Filters and small infants

In January of this year, the Association of Anaesthetists of Great Britain and Ireland (AAGBI) published the third edition of its ‘Checking Anaesthetic Equipment’ glossy [1]. This represents an updated version of the edition published in 1997 with several additions and amendments. Of particular note is a change concerning the use of ‘single-use bacterial/viral filters’. In 1997, the AAGBI made no reference to the use of bacterial/viral filters. However, in the latest edition they recommend that ‘a new, single-use bacterial/viral filter and angle piece/catheter mount must be used for each patient’. For small infants this may not always be the best way to humidify the patients' airway as filter devices are known to increase airway resistance. In an adult volunteer study the use of an HME increased the work of breathing by 70%[2]. Small filters are available for babies but the increase in breathing system resistance will result in an increase in the work of breathing in a group of patients who are poorly equipped to tolerate this. These patients would usually be paralysed and ventilated for surgery but they will have to re-establish spontaneous ventilation adequately prior to tracheal extubation, and this period of spontaneous ventilation under anaesthesia should be made as easy as possible for the patient. In patients weighing less than 3 kg, it may be best to exclude a breathing system filter and use a disposable circuit with a water bath humidifier. We are aware of two single-use bacterial/viral filters for use in babies weighing more than 3 kg. The Clear-Therm Micro HMEF (Intersurgical UK, Berkshire, UK) and the Hygrobaby (DAR-Mallinckrodt, Tyco Healthcare UK Ltd, Hampshire, UK) are both recommended for use in babies down to a weight of 3 kg. These are marketed as bacterial/viral filters and in our institution we use them to protect the breathing circuit from contamination. This is important as we often use a separate circuit for individual patients at induction, maintenance of anaesthesia, and transfer of the patient to recovery. These ‘protected’ circuits are then re-used for subsequent patients and discarded if contamination is thought to have occurred or after a period of use in line with current recommendations. At present, there are no bacterial/viral filters available for babies weighing less than 3 kg. Therefore, unless we use a filter against the manufacturers' advice, it is not possible to adhere to the AAGBI's recommendations for use of filters in this group of patients. We were also confused by an additional recommendation on the AAGBI members' news website discussing highlights from their recent publication [1,3]. It suggested that ‘paediatric filters do not have sufficient internal surface area to afford the same degree of protection’ of breathing circuits and that ‘in paediatric anaesthesia the whole circuit should therefore be changed for each patient’. If we were to implement this recommendation for all paediatric patients in our institution, about 10 000 disposable circuits would be discarded each year. This would represent a four-fold increase in current expenditure on breathing circuits. Drs Moores and Bell raise two very interesting points. I agree entirely with their first point that in infants weighing less than 3 kg it may be best to exclude a breathing system filter and use a disposable circuit with a water bath humidifier. Our recommendation that ‘a new single use…filter…be used for each patient’[1] is to protect the breathing circuit from contamination so that it can safely be reused. If the unprotected disposable circuit is thrown away after single patient use, there should be no risk of contamination. This is completely in line with AAGBI guidelines. Regarding the second point which refers to children over 3 kg and the relative inefficiency of paediatric breathing system filters compared to adult filters, current advice (which appeared on the AAGBI website and also in Anaesthesia News), is that, based on discussions with the British Anaesthetic and Respiratory Equipment Manufacturing Association (BAREMA) and the Medical and Healthcare products Regulatory Agency (MHRA), paediatric filters do not offer the same degree of protection as adult filters, and breathing circuits should be changed for each new patient. This is based on a review of 104 adult and paediatric breathing filters [2] carried out by the Cardiff Evaluation Unit of the MHRA and published on the MHRA website (http://www.medical-devices.nhs.uk). This evaluated the performance of filters by determining the penetrance of salt particles and has led to the release of an MHRA Alert [3] drawing attention to this standardised method of comparison. The test is an in vitro test using flow rates of 30 l.min−1 for adult filters and 15 l.min−1 for paediatric filters – flow rates which are comparable to peak flows attained during respiration. There is no recommendation on a minimum value for penetrance to qualify as a ‘pass’, nor is there any clinical evidence that correlates salt particle penetrance with transmission of infective agents. Furthermore, some adult filters also have a relatively high penetrance value. It is clear from my correspondence that in many paediatric units the whole circuit is not routinely changed between cases, except in special circumstances which may vary from unit to unit, and that if circuits were to be changed for every case there would be both significant cost and manufacturing and disposal implications. On one hand, our patients deserve full protection from transmission of infective agents, while on the other there is no clinical evidence that such transmission is occurring with current practice. This issue will be explored further with the MHRA, BAREMA and the Association of Paediatric Anaesthetists. Until any further advice is available this remains a local risk management issue and individual units should review their practice in the light of existing evidence. J. A. CarterChairman, Safety Committee, AAGBI, UK E-mail: [email protected]