Filtration in Anaesthetic and ICU Breathing Systems

There is now a wealth of clinical papers demonstrating that breathing circuits used during anaesthesia or ICU can become contaminated during use. Contamination of breathing systems can arise either via medical gases or through the patient. Several studies have shown that medical gases are not sterile and it has been observed that these gases can act as vectors transporting organisms around the circuit. However, it is the patient that is the major source of initial contamination in a breathing circuit.

With regard to anaesthetic circuits, both non-rebreathing and rebreathing or circle systems can become contaminated with microbes during use. Studies have shown that the spread of organisms can take place against the gas flow (retrograde flow). Work investigating circle system contamination is interesting from several points of view. In the past many people believed that colonisation and growth of bacteria in circle systems was not a problem as it was thought that soda lime had a bacteriocidal effect and thus “sterilised” the system. However, it has been found that bacteria can be rapidly spread and recovered from all components of a circle system15 and that organisms can pass through a soda lime cannistor and remain viable. Another point of interest with these systems is that the reaction between soda lime and CO2 is highly exothermic, leading to the generation of heat and moisture. Considerable condensation can therefore occur with these systems, particularly during long operations. Microbes may colonize this condensed water and proliferate.

Biofilm formation at the end of endotracheal tubes has been noted, allowing bacterial proliferation to occur. Particles can be projected from the biofilm into the breathing circuit thus allowing this contamination to be easily disseminated. Work has also shown that anaesthetic endotracheal tubes harbour viruses and that viral replication is unaffected by volatile anaesthetic agents such as halothane or isoflurane. Furthermore, studies have shown that 76-86% of endotracheal tubes have some degree of blood contamination (either visible or subvisible) after use. In Australia recently, five patients who all used the same anaesthetic circuit were found to have contracted Hepatitis C. It is believed that a patient carrying Hepatitis C may have coughed up blood stained respiratory secretions into the anaesthetic circuit. Four other patien ts who used this circuit are believed to have subsequently become infected.

Nosocomial infections acquired via the respiratory tract are an expensive, time consuming and potentially fatal complication of anaesthesia and respiratory therapy. Several groups have recommended use of individually clean anaesthesia or regular tubing change practices in the ICU as a potential means of reducing the incidence of these infections. There are several ways in which breathing systems can be decontaminated. However, sterilisation and disinfection methods can be time consuming, expensive and may also be environmentally unfriendly. In addition, there have been a number of reports in the literature that show that these methods are not always completely effective. In some of these studies patients actually acquired infections from equipment following defective decontamination regimes. Recently disposable circuits and filters have become very popular in anaesthesia and intensive care.

Incorporating a filter into the breathing system is an increasingly popular way of providing individually clean anaesthesia or allowing ICU circuits to remain free of patient microflora. In a recent editorial in the British Journal of Anaesthesia, Snowdon stated that there is an argument for using a microbial filter for every anaesthetic, as the anaesthetist will not always be aware that the patient may be carrying a high risk infectious disease. As the patient is the most likely source of initial contamination of a circuit probably the most logical place to position the filter is as near to the patient as possible. Hedley and Allt-Graham have proposed that the “ideal patient end” filter should totally retain contaminated liquids, have a high airborne microbial removal efficiency, a low resistance during clinical use and be able to be left in place during nebulisation. In addition the device showed also act as a heat and moisture e xchanger (HME). Currently there are a whole range of different devices that have been proposed for use at the “patient end” of a breathing circuit and it is extremely difficult to know whether or not a device possesses these attributes. Fortunately things can be simplified to some extent as all of the available products can be divided into three groups.

USA	China	Japan
Toll Free: 1-866-528-4572 Fax: 1-732-412-4040	Toll Free: 400-820-0293 Fax: 86-21-51687551	Toll Free: 81-047-710-7392 Fax: 81-3-6740-1115