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Study Objective: To determine the optimal solution to use when anesthetizing the airway by aspiration of lidocaine. Design: Randomized, double-blind clinical study. Setting: University hospital. Patients: 96 adult ASA physical status 1,II, and III patients, scheduled for diagnostic flexible bronchoscopy. Interventions: Patients were randomized to receive one of 5 solutions of lidocaine: Group A (n = 16): 1% lidocaine, 0.2 mL · kg −1 ; Group B (n = 16): 1.5% 0.2 mL · kg −1 ; Group C (n = 32): 2% 0.2 mL · kg −1 ; Group D (n = 16): 1% 0.3 mL · kg −1 , and Group E (n = 16): 2% 0.3 mL · kg −1 . Fiberoptic bronchoscopy was performed after the airway was anesthetized with this aspiration technique, using the assigned lidocaine solution. The scope was manipulated in the trachea to test for anesthesia. Measurements and Main Results: Successful airway anesthesia was determined by tolerance to bronchoscopy without sustained coughing, and also by the number of lidocaine supplements, if any, that were given via the bronchoscope. Arterial plasma concentrations of lidocaine were measured in 33 patients from Groups C, D, and E. All solutions provided equally effective anesthesia of the airway. All patients tolerated endoscopy through the vocal cords, and 94 patients required no supplementary anesthesia, or only one dose of lidocaine, during bronchoscopy to the carina. The highest peak plasma concentrations of lidocaine were 5.02 and 6.28 μg · mL. No patient had signs of toxicity. Conclusions: This technique produced anesthesia of the airway to the carina, safely, suitable for awake intubation, in 94 of 95 patients. The use of 1% lidocaine, 0.2 to 0.3 mL · kg −1 , so that the volume is 10 to 20 mL, is recommended.