Relative Risk Analysis of Factors Associated with Difficult Intubation in Obstetric Anesthesia

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Difficult tracheal intubation, often unexpected, has been identified as the commonest contributory factor to anesthetic-related maternal death. The ability to predict such cases preoperatively would be of great value. Preoperative airway assessment and potential risk factors for difficult tracheal intubation were recorded in 1,500 patients undergoing emergency and elective cesarean section under general anesthesia. Airway assessment using a modified Mallampati test recorded oropharyngeal structures visible upon maximal mouth opening. Potential risk factors documented were obesity; short neck; missing, protruding, or single maxillary incisors; receding mandible; facial edema; and swollen tongue. Subsequent to induction of anesthesia, the view at laryngoscopy and difficulty at intubation were graded. There was a significant (P < 0.001) correlation between the oropharyngeal structures seen and both the view at laryngoscopy and difficulty at intubation. Univariate analysis demonstrated a significant association between difficult intubation and short neck (P < 0.001), obesity (P < 0.0001), missing maxillary incisors (P < 0.02), protruding maxillary incisors (P < 0.001), single maxillary incisor (P < 0.0001), and receding mandible (P < 0.003). Neither facial edema (P = 0.414) nor swollen tongue (P = 0.141) were found to be associated with difficult intubation. Multivariate analysis removed obesity and missing and single maxillary incisors as risk factors. Obesity was eliminated because of its strong association with short neck. The probability of experiencing a difficult intubation for various combinations of risk factors was determined. The relative risk of experiencing a difficult intubation in comparison to an uncomplicated class I airway assessment was class II, 3.23; class III, 7.58; class IV, 11.3; short neck, 5.01; receding mandible, 9.71; and protruding maxillary incisors, 8.0. Using the probability index and/or relative risk for various combinations of risk factors may allow preoperative prediction of difficult tracheal intubation.

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