What is the optimal volume of administration for endobronchial drugs?

During general anesthesia, three groups of six patients each received 2 mg/kg lidocaine as a marker substance endobronchially in either 10, 5, or 3 mL distilled water. It was found that the group receiving 10 mL initially exhibited the highest lidocaine plasma concentration with a mean of 2.01 micrograms/mL, in comparison with 1.25 micrograms/mL in the 5 mL group and 0.95 micrograms/mL in the 3 mL group. After about 10 minutes, concentration courses were almost the same in the 10 mL and the 5 mL groups. The PaO2 in the 10 mL group dropped initially by approximately 40 mm Hg on average and remained low over 60 minutes. By this time the PaO2 in the 5 mL group (initial drop 46 mm Hg) had come back to the original value (P less than .05). The 3 mL group exhibited even more favorable courses in the PaO2 (initial drop 16 mm Hg on average). However, the lidocaine plasma concentration was at the lowest at all times in this group and, moreover, under the therapeutic level of 1.5 micrograms/mL with the dosage used.