Indirect laryngoscopy with rigid 70-degree laryngoscope as a predictor of difficult direct laryngoscopy

Abstract

Introduction and objectives: The commonly-used predictors for difficult airway management are not very accurate. We investigate the power of indirect laryngoscopy with the rigid 70-degree laryngoscope as a predictor of difficult visualisation of the larynx with direct laryngoscopy.

Methods: We performed preoperative indirect laryngoscopy with the rigid laryngoscope on 300 patients. The vision obtained was classified into four grades: 1 (vocal cords visible), 2 (posterior commissure visible), 3 (epiglottis visible) and 4 (no glottic structure visible). Grades 3 and 4 were considered predictors of difficult larynx visualisation. Next, direct laryngoscopy with the Macintosh laryngoscope was carried out on the patients under general anaesthesia. Positive value was defined as a Cormack and Lehane III and IV. Other common clinical predictors were also analysed. A logistic regression model using the relevant variables was elaborated. We also investigated predictors of difficult visualisation of the larynx with indirect laryngoscopy.

Results: The model found and the coefficients for preparing it were: f(x)= -10.097+5.145 indirect laryngoscopy (3-4)+3.489 retrognathia+2.548 mouth opening <3.5 cm+1.911 thyromental distance <6.5 cm+.352 snorer+(0.151 cm neck thickness). This model provided a correct result in 94.3% of cases. In the case of indirect laryngoscopy, the model found was: f(x)=-2.641+0.920 snorer+0.875 cervical mobility.

Conclusions: Indirect laryngoscopy was the independent variable with the greatest predictive power. Snoring is a common predictor in both laryngoscopy models.