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The Rational Clinical Examination
February 5, 2019

Will This Patient Be Difficult to Intubate?The Rational Clinical Examination Systematic Review

Author Affiliations
  • 1Department of Medicine, Sinai Health System, Toronto, Ontario, Canada
  • 2Interdepartmental Division of Critical Care Medicine and Department of Medicine, University of Toronto, Toronto, Ontario, Canada
  • 3Institute for Health Policy, Management and Evaluation, University of Toronto, Toronto, Ontario, Canada
  • 4Department of Anesthesia, University of Toronto, Toronto, Ontario, Canada
  • 5Department of Critical Care Medicine, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada
  • 6Departments of Critical Care Medicine and Medicine, St Michael’s Hospital, Toronto, Ontario, Canada
  • 7Li Ka Shing Knowledge Institute, St Michael’s Hospital, Toronto, Ontario, Canada
  • 8Department of Medicine, Duke University School of Medicine, Durham, North Carolina
  • 9Durham Veterans Affairs Medical Center, Durham, North Carolina
  • 10Department of Anesthesia and Pain Management, University Health Network, Toronto, Ontario, Canada
JAMA. 2019;321(5):493-503. doi:10.1001/jama.2018.21413
Key Points

Question  Which risk factors and physical findings can help predict difficult endotracheal intubation?

Findings  In this systematic review, several physical findings increased the likelihood of difficult intubation. The best predictors were an inability to bite the upper lip with the lower incisors, a short hyomental distance, retrognathia, or a combination of findings based on the Wilson score. No risk factor or physical finding consistently ruled out a potentially difficult intubation.

Meaning  Although a variety of tests are helpful in identifying a potentially difficult intubation, the inability to bite the upper lip with the lower teeth was the best predictor.


Importance  Recognizing patients in whom endotracheal intubation is likely to be difficult can help alert physicians to the need for assistance from a clinician with airway training and having advanced airway management equipment available.

Objective  To identify risk factors and physical findings that predict difficult intubation.

Data Sources  The databases of MEDLINE and EMBASE were searched from 1946 to June 2018 and from 1947 to June 2018, respectively, and the reference lists from the retrieved articles and previous reviews were searched for additional studies.

Study Selection  Sixty-two studies with high (level 1-3) methodological quality that evaluated the accuracy of clinical findings for identifying difficult intubation were reviewed.

Data Extraction and Synthesis  Two authors independently abstracted data. Bivariate random-effects meta-analyses were used to calculate summary positive likelihood ratios across studies or univariate random-effects models when bivariate models failed to converge.

Results  Among the 62 high-quality studies involving 33 559 patients, 10% (95% CI, 8.2%-12%) of patients were difficult to intubate. The physical examination findings that best predicted a difficult intubation included a grade of class 3 on the upper lip bite test (lower incisors cannot extend to reach the upper lip; positive likelihood ratio, 14 [95% CI, 8.9-22]; specificity, 0.96 [95% CI, 0.93-0.97]), shorter hyomental distance (range of <3-5.5 cm; positive likelihood ratio, 6.4 [95% CI, 4.1-10]; specificity, 0.97 [95% CI, 0.94-0.98]), retrognathia (mandible measuring <9 cm from the angle of the jaw to the tip of the chin or subjectively short; positive likelihood ratio, 6.0 [95% CI, 3.1-11]; specificity, 0.98 [95% CI, 0.90-1.0]), and a combination of physical findings based on the Wilson score (positive likelihood ratio, 9.1 [95% CI, 5.1-16]; specificity, 0.95 [95% CI, 0.90-0.98]). The widely used modified Mallampati score (≥3) had a positive likelihood ratio of 4.1 (95% CI, 3.0-5.6; specificity, 0.87 [95% CI, 0.81-0.91]).

Conclusions and Relevance  Although several simple clinical findings are useful for predicting a higher likelihood of difficult endotracheal intubation, no clinical finding reliably excludes a difficult intubation. An abnormal upper lip bite test, which is easily assessed by clinicians, raises the probability of difficult intubation from 10% to greater than 60% for the average-risk patient.