End-tidal carbon dioxide measurement and breathing system filters

Breathing system filters are recommended for use during general anaesthesia. Manufacturers of gas sampling equipment recommend that gas is sampled from the 'machine side' of these filters, thereby avoiding contamination of the sampling line and waterlogging in the case of heat and moisture exchangers. The aim of this study was to investigate differences between the measured end-tidal carbon dioxide values at either side of the filter. Fifteen adults were studied during mechanical ventilation and 15 during spontaneous ventilation under general anaesthesia. End-tidal carbon dioxide values were significantly lower at the machine side of the filter in both groups (p = 0.00001). The measurement error induced by the inclusion of the breathing system filter was significantly greater in the spontaneously breathing group (p = 0.0004).     

Filters in anaesthesia and intensive care

The use of various types of filters in anaesthesia and intensive care seems ubiquitous, yet authentication of the practice is scarce and controversies abound. This review examines evidence for the practice of using filters with blood and blood product transfusion (standard blood filter, microfilter, leucocyte depletion filter), infusion of fluids, breathing systems, epidural catheters, and at less common sites such as with Entonox inhalation in non-intubated patients, forced air convection warmers, and air-conditioning systems. For most filters, the literature failed to support routine usage, despite this seemingly being popular and innocuous. The controversies, as well as guidelines if available, for each type of filter, are discussed. The review aims to rationalize the place of various filters in the anaesthesia and intensive care environment.     

Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 2 - practical use, including problems, and their use with paediatric patients

Heat and moisture exchangers and breathing system filters are intended to replace the normal warming, humidifying and filtering functions of the upper airways. The first part of this review considered the history, principles of operation and efficiency of these devices. The aim of this part of the review is to summarise recent guidelines on the use of these devices and outline the problems that can occur. In particular, the effect of these devices on gas analysis, dead space, resistance to gas flow and blockage of the breathing system is considered. In children, it is important to consider the addition of dead space and resistance to gas flow. A body weight of 2.5 kg is probably the lower weight limit for use with heat and moisture exchangers, and 3 kg for filters. The resistance to gas flow of a heat- and moisture-exchanging filter added to a Mapleson F breathing system can cause a delay in the induction of anaesthesia.     

The influence of breathing system filters on paediatric capnography

Breathing system filters are in common use during paediatric anaesthesia. Expired gas sampling from the patient side of these filters may contaminate and saturate the sampling line, while sampling from the machine side may cause underestimation of end-tidal carbon dioxide (PECO 2). The aim of this investigation was to elucidate the degree of underestimation of PECO 2 induced by sampling from the machine side of a breathing system filter. Ten spontaneously breathing children and ten children receiving mechanical ventilation under general anaesthesia were studied. PECO 2 was higher at the patient side of the filter in both ventilated and spontaneously breathing groups (P<0.002 for each). The bias in measuring at the machine side of the filter was significantly greater in the spontaneously breathing children as compared with the mechanically ventilated children (-1.8 vs -0.7 kPa; P<0.004).     

Efficacy of HME filters used in children and factors responsible for contamination of breathing systems

Background: Adenosine triphosphate (ATP) bioluminescence has been used to assess the efficacy and factors that increase the chance of cross‐contamination of HME (heat and moisture exchanging) filters in adults [1, 2]. This method is recommended for testing in hygiene monitoring [3]. The aim was to evaluate the efficacy of breathing system filters and ascertain potential factors responsible for contamination of breathing system used in children. Method: Breathing system filters were collected over 10 days from theatres and swabbed on both the patient and machine sides. The breathing system used in the anaesthetic rooms and theatres were swabbed twice daily. The contamination was assessed using the Bio trace Clean‐Trace^® (3M Health Care Limited, Loughborough, UK). Data collected included age and weight of the patient, type of surgery, duration of use of the HME filter and breathing system, type of airway device and ventilation, position and episodes of cough at induction and extubation. Values of less than 50 relative light units (RLU) were considered as background contamination, >50 RLU as contamination [1]. Values >100 RLU are indicative of contamination in the food industry [3]. Results: One hundred and nineteen breathing system filters (60 Dar Hygroboy^®, 17 Dar Hygrobaby^®, 33 Humid‐Vent^® Filter Pedi, 5 Pall Ultipor^®, 4 HMEF/750^®) were tested. The age of the children ranged from 2 days to 15 years (mean 7.4 years). The median [IQR] {range} for duration of use of the filters in minutes were 40 [23–60] {8–330} respectively. On the patient side, the median [IQR] {range} RLU of the filters were 65 (30–125) {5–47006}. A total of 67 (56%) and 40 (34%) of these filters had RLU levels greater than 50 and 100 respectively. On the machine side, the median (IQR) [range] RLU of the filters were 12 (5–45) [2–202]. 27 (23%) and 7 (6%) of these filters had RLU levels greater than 50 and 100 respectively. Six of these seven filters were from patients either second or third on the list. There were mild associations between contamination on the patient and machine sides (Spearman's ρ = 0.26, P = 0.004) and duration and contamination on the patient side (ρ = 0.21, P = 0.022), but not between duration and contamination on the machine side (ρ = ?0.02, P = 0.81). At start of the day, the median (IQR) [range] RLU of the breathing systems were 10 (6–30) [1–144]. 3 (12%) and 1 (4%) of these had RLU levels greater than 50 and 100 respectively. At the end of the day, the median [IQR] {range} RLU of the breathing systems were 84 (54–162) [14–331]. 16 (76%) and 8 (38%) of the breathing system had RLU levels greater than 50 and 100 respectively. Ten children (8.4%) coughed, this caused significant higher RLU on the patient side of the filter with median (IQR) [range] being 179 (107–400) [29–492]. However, RLU was 50 or less on the machine side in 90% of these filters. Discussion: HME filters are changed with every case, whereas change of breathing systems is variable from every list to every day to every week. Our results show that a majority of the breathing systems in the operating theatres were contaminated by the end of the day. We suggest using a bioluminescence test before reusing the breathing system for another list. Acknowledgement: 3M Health Care Limited, Loughborough, UK for providing the Biotrace Clean‐Trace^® equipment for this study.     

Gas exchange with a reflecting system for inhalational anaesthesia

In order to reduce comsumption of inhalational anaesthetsics during high‐flow anaesthesia, a system open in regard to oxygen, nitrogen and nitrous oxide, but closed to inhalational anaesthetics, was developed. This was achieved by a reflecting filter for inhalational anaesthetics made of active carbon. The principle has been developed into the commercially available Anesthetic Conserving Device (ACD, AnaConDa®). The ACD is a modified heat and moisture exchanger containing a bacterial and viral filter as well as a carbon filter. The ACD can be used for administering the inhalational agents isoflurane and sevoflurane as an alternative to low‐flow anaesthesia systems during surgery. It can also be used in intensive care units to administer sedation using isoflurane or sevoflurane to critically ill patients.     

The moisture-conserving performance of breathing system filters during the first three minutes of simulated use

Breathing system filters are recommended for use during anaesthesia to protect the patient from inhaling gas-borne particles. Filters placed at the patient connection port of the breathing system can also humidify the inspired gases. The end-inspired moisture content was measured when using five different filters with two different ventilatory test conditions on a patient model during a typical pre-oxygenation period of 3 min. The moisture content of the end-inspired air at the end of the 3-min period varied from 6.4 to 27.8 and from 4.4 to 25.9 g.m-3 for tidal volumes of 0.5 and 1.0 l, respectively (p < 0.0001 for all pairwise comparisons of the five filters and for the two tidal volumes). Those breathing system filters that have at least an adequate level of performance (at least 20 g.m-3) will generally achieve this level within the 3-min pre-oxygenation period.     

Comparison of manufacturers' specifications for 44 types of heat and moisture exchanging filters

Background. Although heat and moisture exchanging filters (HMEF)are recommended for use during anaesthesia, the criteria forchoosing a filter are not clearly defined. Manufacturers offermany different types of HMEF with various technical characteristics.We compared the technical specifications provided by the manufacturersfor different types of HMEF. Methods. Filter manufacturers were asked to provide technicalinformation. Additional information was obtained from websites.Information about 44 filters (16 mechanical and 28 electrostatic)was collated. Results. Filter performances were estimated with different sizesof microorganism and durations of challenge. Twenty-eight filtershad not been tested by independent laboratories. For 12 of thefilters, information obtained from websites and from the manufacturersdiffered. Most filter specifications claimed high efficiency,particularly for filtration, microbial challenge number andtest duration. Electrostatic filters used in anaesthesia wereclaimed to have high filtration efficiency, similar to the efficiencyprovided by mechanical filters. Excluding moisture output valuesdid not alter the general conclusions. Conclusions. Technical aspects of the tests, international standards,and independent validation should be considered when a filteris chosen.     

The effect of volatile anaesthetic agents on the filtration performance of paediatric breathing system filters

The aim of this study was to determine the filtration performance of five commonly used paediatric breathing system filters following exposure to desflurane, isoflurane and sevoflurane. It has been suggested that oil may degrade the performance of filter material. Volatile anaesthetic vapours are organic and hence may affect the filtration performance of breathing system filters during anaesthesia. This has not been tested for various concentrations of volatile agent, type and duration of exposure. The filtration performance of the filters was measured following exposure to desflurane, isoflurane and sevoflurane at 1 and 2 minimum alveolar concentration (MAC) for 1 and 4 h. Penetration of particles through the Clear-Therm Micro, Clear-Therm Mini and Humid-Vent Filter Pedi increased by between 2.4 and 2.8 times after exposure to desflurane at 2 MAC for 4 h compared to that through unexposed filters (p < 0.0001 for all three filters). Further investigation is required to determine whether this reduction in filtration performance by desflurane is clinically significant.     

Assessment of 44 heat and moisture exchange filters. What to choose?

OBJECTIVES: If the use of heat and moisture exchange filter (HMEF) in anaesthesia is recommended by the French Society of Anaesthesia and Intensive Care (SFAR), the criteria's choice are not clearly defined. Many HMEF are proposed by manufacturers which technical characteristics are different. STUDY DESIGN: The aim of this study was to evaluate the HMEF using items of the American Association of Respiratory Care (AARC) and technical dossiers. METHODS: All manufactures producing filters have been contacted to give their technical dossiers. Forty-eight filters have been analyzed (13 mechanical filters, 31 electrostatic filters). Each item has been scaled 0, 5 or 10. The final result was on 10. RESULTS: Seventeen filters had a note superior to 5. There were 8 mechanical filters and 9 electrostatic filters. The difference between the filters was the size of the micro-organisms tested and the duration of the test. Some filters were not tested by independent laboratories (N = 8). There were differences between the commercial documentation and on Internet and the technical dossiers (N = 12). DISCUSSION: We noted the good quality of the filters particularly concerning criters recommended by the Sfar (filter medium, filtration efficiency, microbial challenge number and duration of the test). The electrostatic filters recently used in anaesthesia have high performance concerning filtration efficiency. To supprime the moisture output criteria did not change the results. CONCLUSIONS: Criteria's used by manufactures to evaluate there filters are not always precised or too restrictive. The technical tests, the international norms, the certificates of validation, the ergonomic qualities and the definition of our needs are the main elements of choice of a filter.     

Wasserdurchlässigkeit von Beatmungsfiltern

Breathing filters or heat and moisture exchangers (HME), which are placed between a tracheal tube and the Y-piece, allow reuse of breathing tubes without changing between operations. During low-flow and minimal-flow anaesthesia, condensed water accumulates in the breathing circuit. An evaluation of the volume of condensed water is given (Fig.?1). It may be possible that water flows onto the filter surface, for example, when the breathing tubes are lifted. The water permeability of such breathing filters and HME was tested. For the experiments, a commercial breathing circuit and ventilator (Dräger Sulla 808+Ventilog) and patient model (Fig.?2) were used; 12 breathing filters/HMEs of different manufacturers were tested. Only 3 filters were not permeable to the test volume of 20?ml water. The authors suggest the water volume be checked routinely and the breathing tubes be emptied if necessary.     

Passage of pathogenic microorganisms through breathing system filters used in anaesthesia and intensive care

Invasive ventilation poses a risk of respiratory infection that can be drug‐resistant. One means of reducing transmission of infection is the use of a breathing system filter. Filters are intended to be used with dry gas. Current international standards do not require that filters prevent bacterial transfer when wet. It is not known whether microorganisms pass through wet filters, but theory predicts that this might occur. We tested six filters from three different manufacturers. We passed a suspension of microorganisms through the filters using the least pressure necessary, and incubated a sample of the filtrate on blood agar. All the filters tested allowed free passage of both Candida albicans and coagulase‐negative staphylococci. The median (IQR [range]) pressure required for fluid to flow across the filter varied greatly between different filter types (20 (0–48 [0–138]) cmH₂O). We conclude that even large microorganisms pass across moist breathing system filters in conditions that are found in clinical practice.     

The use of breathing system filters as oxygen-delivery devices.

A breathing system filter can remain connected to a laryngeal mask airway during recovery from anaesthesia. The Luer-lock monitoring port on the filter could be used for the delivery of supplementary oxygen. The efficacy of this technique was assessed in vitro by comparing the performance of two filters with four other devices, including a T-piece with a 40% oxygen injector. The minimum inspired oxygen fraction and pressure drop were measured over a range of ventilatory conditions and oxygen flows. The minimum inspired oxygen fraction measured with the remaining devices was greater than with the T-piece at an oxygen flow of 10 l.min-1. The pressure drop was greater with the breathing system filters than with the T-piece. A potential advantage in using filters is that no additional equipment is required. However, if using the filters as oxygen supplementation devices, there is only one port open to the atmosphere in the system and inadvertent obstruction of this port could cause barotrauma. It must be remembered that the nonstandard use of such equipment contravenes the recently introduced Medical Device Directive regulations.     

An evaluation of the filtration performance of paediatric breathing system filters at low flows

The filtration performance of five different types of filter intended for use in paediatric anaesthesia was measured. A total of 120 unused filters (24 samples of each filter type) were evaluated. The pressure drop and filtration performance, using challenges of sodium chloride particles, were measured for each filter at 3 l min(-1) and 15 l min(-1). The pressure drop was less at the lower flow; there was a wide variation in the pressure drop across some filters. The filtration performance of all filter types showed an improvement at 3 l min(-1) compared to 15 l min(-1). Four filter types had filtration efficiencies greater than 95% at 15 l min(-1) and greater than 99% at 3 l min(-1). The remaining filter type had a filtration efficiency less than 90% at 15 l min(-1) and greater than 95% at 3 l min(-1). These levels of performance are comparable to that of breathing system filters intended for use in adult anaesthesia using flows representing mean inspiratory flow.     

Contamination control in long-term ventilation

Twenty-eight patients who required periods of mechanical ventilation for up to 22 days in the intensive therapy unit were studied to evaluate the clinical use of the Pall Ultipor Breathing System Filter (BB50T) as a heat- and moisture-exchanging bacterial filter. Results in this group of patients showed that there was no longer any need to sterilise breathing systems or decontaminate ventilators if these filters were used. They also performed satisfactorily as a heat and moisture exchanger in patients in need of long-term ventilation, and their use appears to offer substantial advantages as regards cost, ease of use and patient safety.     

Influence of patient factors on the efficacy of breathing system filters at preventing contamination of breathing systems*

We measured the level of contamination in 207 breathing system filters of five different models. The median (IQR [range]) levels of contamination measured in relative light units on the machine side of the filters were: HMEF 750/S 27 (16–56 [4–13 615]); Ultipor BB25 26 (13–40 [9–66]); Humid‐vent filter pedi 19 (15–34 [11–48]); Hygroboy 11 (7–19 [3–113]); and Hygrobaby 9 (6–14 [4–21]). A total of 41/138 (30%) of the HMEF 750/S (the most commonly used filter) had measured values > 50, indicating excessive contamination on the machine side of the filter. The incidence of coughing and duration of the case were significantly associated with the incidence of excessive contamination on the machine side (p = 0.034 and p = 0.024, respectively). Excessive contamination on the machine side of the filter could be from the patient or from the re‐used breathing system and could result in cross‐infection. The level of contamination may need to be checked routinely during each list.     

The effect of a heat and moisture exchanger on gas flow in a Mapleson F breathing system during inhalational induction

Heat and moisture exchangers (HMEs) humidify, warm and filter inspired gas, protecting patients and apparatus during anaesthesia. Their incorporation into paediatric anaesthetic breathing systems is recommended. We experienced delays in inhalational induction whilst using a Mapleson F breathing system with an HME. We have demonstrated that the HME significantly alters gas flow within the breathing system. Approximately half of the fresh gas flow is delivered to the patient, the remainder being wasted into the expiratory limb of the breathing system. We suggest that the HME should be removed from the Mapleson F breathing system until inhalational induction is complete, or that the reservoir bag is completely occluded until an effective seal is obtained with the mask.     

Vergleichende Untersuchung über die Abscheidungsleistung von Bakterienfiltern unter Beatmungsbedingungen

Two commercially available bacterial filters to be used as part of the mechanical ventilation unit during anaesthesia were tested for hygienic criteria. Manufacturers claim that bacterial breathing filters have a filtration capacity of about 99.995%, so that there would be no need for thermal disinfection of tubing and ventilation circuits after each use. One filter is designed for a single use only, the other can be used up to 24 times after sterilisation. Both filters consist of hydrophobic glass fibres. Methods. During simulated mechanical ventilation for 24 h, an alcoholic suspension of Bacillus subtilis was atomised in front of the filters tested. A gelatine membrane filter was integrated in the ventilation circuit and captured the filtered gas behind the test filter. Results. During simulated mechanical ventilation for 24 h, the filtration capacity of both the disposable and reusable filters (Table 2) did not confirm the manufacturers' short-term technical findings over 8 s (DIN-EN 143). Conclusions. The use of bacterial filters during mechanical ventilation reduces the probability of bacterial contamination, but does not make sterilisation of the tubes and ventilation circuit unnecessary.     

The effect of desflurane on filtration performance of breathing system filters*

This study assessed the effect of desflurane on the filtration performance of six breathing system filters intended for use with adults. Three filters contained an electrostatic filter material and three contained a pleated glass fibre filter material. Five samples of each model of filter were exposed to 6% v/v of desflurane for 1 h, 12% v/v of desflurane for 1 h, 12% v/v of desflurane for 4 h and air only for 1 h. Five samples of each filter were also tested without exposure to any vapour or air. The filtration performance was measured by challenging each filter with an aerosol of sodium chloride particles using a Moore’s test rig. Penetration of particles through the electrostatic filters increased following exposure to a higher concentration of desflurane for a longer duration (p < 0.001). The effect on two of the pleated filters was not significant (p = 0.55 and p = 0.64). The effect on the remaining pleated filter was significant (p < 0.001) but small. The efficiency of some filters decreases when they are exposed to high concentrations of desflurane for a long duration. This effect appears more marked in electrostatic filters compared with pleated filters.     

Sedierung mit volatilen Anästhetika auf der Intensivstation

The use of volatile anaesthetics in intensive care medicine has so far been limited by the lack of equipment suitable for daily routine use and the need for an anaesthetic machine. The new Anaesthetic Conserving Device (AnaConDa) enables the routine use of volatile anaesthetics for long-term sedation via intensive care ventilators. The Anaesthetic Conserving Device replaces the common heat and moisture exchanger in the ventilation circuit. The volatile anaesthetic is continuously applied in liquid status via a syringe pump to a form of mini-vaporiser where the anaesthetic agent is vaporised. The expired anaesthetic gas is stored in the carbon filter and approximately 90% of the gas is resupplied into the breathing cycle. The current experiences suggest that volatile anaesthetics present an alternative for long-term sedation in intensive care units, providing optimised pathways, from a medical as well as from an economical point of view. It must, however, be emphasized that the use of volatile anaesthetics for longer periods of time is an off-label use and should only undertaken by medical professionals at their own risk.