Predicting Difficult Intubation – What RTs Need to Know

By Sara Zaib Khan, MD; Atheena Saran Antony, BDS; Rammohan Gumpeni, MD; Arunabh Talwar, MD, FCCP

Health care providers are concerned about difficult airway management because failure to secure an airway can result in difficulties, as well as morbidity, and even death. According to the American Society of Anesthesiologists, Intubation is called ‘difficult’ if a certified anesthesiologist needs more than three attempts or more than 10 minutes to achieve thorough intubation. Difficult laryngoscopy and difficult endotracheal intubations occur in 1.5% to 13% of patients who receive general anesthesia.¹

There are many ways of predicting difficult intubation. Healthcare providers should be aware of the simple methods which predict difficult intubations. Here are some of the commonly used parameters to predict the pretest possibility of difficult intubation.

Mallampati Score

Mallampati score is a categorization based on oropharyngeal anatomic characteristics. The oropharynx is visually graded on a scale of 1-4.² Mallampati score is commonly used to predict the possibility of intubation difficulties during general anesthesia. This scoring method has also been shown to be effective in predicting Obstructive Sleep Apnea (OSA).³

The Mallampati classification system was first described by Dr. Sheshagiri Rao Mallampati, an anesthesiologist, in 1983.⁴ The score is based on healthcare providers’ visual examination of the oropharynx to find if the tongue obstructs visualization of the uvula and faucial pillars.²

To calculate the Mallampati score the clinician tries to view oropharyngeal structures with the patient’s mouth open and the tongue protruded as follows:⁵

Class I: The entire tonsillar pillars, uvula, hard and soft palates are visualized

Class II: Partial uvula and soft palate are visualized

Class III: Only the soft palate is visualized

Class IV: No visualized structures beyond the tongue

The scores of 1 and 2 are considered low risk, whereas higher Mallampati scores of 3 and 4 indicate difficulty in intubation. This evaluation is non-invasive, and no special tools are needed, giving it the upper hand.⁵

Today Mallampati scoring has been adopted as a standard assessment step prior to anesthesia and sedation during treatment because it shows a positive correlation in finding difficult intubations.  We would like to emphasize that the Mallampati score is used as a helpful aid but does not replace the typical airway evaluation prior to intubation that is performed.² It also has an independent association between the presence and severity of OSA.

The upper airway abnormalities predispose to difficult tracheal intubation in some ways also predispose to obstructive sleep apnea (OSA). Difficult intubation and OSA are related significantly.⁶ They share anatomical features which act to reduce the skeletal confines of the tongue.³ The potential association is important as both conditions increase perioperative risk and patients with a trachea that is difficult to intubate may need assessment for OSA.

It has been proven that the Mallampati score is an independent predictor of both the presence and severity of obstructive sleep apnea. On average, for every 1-point increase in the Mallampati score, the odds of having obstructive sleep apnea (apnea-hypopnea index> or = 5) increased more than 2-fold.⁵ In addition it is well known that nasal obstruction is associated with higher incidences of sleep apnea, primarily by increasing the negative pressure on the airway during inspiration. The combination of a high Mallampati score and nasal obstruction represents a greater risk factor for worsening OSA.⁷

Sleep apnea is probably the most prevalent of all sleep disorders. Nocturnal attended polysomnography is the gold standard for diagnosing OSA. However, Mallampati scoring is a simple, inexpensive, way to suspect OSA in someone by routinely examining the oral cavity.⁸

In addition to the Mallampati score, various other oropharyngeal parameters including the upper lip bite test, thyromental distance, hyomental distance, inter-incisal gap, non-compliant submandibular space, and retrognathia are used to determine intubation difficulties.¹

Upper lip bite test [ULBT]

The ULBT test is performed in a neutral position by asking the patient to bite their upper lip. This test is performed by assessing the mandibular skeletal movements, and the movements of muscles, ligaments, and soft tissues as well. The Upper Lip Bite Test is classified as CLASS I, II OR III.^1,9 ULBT is shown to be helpful in predicting intubation difficulty where Class 3 is the poor outcome in intubation.

Class I: the patient can fully cover the upper lip with lower incisors

Class II: The patient can partially cover the upper lip with lower incisors

Class III: The patient cannot reach the upper lip with lower teeth

Thyromental distance

Thyromental distance is another orofacial measurement used to rate the difficulty in intubation. This test is carried out by measuring the distance from a point from the bulge of the thyroid cartilage to the mental protuberance of the mandible. If the distance between these points is shorter, greater will be the potential for difficult intubation. It is classified as ^1,9,10

Class I- distance is greater than 6.5cm

Class II- distance is between 6-6.5 cm

Class III – distance less than 6 cm

Hyomental distance

This measurement is also a predictor of difficulty in intubation and laryngoscopy. Hyomental distance is the distance between the body of the hyoid bone and the mentum of the mandible. The hyomental distance ratio is the ratio between the hyomental value when the head is at maximum extension and in a neutral position. This measurement has the sensitivity to predict intubation difficulties. If the value of the Hyomental distance ratio (HMDR) is 1.2 or lesser indicates difficulty in intubation. This test is irrespective of age or sex.^1,9

Inter-Incisal Gap

This measures the distance between the incisal edges of maxillary and mandibular teeth. The gap is less than 3 cm, the prospect of difficult intubation. The gap of less than 2 cm is presumably difficult to intubate.^1,9

Non-compliant submandibular space

This method is often used to predict difficulty in intubation. The submandibular space will be studied while a patient is in a supine position. This method checks for the compliance of submandibular space. A bulky non-compliant space is called a submental sign. A hyomental distance of less than 2 cm is considered a positive submental sign, which is suggestive of difficulty in intubation. In this condition, the larynx should be placed anteriorly.¹

Retrognathia

Retrognathia refers to the unusual backward position of the mandible. This may also be due to the deficient growth of the mandible. It can be identified by measuring from the angle of the mandible to the mental protuberance. Any measurement is less than 9 cm consider retrognathia which is an indication of difficult intubation.^1,9

Cervical Spine Mobility and Sternomental Distance

The degree of flexion and extension of the cervical spine, and the neurological symptoms resulting from neck movement should be evaluated prior to intubation. Patients with good cervical spine mobility have a longer sternomental distance which is the distance between the upper part of the sternum and the tip of the jaw with the neck fully extended. Poor mobility of the cervical spine can make intubation difficult.⁹

Neck circumference

A short thick neck is suggestive of difficult intubation. A neck circumference greater than 40 cm puts patients at risk for an intricate intubation. (Dawood)

Abnormal teeth Alignment

In addition, crooked or malposition teeth can reduce the visibility of vocal folds which may lead to difficult intubation. ⁹

The greater the number of positive findings, the more likely intubation by direct laryngoscopy will be difficult. The highest positive predictive value comes from a history of difficulty with intubation, or findings of a short thyromental distance or decreased range of motion of the neck

Conclusion

Incorporating a head and neck quick physical exam, including calculating the Mallampati score can prepare healthcare providers in predicting difficulty in intubation and ensure that the procedure is performed without any complications.

Predictors of difficult endotracheal intubation

• Prior difficult intubation
• Interincisal (inter gingiva in edentulous patients) gap <3 cm
• Thyromental distance <6 cm
• Sternomental distance <12
• Hyomental Distance
• Head and neck extension <30 degrees from neutral
• Mallampati oropharyngeal classifications class III or IV
• Inadequate mandibular protrusion (inability to place lower incisors in front of upper incisors)
• Upper Lip Bite Test Class III
• Neck circumference >40 cm

Lack of submental compliance (hard and non-compliant)

References

1. Dawood AS, Talib BZ, Sabri IS. Prediction of Difficult Intubation by Using Upper Lip Bite, Thyromental Distance and Mallampati Score in Comparison to Cormack and Lehane Classification System. Wiad Lek;74:2305-14.
2. Green SM, Roback MG. Is the Mallampati Score Useful for Emergency Department Airway Management or Procedural Sedation? Ann Emerg Med;74:251-9.
3. Liistro G, Rombaux P, Belge C, Dury M, Aubert G, Rodenstein DO. High Mallampati score and nasal obstruction are associated risk factors for obstructive sleep apnoea. Eur Respir J 2003;21:248-52.
4. Mallampati SR. Clinical sign to predict difficult tracheal intubation (hypothesis). Can Anaesth Soc J 1983;30:316-7.
5. Nuckton TJ, Glidden DV, Browner WS, Claman DM. Physical examination: Mallampati score as an independent predictor of obstructive sleep apnea. Sleep 2006;29:903-8.
6. Hiremath AS, Hillman DR, James AL, Noffsinger WJ, Platt PR, Singer SL. Relationship between difficult tracheal intubation and obstructive sleep apnoea. Br J Anaesth 1998;80:606-11.
7. Rodrigues MM, Dibbern RS, Goulart CW. Nasal obstruction and high Mallampati score as risk factors for Obstructive Sleep Apnea. Braz J Otorhinolaryngol;76:596-9.
8. Votolato GS, Henry JK, Brooks JK, Cohen L, Bashirelahi N. What every dentist needs to know about obstructive sleep apnea. Gen Dent;68:30-4.
9. Detsky ME, Jivraj N, Adhikari NK, et al. Will This Patient Be Difficult to Intubate?: The Rational Clinical Examination Systematic Review. JAMA;321:493-503.
10. Rao KVN, Dhatchinamoorthi D, Nandhakumar A, Selvarajan N, Akula HR, Thiruvenkatarajan V. Validity of thyromental height test as a predictor of difficult laryngoscopy: A prospective evaluation comparing modified Mallampati score, inter-incisor gap, thyromental distance, neck circumference, and neck extension. Indian J Anaesth;62:603-8.

 

Sara Zaib Khan, M.D.; Atheena Saran Antony, BDS; Arunabh Talwar, M.D., F.C.C.P., Northwell Health Department of Pulmonary, Critical Care and Sleep Medicine. Rammohan Gumpeni, M.D., New York Hospital of Queens, Department of Pulmonary Medicine.