In vitro diffusion of lidocaine across endotracheal tube cuffs

Purpose: Lidocaine diffuses across endotracheal tube cuffs, which may serve as a reservoir for local anesthetic to assist in the prevention of ETT- induced cough while emerging from general anesthesia. However, the rate of diffusion is slow. Two techniques, alkalization and warming, may increase the proportion of uncharged drug available for diffusion. The purpose of this study is to determine the effectiveness of warming alkalization or warming with alkalization on diffusion. Methods: Four preparations of lidocaine 4% were studied. Group (Gr) L -lidocaine (24°C), Gr WL - warmed lidocaine (38°C), Gr AL - alkalized lidocaine (24°C), Gr WAL - warmed, alkalized lidocaine (38°C). Twenty-four Mallinckrodt 8.0 ID (Mallinckrodt Critical Care Division of Mallinckrodt, Inc., Glens Falls, New York) endotracheal tube cuffs were filled with 6 ml of one of the four preparations. They were then placed in a 20 ml water bath at 38°C and samples were drawn from the water bath at intervals for up to 360 min. The lidocaine concentration in each sample was determined by gas chromatography. Results: The highest lidocaine concentration was reached in Gr WAL (410.98 ± 8.53 μg · ml^-1) after 300 min and then decreased to 376.18 ± 4.59 μg · ml^-1 after 360 min. In Gr AL the highest concentration (235.05 ± 2.99 μg · ml^-1) was reached after 360 min. Lidocaine concentrations in Gr L and WL after 360 min were 3.19 ± 1.16 μg · ml^-1 and 4.32 ± 2.02 μg · ml^-1 respectively. Conclusion: Alkalization with or without warming, but not warming alone, promotes lidocaine diffusion from endotracheal tube cuff.