Failure of bacterial filters to reduce the incidence of pneumonia after inhalation anesthesia

Respiratory Filters and Ventilator-Associated Pneumonia: Composition, Efficacy Tests and Advantages and Disadvantages

Respiratory filters are devices with a high capacity to prevent the passage of microorganisms. The use of respiratory filters interposed in respiratory circuits to avoid ventilator-associated pneumonia (VAP) was proposed after reports between 1952 and 1972 of several outbreaks of respiratory infections attributed to contamination of anesthesia machines; however, none of the reports presented a bacteriological demonstration of a cause-and-effect relationship. The use of respiratory filters has not decreased the incidence of VAP in patients on anesthesia machines and in critically ill patients. Besides, respiratory filters could have some undesirable effects such as the increase of resistance to inspiratory airflow, increase of resistance to expiratory airflow and increase of dead space in the breathing circuit. Thus, the use of respiratory filters is not routinely necessary; however, they should be used in patients with suspected or confirmed highly communicable respiratory infections (such as bacillary pulmonary tuberculosis) and who require mechanical ventilation).

Contamination of anesthesia circuits by pathogens

BACKGROUND AND OBJECTIVES

Evaluation of contamination of anesthesia circuits by collecting 56 culture samples from the circular system; previously reprocessed tracheas by disinfection with 1% hypochlorite or 2% glutaraldehyde after being washed in non-sterile water and soap and dried by using compressed air and stored in surgical grade paper; and from other places of the non-reprocessed respiratory circuit, before anesthetic procedures.

METHODS

Samples from the inspiratory and expiratory branches of the tracheas, canister, soda-lime, and collector jar (drain) through swab in Stuart medium and streaked in Agar blood, Mac Conkey, and Sabouraud growth media.

RESULTS

The level of contamination with fungus and bacteria in the inspiratory and expiratory branches of tracheas reached up to 39.3% in some sites; in some cases, more than one microorganism was present, 75% fungal and 25% bacterial contamination. Cultures were positive for Candida sp., Dermatophytus sp., Penicillium sp., Aspergillus sp., Staphylococcus aureus, Staphylococcus saprophyticus, and Staphylococcus epidermidis. Contamination was observed in 25% of the canisters with growth of Candida sp., Penicillium sp., Dermatophytus sp., Aspergillus sp., and Fusarium sp. In the collector jar, a contamination rate of 36% was observed with growth of Candida sp., Dermatophytus sp., S. saprophyticus and Acinetobacter baumannii. Microorganisms did not grow in soda-lime cultures.

CONCLUSIONS

In all sites investigated except for soda-lime growth of microorganisms was observed with the possibility of cross infection.

Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 1 - history, principles and efficiency

Heat and moisture exchangers and breathing system filters are intended to replace the normal warming, humidifying and filtering functions of the upper airways when these structures are bypassed during anaesthesia and intensive care. Guidance on their use continues to evolve. The aim of this part of the review is to describe the principles of their action and efficiency and to summarise the findings from clinical and laboratory studies. Based on previous studies, an appropriate minimum target for moisture output is 30 and 20 g.m⁻³ for long-duration use in intensive care and short-duration use in anaesthesia, respectively. The practice of reusing a breathing system in anaesthesia, provided it is protected by a filter, assumes that the filter is effective. However, there is wide variation in the gas-borne filtration performance, and contaminated condensate can potentially pass through some filters under typical pressures encountered during mechanical ventilation.

An investigation of the bacterial contamination of small animal breathing systems during routine use

OBJECTIVE

To investigate the need for sterilization of anaesthetic breathing systems to prevent cross-infection between animals due to the re-use of anaesthetic circuit tubing.

STUDY DESIGN

Prospective microbiological study.

METHODS

Bacteriology samples were taken from 37 sterile breathing systems, each used for 1 day, at two sampling sites (one proximal and one distal to the animal). The number of patient connections, cumulative anaesthesia time, culture results, number of colony-forming units and the number of different species were recorded. Secondly, four sterile breathing systems were used for 2 months under routine conditions and sampled every 2 weeks and the same parameters recorded. Finally, the inner surfaces of four sterile breathing systems were inoculated with a known load of canine oropharyngeal bacteria. Bacteriology samples were taken at 1 minute, 1 hour and 1 day post-deposition. The number of colonies identified was compared with the initial load.

RESULTS

Only a very small number of micro-organisms were isolated and these were generally organisms of low pathogenic potential. The proximal site was found to be significantly more colonized than the distal site (p < 0.001). Neither longer daily connection time (p = 0.54), nor a higher number of connections (p = 0.81) increased the incidence of proximal site colonization. Over the 2-month study period, the bacterial population did not increase. There was no correlation between cultures isolated from successive samples taken from the same tubing. There was rapid loss of viability of the micro-organisms deliberately inoculated onto the tubing surface: the number of colonies isolated from the breathing system after 1 minute was significantly lower than in the inoculum (p = 0.042).

CONCLUSIONS AND CLINICAL RELEVANCE

Sterile anaesthesia breathing systems were colonized by environmental micro-organisms of low pathogenicity. Although long-term survival of recognized pathogens in a dry environment is still possible, the use of a bacterial filter or a sterilized breathing system for routine veterinary anaesthesia cannot be supported by current evidence.

Ventilator-associated pneumonia using a heated humidifier or a heat and moisture exchanger: a randomized controlled trial [ISRCTN88724583]

INTRODUCTION

Some guidelines to prevent ventilator-associated pneumonia (VAP) do not establish a recommendation for the preferential use of either heat and moisture exchangers (HMEs) or heated humidifiers (HHs), while other guidelines clearly advocate the use of HMEs. The aim of this study was to determine the incidence of VAP associated with HHs or HMEs.

METHODS

A randomized study was conducted in the intensive care unit of a university hospital involving patients expected to require mechanical ventilation for >5 days. Patients were assigned to two groups; one group received HH and the other group received HME. Tracheal aspirate samples were obtained on endotracheal intubation, then twice a week, and finally on extubation, in order to diagnose VAP. Throat swabs were taken on admission to the intensive care unit, then twice a week, and finally at discharge from the intensive care unit in order to classify VAP as primary endogenous, secondary endogenous, or exogenous.

RESULTS

A total of 120 patients were assigned to HMEs (60 patients) and HHs (60 patients); 16 patients received mechanical ventilation for less than five days and were excluded from the analysis. Data analysis of the remaining 104 patients (53 HMEs and 51 HHs) showed no significant differences between groups regarding sex, age, Acute Physiology and Chronic Health Evaluation II score, pre-VAP use of antibiotics, days on mechanical ventilation, and diagnosis group. VAP was found in eight of 51 (15.69%) patients in the HH group and in 21 of 53 (39.62%) patients in the HME group (P = 0.006). The median time free of VAP was 20 days (95% confidence interval, 13.34-26.66) for the HH group and was 42 days (95% confidence interval, 35.62-48.37) for the HME group (P <0.001). Cox regression analysis showed the HME as a risk factor for VAP (hazard rate, 16.2; 95% confidence interval, 4.54-58.04; P < 0.001).

CONCLUSION

The patients mechanically ventilated during more than 5 days developed a lower incidence of VAP with a heated humidifier than heat and moisture exchanger.

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.

Bacterial filters in respiratory circuits: an unnecessary cost?

OBJECTIVE

To evaluate the efficacy of bacterial filters (BF) to decrease pneumonia associated with mechanical ventilation (MV).

DESIGN

Prospective, randomized study.

SETTING

A 24-bed medicosurgical intensive care unit in a 650-bed tertiary hospital.

PATIENTS

A total of 230 patients who needed MV for >24 hrs.

INTERVENTIONS

A total of 114 patients were ventilated with BF and 116 without BF.

MEASUREMENTS

Throat swab and tracheal aspirate were taken at the moment of admission and twice a week until discharge. We considered the following infectious events: pneumonia, respiratory infection, which comprises pneumonia or tracheobronchitis, and respiratory colonization-infection complex, which comprises respiratory infection or colonization. All infectious events were classified as endogenous or exogenous based on throat flora.

MAIN RESULTS

Both groups of patients (ventilated with and without filters) were similar in age, sex, Acute Physiology and Chronic Health Evaluation II score, diagnostic group, days of MV, and mortality. There was no difference in the percentage of patients who developed pneumonia (24.56% with BF and 21.55% without BF), respiratory infection (33.33% vs. 28.44%), or colonization-infection (42.10% vs. 43.96%). The number of infectious events per 1000 days of MV were also similar in both groups: pneumonia (17.41 with BF and 16.26 without BF), respiratory infection (24.62 vs. 21.48), and colonization-infection (36.63 vs. 36). There were also no differences in incidence of infectious events by MV duration. Likewise, we did not find any differences in the number of exogenous events per 1000 days of MV: pneumonia, 2.40 with BF vs. 1.74 without BF; colonization-infection, 4.20 vs. 4.05.

CONCLUSIONS

Bacterial filters in ventilation circuits neither reduce the prevalence of respiratory infections associated with MV nor decrease exogenous infectious events; thus, their usage is not necessary.

The potential for dissemination of Mycobacterium tuberculosis through the anesthesia breathing circuit

BACKGROUND

Respiratory pathogens that pass through the anesthesia breathing system potentially can infect other patients. This study was designed to determine if bacteria can pass through contemporary anesthesia breathing systems and if the environment within the machine is hostile to these organisms.

METHODS

Staphylococcus aureus, Pseudomonas aeruginosa, and Mycobacterium tuberculosis were nebulized into the expiratory limb of an anesthesia breathing circuit and collected from the inspiratory and expiratory limbs in an impinger system that provided a quantitative determination of the number of organisms entering the circuit and the number that would reach the patient in the inspiratory gas. Bacteria were collected before, during, and after nebulization. A second experiment determined if a saturated solution of soda lime was bactericidal.

RESULTS

When the gas flow through the circuit was interrupted for < 1 h following the nebulization period, large numbers of microorganisms (1 x 10(3) to 1 x 10(5), around 100% of the nebulized organisms) were collected from the inspiratory gas. Soda lime itself was not bactericidal for any of the organisms tested, but solutions of this material with a pH of 12 were bactericidal.

CONCLUSION

Cross contamination between patients may occur unless the gas flow through the anesthesia breathing system is interrupted for > 1 h.

Bacterial filters in anesthesia: results of 9 years of surveillance

In 9 years of surveillance of postoperative lower respiratory infections, the infection rate in patients following regional anesthesia was 0.2% and 0.1% in patients following general anesthesia. No bacterial filters in the breathing circuit were used. Infected patients had risk factors such as type of surgery, American Society of Anesthesiologists class > or =2, old age, chronic obstructive pulmonary disease, or smoking habits. Infections were not clustered. This suggests that, in our setting, patient factors are most important in the development of postoperative lower respiratory infections and that the role of bacterial filters as a preventive measure is negligible.

How useful are microbial filters in respiratory apparatus?

Following an outbreak of hepatitis C in surgical patients in Australia, it has been suggested that transmission can take place as a result of contaminated anaesthetic circuits. It has therefore been recommended that filters should be placed between patients and breathing systems with a new filter being used for each patient. Although nosocomial pneumonia is a major manifestation of hospital-acquired infection, it is unclear whether contamination of ventilator circuits is implicated in the aetiology of this condition. Some data suggest that bacteria cannot survive well in anaesthetic circuits and several studies have failed to demonstrate significant contamination of circuits in clinical situation. Several outbreaks of pneumonia related to contaminated anaesthetic equipment have been described, but many of these were controlled by appropriate decontamination of the respiratory equipment. Although ventilator filters are used by the majority of intensive care units and filters do have the ability to filter bacteria and viruses, there are few data suggesting that the use of filters reduce the rate of pulmonary infections in long-term ventilated patients. Furthermore, to change filters between operations would have significant financial implications, and there is no conclusive evidence that they would reduce cross infection. Until more data are available on the role of filters in both long-term ventilated patients and operations, standard hygienic measures such as appropriate disinfection protocols are still the most effective way of reducing ventilator-associated infections.

Revision of the anaesthetic aspects of an infection control policy following reporting of hepatitis C nosocomial infection

Following the report by the N.S.W. Health Department of five cases of nosocomial hepatitis C infection allegedly caused by contaminated circuitry, a revision of the existing infection control policy was undertaken. Review of the existing policies of other organizations and the published scientific data in this area was the basis of the revision. The final policy as it pertains to anaesthesia requires universal precautions, disposal or decontamination and high level disinfection of instruments and apparatus that come into contact with patients or blood or body substances and protection of the breathing circuit by a filter with disposal or high-level disinfection of all parts of the circuit not so protected for every patient.

Anaesthetic machine and breathing system contamination and the efficacy of bacterial/viral filters

Contamination of the anaesthetic machine and breathing system by the environment and by patient exposure has been shown to occur. Outside the intensive care setting, however, it is difficult to demonstrate that the anaesthetic machine and breathing system are a vector for patient cross-infection. Bacterial and viral filters for use within the breathing system have been shown to be very effective for filtration, yet their use has not been demonstrated to be of benefit in the prevention of cross-infection between patients. Several instances of patient morbidity are a direct consequence of filter use. The use of bacterial/viral filters may represent another step towards defensive medical practice.

Bacterial filters protect anaesthetic equipment in a low-flow system

We have recently described a low-flow system with mechanical ventilation in which there was an open connection between the circle and ventilator. In the present study bacterial contamination and the role of bacterial filters at different points in the system were studied. Filters between the tracheal tube and circle are an effective barrier, but their absence did not increase contamination of either the circle or the ventilator. Some filters are also effective as heat and moisture exchangers if situated at the tracheal tube. Because of the lack of bacterial contamination, a prolonged interval between disinfection of the open connection and ventilator is acceptable, which reduces wear and costs.

Heat and moisture exchangers and vaporizing humidifiers in the intensive care unit

A prospective, randomized, controlled study was undertaken to compare the Pall Ultipor breathing circuit filter (PUBCF), a heat-and-moisture exchanger, and heated hot water systems (HHWSs) in ICU patients submitted to controlled mechanical ventilation. Humidification of inspired gas and bacterial contamination of breathing circuits were evaluated. During the study, there were six episodes of tracheostomy tube (TT) occlusion in six patients included in the PUBCF group. No patient out of 42 included in the HHWS group experienced this complication (p less than 0.01). There were 4 percent of days with thick and tenacious bronchial secretions in the PUBCF group and no case in the HHWS group (p less than 0.02). In the PUBCF group, 23 percent of days with hypothermia were noted as opposed to 12 percent in the HHWS group (p less than 0.01). Fewer breathing circuits were found to be contaminated in the PUBCF group (11 percent) than in the HHWS group (54 percent, p less than 0.01). In patients with an organism growing in bronchial specimens, the same organism was found to contaminate the breathing circuit in 10 percent of cases in the PUBCF group and 77 percent of cases in the HHWS (p less than 0.01). We conclude that, in the conditions of this study, the PUBCF did not provide sufficient humidification of inspired gas in ICU patients. Protection against contamination of breathing circuits was effective, but 10 percent of patients remained at risk for this complication.

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

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.

Postoperative pneumonia and urinary-tract infection: epidemiology and prevention

Hospital-acquired pneumonias and urinary-tract infections are important causes of morbidity and mortality in surgical patients, and a great deal of effort has been expended on infection control strategies to prevent their occurrence. Prophylactic antibiotics, used either systemically or topically, are not routinely recommended for the prevention of either of these infections. The beneficial effects of these agents are transient, and they are often in association with the acquisition of colonization or infection with resistant bacteria. New approaches for infection control, not involving antibiotic agents, are being developed to lower the infection rates of both hospital-acquired pneumonias and urinary-tract infections to an irreducible minimum.

Nosocomial bacterial pneumonia: an overview

Nosocomial pneumonia accounts for 10% to 20% of all nosocomial infections and represents one of the most serious complications of hospitalization. This review focuses on the etiology, pathogenesis, and prevention of nosocomial pneumonia, with emphasis on infection control procedures to prevent and minimize its occurrence.

Comparison of microbiological contents of compressed air in two Danish hospitals. Effect of oil and water reduction in air-generating units

In a comparative study microbiological contamination of compressed air for medical use produced in oil-lubricated and oil-free compressors was investigated. Significantly lower levels of bacterial contamination were observed in the air produced by oil-free compressors; but if the air is transported to operating rooms and intensive care units through extensive pipeline systems previously contaminated by oil-lubricated compressors, the bacterial count at peripheral air outlets remains unchanged.

Bacterial contamination of compressed air for medical use

The present study demonstrates a previously unnoticed source of bacterial contamination of locally manufactured compressed air for medical use. Air samples were drawn into a specially constructed device, and bacterial contents were identified from growth on agar plates. Various factors contributing to bacterial contamination of compressed air during production are mentioned and preventive measures are discussed.

Randomized controlled trials in hospital epidemiology. Sixth annual National Foundation for Infectious Diseases lecture

Randomized controlled trials (RCTs) are useful in the evaluation of both new and well-established hospital infection control practices for which the benefit is unproven and not obvious. RCTs are often undertaken to resolve controversies, and many of them have a "negative" or inconclusive outcome. Well-designed RCTs with a negative outcome have the potential for reducing health care costs by identifying widely used preventive measures that are not cost effective. ICPs should appreciate that RCTs have important limitations, may contain serious flaws in design, and are subject to exploitation by pharmaceutical and equipment manufacturers. Knowledge of the principles of RCTs can lead to improved scientific standards for all research--nonrandomized as well as randomized--in hospital epidemiology.