Factors Predicting Difficult Endotracheal Intubation

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Factors Affecting ETT Placement
History: History of difficult intubation 16-19
Signs: Upper lip bite test grade 3 14 (95% CI, 8.9-22)
Signs: Shorter hyomental distance 6.4 (95% CI, 4.1- 10)
Signs: Retrognathia 6 (95% CI, 3.1-11)
Signs: Combination of findings on Wilson score 9.1 (95% CI, 5.1-16)
Signs: Impaired neck mobility 4.2 (95% CI, 1.9-9.5)
Signs: Modified Mallampati score ≥3 4.1 (95% CI, 3.0-5.6)

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Factors Affecting ETT Placement
History: Absence of a history of difficult intubation 0.72-0.82
Signs: Upper lip bite test grade 3 0.42
Signs: Shorter hyomental distance 0.84
Signs: Retrognathia 0.85
Signs: Combination of findings on Wilson score 0.60
Signs: Impaired neck mobility 0.77
Signs: Modified Mallampati score ≥3 0.52


Endotracheal intubation is a common procedure in emergency medicine, and recognizing a potentially difficult intubation is imperative in planning for the procedure. While the “can’t intubate, can’t ventilate” scenario is rare, it is catastrophic if the airway operator is not prepared.1, 2, 3 Thus, predicting factors associated with difficult endotracheal intubation is important for emergency clinicians, with consideration of airway adjuncts such as video laryngoscopy, supraglottic airway devices, and cricothyrotomy.4 Some of the factors associated with intubation failure (or difficult intubation) include a history of prior difficult intubation, limited upper lip bite test (the patient bites the upper lip with his/her lower incisors), retrognathia, short thyromental and hyomental distance, decreased cervical spinal motion, higher modified Mallampati classification (defined by visibility of oropharyngeal structures with maximal mouth opening and tongue protrusion), and composite scores such as the Wilson Score (incorporating weight, mobility of the cervical spine and jaw, retrognathia, and incisor appearance).4, 5, 6, 7

The systematic review discussed here included studies evaluating risk factors (based on medical history or physical examination) or clinical tests that could predict difficult intubation (outcome) in adults (> 18 years) undergoing endotracheal intubation with direct laryngoscopy.8 Authors assessed the quality of the included trials using the Rational Clinical Examination (RCE) series quality checklist.9

The authors of the meta-analysis identified 62 relevant studies (n = 33,559 patients), which were all performed in the operating room (OR). The overall prevalence of difficult intubation was 10% (95% confidence interval [CI], 8.2% - 12%) which was most commonly defined by Cormack-Lehane grade 3 or 4.10 Cormack-Lehane grade 3 is defined as only the epiglottis visualized and grade 4 by neither glottis nor epiglottis seen on direct laryngoscopy.10 Other definitions included combination of Cormack-Lehane grade with additional requirements such as number of intubation attempts, time, use of bougie in 6 studies; percentage of glottic opening in 1 study; Intubation Difficulty Scale score > 5 in 3 studies; or minimum intubation time or number of attempts in 5 studies. History of prior difficult intubation was associated with an increased likelihood of difficult intubation (positive likelihood ratio [LR+]: 16 - 19).

Clinical examination findings including upper lip bite test class 3, defined as inability to bite any part of the upper lip with lower incisors, was a strong predictor of difficult intubation (LR+: 14; 95% CI, 8.9 - 22). Other findings, such as retrognathia (LR+: 6.0; 95% CI, 3.1 - 11), hyomental distance <3 - <5.5 cm (LR+: 6.4; 95% CI, 4.1 - 10), impaired neck mobility (LR+: 4.2; 95% CI, 1.9 - 9.5), impaired mouth opening (LR+: 3.6; 95% CI, 2.1 - 6.1), and the modified Mallampati score > 3 (LR+: 4.1; 95% CI, 3.0 - 5.6) also predicted difficult intubation. The Wilson score was also a strong predictor of difficult intubation (LR+: 9.1; 95% CI, 5.1 - 16). However, no clinical factor or composite score was useful in excluding difficult intubation. Sensitivity analyses did not change interpretation of results.8


The trials included in the systematic review (rated as high-quality) identified certain findings are associated with an increased risk of difficult intubation. However, none of the findings were sufficient to exclude this. There was some variability in the reference standard used among studies to define a difficult airway, though the majority of studies incorporated the Cormack-Lehane classification system.10 In addition, studies that used the time of intubation or number of intubation attempts to define a difficult airway might have been influenced by the individual clinician’s ability or experience in intubation. Several predictors such as impaired cervical motion and retrognathia are subjective and vulnerable to inter-observer variability.

Authors of the systematic review limited their analysis to studies with independent assessments of predictors and outcomes in order to reduce bias. This led to exclusion of studies conducted in emergency settings. Therefore, all studies included in the systematic review were performed in the OR setting, limiting the applicability to the emergency department (ED) setting. Endotracheal intubation in the OR setting is more commonly associated with a nonemergent need for endotracheal intubation. While ED patients may differ with regard to mental and hemodynamic status, presence of gastric contents or vomiting, and ability to cooperate well with the assessments, knowledge of factors associated with difficult intubation and adequate preparation are still essential. Finally, this analysis evaluated only direct laryngoscopy. Therefore, the results of this review may not reflect current airway technology incorporating video laryngoscopy, extraglottic airway devices, and other advanced techniques.

In summary, the existing evidence indicates that several findings predict a difficult endotracheal intubation, but their absence cannot reliably exclude this scenario. The most accurate assessment was the upper lip bite test, followed by shorter hyomental distance, retrognathia, impaired neck mobility, modified Mallampati score > 3, and the Wilson score. Future studies should incorporate new airway technology such as video laryngoscopy and include emergency situations.

The original manuscript was published in Academic Emergency Medicine as part of the partnership between TheNNT.com and AEM.


Brit Long, MD; Alex Koyfman, MD; Michael Gottlieb, MD, RDMS
Supervising Editor: Shahriar Zehtabchi, MD


July 1, 2019

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)