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

Educational Objective
To recognize patients who may be difficult to intubate.
1 Credit CME
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.

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Article Information

Corresponding Author: Damon C. Scales, MD, PhD, 2075 Bayview Ave, Room D108, Toronto, ON M4N 3M5, Canada (damon.scales@sunnybrook.ca).

Correction: This article was corrected on March 24, 2020, to fix an incorrect percentage in the Scenario Resolution section.

Accepted for Publication: December 13, 2018.

Author Contributions: Drs Detsky and Scales had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Detsky, Adhikari, Friedrich, Wijeysundera, Scales.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Detsky, Jivraj, Simel, Scales.

Critical revision of the manuscript for important intellectual content: Jivraj, Adhikari, Friedrich, Pinto, Simel, Wijeysundera, Scales.

Statistical analysis: Detsky, Pinto, Simel, Scales.

Administrative, technical, or material support: Jivraj, Scales.

Supervision: Adhikari, Simel, Scales.

Conflict of Interest Disclosures: Dr Simel reported receiving honoraria for contributions to JAMAEvidence.com; and is supported by the Durham Center of Innovation to Accelerate Discovery and Practice Transformation (CIN 13-410) at the Durham VA Health Care System. Dr Wijeysundera is supported in part by a New Investigator Award from the Canadian Institutes of Health Research and an Excellence in Research Award from the Department of Anesthesia at the University of Toronto. No other disclosures were reported.

Additional Contributions: We acknowledge Jamie Spiegelman, MD (Humber River Hospital, Toronto, Ontario, Canada), for help with a preliminary search and related data abstraction and we thank Daniel Nishijima, MD (University of California-Davis School of Medicine), Karen Welty-Wolf, MD (Durham Veterans Affairs Medical Center and Duke University), and Jonathan Mark, MD (Durham Veterans Affairs Medical Center and Duke University, Durham, NC) for helpful comments on an earlier version of the manuscript. None of those acknowledged were compensated for contributing.

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