Vapourisers: physical principles and classification

Uncontrolled delivery of liquid volatile anaesthetic when using the anaesthetic conserving device

During patient sedation with liquid volatile anaesthetic, some problems may occur through a process called auto-pumping, defined as an expansion of bubbles inside the syringe, which can lead to uncontrolled anaesthetic delivery. The study examined how the temperature of liquid volatile anaesthetics (sevoflurane and isoflurane) and the presence of gas bubbles in the syringe affect the occurrence of auto-pumping when using the anaesthetic conserving device (ACD, AnaConDa™, Sedana Medical, Uppsala, Sweden). Four different circumstances for each volatile anaesthetic were tested with a bench study: volatile anaesthetic at room temperature or precooled with and without the presence of gas bubbles in the syringe. Liquid volatile anaesthetic was infused into the ACD via a syringe pump at a fixed rate and heated gradually until the temperature of the syringe surface reached 50 °C. A main-stream gas monitor was used to measure the expired fraction of volatile anaesthetic (F_E vol%). The occurrence of auto-pumping was observed only in the subgroups containing gas bubbles, with both anaesthetics. In these subgroups, the values of the expired anaesthetic gas fraction increased dramatically with the expansion of gas bubbles in the syringe (ΔF_E ranged from +1.6 to 2.4 vol% for sevoflurane and +2.3 to 3.4 vol% for isoflurane). Furthermore, when the heat source was removed, a substantial decline in anaesthetic agent values below the baseline was observed with both anaesthetics. The presence of gas bubbles in the syringe, especially when exposed to a heat source, may provoke auto-pumping with uncontrolled excessive anaesthetic delivery. If auto-pumping is suspected, the syringe pump must be stopped and the ACD removed from the breathing circuit at once.