The analysis of fresh gas flow in a circle anaesthetic breathing system, and its influence on CO2 absorbent desiccation

Abstract

Background: If at the end of anaesthesia, the gases on the anaesthetic machine are not turned off, they continue to flow through the breathing circuit and may dry out the carbon dioxide absorbent in the absorber. At the beginning of the next anaesthetic procedure, the desiccated absorbent decomposes the volatile anaesthetic, resulting in the formation of toxic gas mixtures, which are dangerous for the patient. In such circumstances, the measured level of the volatile anaesthesic in the breathing circuit differs from the selected value. In the paper, we analyse potentially dangerous possible directions of continuous flow of fresh gas mixture in different anaesthetic breathing circuits. Methods: In the first part of the study, we simulated the continuous flow of gases in a Sulla anaesthesia machine using an experimental method in a simulation setting. In the second part, we analysed whether a retrograde flow through the absorber was possible in a convential anaesthetic breathing circuit and in the Fabius GS, Cato, Julian and Primus circuits with the Y-piece connector open or closed. Information on users’ requests for maintenance services due to discrepancies between the desired and measured levels of the volatile anaesthetic in the breathing circuit were obtained from the manufacturer’s local servicing agent. Results: In a Dräger ISO 8 circle system (Sulla anaesthetic machine) with a standard gas supply connector, a retrograde flow is not possible. If the Y-piece connector is left open, the gases are vented to the outside via the absorber and the inspiratory tube. If the Y-piece connector is closed or open, the absorbent in the canister dries because of the continuous flow of gas. A retrograde flow is possible only in those Sulla anaesthetic machines where the fresh gas inlet is located below the inspiratory valve. A retrograde flow of gases causing absorbent desiccation can occur in a Cato, Julian, Fabius GS, or Primus anaesthetic machines with the Y-piece connector closed. Maintenance engineers point out that problems are reported mostly on Monday mornings before the first operation. Conclusions: After completed anaesthesia, the fresh gas inlets on the anaesthetic machine must always be closed. Especially in older machines, the carbon dioxide absorber must be checked before the first operation on a Monday morning. In new-generation machines, we must make sure that the emergency ventilation valve is closed after the end of anaesthesia.