Low–flow anaesthesia does not increase the risk of microbial contamination through the circle absorber system

Abstract

In the circle absorber system, a decrease in fresh gas flow means a higher degree of rebreathing, and, consequently, a higher temperature and humidity within the system. With our present hygienic routines, the circle system tubings are changed and decontaminated once daily. Thus, the same circle system is used for several patients each day. In order to evaluate whether the risk of bacterial contamination increased with the introduction of low-flow anaesthesia, 122 patients anaesthetized with either a low-flow technique (less than 1.5 l fresh gas flow/min) or with medium fresh gas flows (3-6 l/min) were studied. Bacterial samples were taken preoperatively from the oropharynx and postoperatively from five locations in the circle system. The patients were studied postoperatively for signs of respiratory tract infection. There were few positive bacteria cultures from the tubings in the circle system, regardless of fresh gas flow. No pathogens were found in the inspiratory tubings and no cases of postoperative respiratory tract infection could be related to bacterial spread from the anaesthesia machine. There were no indications that the present hygienic management was insufficient for the medium- or the low-flow circle system techniques.

Effects of sevoflurane and/or nitrous oxide on bacterial growth in in vitro culture conditions

“…The contact between the gases and the bacteria was performed in liquid at 37°C, which corresponds to clinical conditions and gives optimal conditions for bacterial growth. Soda lime was not used in the circuit because of its controversial bactericidal effects [5,7]. Furthermore, it is known that soda lime is a carbon dioxide absorbant and the use of soda lime is not necessary in clinical pediatric anesthesia practice, unless carbon dioxide is present in the anesthetic circuit.…”

Bacterial and viral contamination of breathing circuits after extended use – an aspect of patient safety?

“…Particularly during low-flow anaesthesia, when increased temperature and humidity within the circle system occurs, the risk of endoluminal bacterial growth during prolonged use of ABC has to be considered, as the CO2-absorber might provide a very hostile environment for microorganisms. After 1 day of use, Bengtson et al 29 found a few positive bacteria cultures in the circle system regardless of fresh-gas flow. Our microbiological results were obtained in ABC applied for low-flow anaesthesia; samples obtained from the y-piece and the water trap indicated no difference in the rate of endoluminal contamination of ABC after 1 day compared to 7 days of use.…”

Effects of isoflurane on bacterial growth

“…The fresh gas was air and oxygen; we avoided nitrous oxide because of a possible bias in interpretation [18]. Soda lime was not included in the circuit because of its controversial bactericidal effects [19,20]. Isoflurane was delivered at a concentration equivalent to a MAC of 1.5, which is a normal clinical concentration in air and which is also the concentration at which there are effects on the immune system [8,9,18].…”