning sign of delicate is performed by placing the index finger of each hand, one submental, under the chin, and the other under the inferior occipital prominence with the head in the neutral position. The patient is then asked to extend their head and neck fully. If the submental finger is seen to be higher than the inferior occipital prominence finger, there should be no difficulty with intubation. If the finger on the inferior occipital prominence is still higher than the submental finger, the clinician can anticipate a difficult airway.

Prayer sign is positive when the patient cannot approximate the palmar surfaces of the phalangeal joints while pressing the hands together. This sign presents in advanced diabetes and has a very high positive predictive value for cervical spine immobility and thus difficult endotracheal intubation.

Recognizing that a patient’s airway will be difficult allows the clinician to plan for and minimize the risks of airway-related morbidity. A prospective observational study of 156 patients undergoing intubation in the emergency department found the LEMON scale evaluation accurately stratified patients according to the risk of difficult intubation. The 3-3-2 rule plays a crucial role in planning as a component of the LEMON scale.[4] LEMON stands for:

L: Look externally

Look for external indicators of difficult endotracheal intubation. Which can include the abnormal shape of the face, extreme cachexia, poor dentition, edentulous mouth, morbid obesity, high arching palate, short neck, large front teeth, surgical scar indicating previous tracheostomy scar, indicating patient might have tracheomalacia, narrow mouth, face, or neck pathology.

E: Evaluate 

This is where the 3-3-2 rule is important. It is the estimated measurement of 3 separate distances on the patient using the examiner's fingers.

3: Measurement of the Inter-incisor space, which should be greater than three fingers distance between the upper and lower teeth of the open mouth of a patient.

3: Measurement of the hyoid-mental distance, which should be greater than three fingers from the anterior tip of the mandible to the anterior neck on the hyoid bone.

2: Measurement of the hyoid-thyroid cartilage distance, which should be greater than two fingers between the floor of the mandible at the hyoid bone to the thyroid notch on the anterior neck.

M: Mallampati Scoring

Mallampati scoring is a system based on the anatomy of the mouth and the view of various anatomical structures when one opens his or her mouth as wide as possible. The scoring is done in a sitting position and is not performable in an emergency. A class I score is interpreted as easy, and class IV is the most difficult.[5][6][7]

• Class I: Structures visualized- soft palate, uvula, fauces, anterior and posterior pillars
• Class II: Structures visualized- soft palate fauces and uvula 
• Class III: Structures visualized- soft palate and the base of the uvula
• Class IV: Soft palate is not visible

O: Obstruction

One should assess if the airway could be obstructed with the foreign body, abscess, tumor, soft tissue swelling such as in a burn victim or expanding hematoma in a trauma patient.

N: Neck Mobility

In alert and awake patients, see if the patient can place their chin on their chest and how far back are they able to tilt their head. Decreased neck mobility is a negative predictor of intubation complication.

Pearls

• It is essential to anticipate when a difficult airway may occur. Many patients that otherwise appear normal/easy to intubate may prove difficult. The 3-3-2 rule can help to anticipate complications.
• The 3-3-2 rule is useful, but its significance is greater when combined with the Mallampati score.
• The 3-3-2 rule, along with other estimations, does not play a role in emergent intubations.