Purpose: Exchanging the tracheal tube for the laryngeal mask airway (the TT/LMA exchange) while the patient is still anesthetized avoids coughing associated with tracheal extubation. This study was conducted to determine the end-tidal concentration of sevoflurane that prevented response to this exchange in 50% patients (MAC(TT/LMA)).
Methods: We studied 21 adult male patients, ASA I, aged 21-54 yr. Anesthesia was induced with thiamylal and sevoflurane, and the trachea was intubated following vecuronium neuromuscular blockade. Anesthesia was maintained with sevoflurane and nitrous oxide in oxygen. After surgery, a predetermined end-tidal sevoflurane concentration was achieved and a steady state was maintained for at least 20 min. The concentration at which the TT/LMA exchange was attempted was determined by a modification of Dixon's up-and-down method with 0.25% as the step size. At the time of the TT/LMA exchange, no residual nitrous oxide > 3% was detected, and the return to normal neuromuscular function was confirmed. When the TT/LMA exchange was accomplished without coughing, movement, or airway obstruction, it was considered a smooth exchange.
Results: Sevoflurane MAC(TT/LMA) determined using the up-and-down method was 2.63% +/- 0.14%. The 50% effective dose obtained using a probit analysis was similar [2.53% (95% confidence limits, 2.13-2.82%)].
Conclusion: Sevoflurane MAC(TT/LMA) in adult male patients was 2.63% (1.54 MAC) and may be useful for the smooth exchange of the tracheal tube for the LMA in a clinical setting.