Incidence and Predictors of Difficult and Impossible Mask Ventilation
Sachin Kheterpal, Richard, Han, Kevin K. Tremper, Amy Shanks, Alan R. Tait, Michael O'Reilly, and Thomas A. Ludwig
(Anesthesiology, 105:885-891, 2006)
Department of Anesthesiology, University of Michigan Medical School, Ann Arbor, MI
Successful mask ventilation (MV) provides a rescue technique during unsuccessful attempts at laryngoscopy and in cases of an unanticipated difficult airway. Because few studies have focused on MV, investigators reviewed a large number of attempted procedures to determine the incidence and predictors of difficult or impossible MV.
All adult patients undergoing general anesthesia at the study institution during a 24-month period were prospectively included in the study. Among the elements of anesthetic cases reviewed were results of a standard airway physical examination, physical features that may affect mask fit, patient history suggesting anatomical pathology, and general patient and operation features. The 4-point scale used at the study institution was applied to identify cases of grade 3 MV (inadequate, unstable, or requiring 2 providers), grade 4 MV (impossible to ventilate), and difficult intubation. Independent predictors of grade 3 or 4 MV were sought using univariate and multivariate analyses.
Of the 61,252 anesthetic cases reviewed, 22,600 cases included an attempt at MV. Thirty-seven cases (0.16%) were grade 4 MV (impossible to ventilate), and 313 cases (1.4%) were grade 3 MV (difficult to ventilate). There were 84 cases (0.37%) of grade 3 or 4 MV and difficult intubation. Independent predictors for grade 3 MV were body mass index of 30 kg/m2 or greater, presence of a beard, Mallampati classification III or IV, age of 57 years or older, severely limited jaw protrusion, and snoring. These 6 variables were used to create a prediction score. Snoring and a thyromental distance of less than 6 cm were independent predictors for grade 4 MV. Variables independently predictive of grade 3 or 4 MV and difficult intubation were limited or severely limited mandibular protrusion, abnormal neck anatomy, sleep apnea, snoring, and body mass index of 30 kg/m2 or greater.
Patients with 3 or more of 6 independent predictors had a grade 3 MV incidence of 5%, which was almost 20 times the baseline incidence of 0.26% for patients with none of these variables. Overall, the incidence of grade 3 MV (1.4%) was similar to that reported in studies using the same definition of difficult MV. The presence of a beard is the only easily modifiable risk factor. Because of the small number of cases of impossible MV, risk factors for this situation are less certain. Defects in mandibular protrusion may be a common abnormality in cases of difficult MV and difficult intubation.