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

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.

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.

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.

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