Rebounds of sevoflurane concentration during simulated trigger-free pediatric and adult anesthesia

In trigger-free anesthesia a volatile anesthetic concentration of 5 parts per million (ppm) should not be exceeded. According to European Malignant Hyperthermia Group (EMHG) guideline, this may be achieved by removing the vapor, changing the anesthetic breathing circuit and renewing the soda lime canister followed by flushing with O2 or air for a workstation specific time. Reduction of the fresh gas flow (FGF) or stand-by modes are known to cause rebound effects. In this study, simulated trigger-free pediatric and adult ventilation was carried out on test lungs including ventilation maneuvers commonly used in clinical practice. The goal of this study was to evaluate whether rebounds of sevoflurane develop during trigger-free anesthesia.

A Dräger® Primus® was contaminated with decreasing concentrations of sevoflurane for 120 min. Then, the machine was prepared for trigger-free anesthesia according to EMHG guideline by changing recommended parts and flushing the breathing circuits using 10 or 18 l⋅min- 1 FGF. The machine was neither switched off after preparation nor was FGF reduced. Simulated trigger-free ventilation was performed with volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) including various ventilation maneuvers like pressure support ventilation (PSV), apnea, decreased lung compliance (DLC), recruitment maneuvers, prolonged expiration and manual ventilation (MV). A high-resolution ion mobility spectrometer with gas chromatographic pre-separation was used to measure sevoflurane in the ventilation gas mixture in a 20 s interval.

Immediately after start of simulated anesthesia, there was an initial peak of 11-18 ppm sevoflurane in all experiments. The concentration dropped below 5 ppm after 2-3 min during adult and 4-18 min during pediatric ventilation. Other rebounds of sevoflurane > 5 ppm occurred after apnea, DLC and PSV. MV resulted in a decrease of sevoflurane < 5 ppm within 1 min.

This study shows that after guideline-compliant preparation for trigger-free ventilation anesthetic machines may develop rebounds of sevoflurane > 5 ppm during typical maneuvers used in clinical practice. The changes in rate and direction of internal gas flow during different ventilation modes and maneuvers are possible explanations. Therefore, manufacturers should provide machine-specific washout protocols or emphasize the use of active charcoal filters (ACF) for trigger-free anesthesia.