Risk of cross-infection from inhalation anaesthetic equipment

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).

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.

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.

Ventilator-associated pneumonia

Mechanically ventilated patients are at a substantially higher risk for developing nosocomial pneumonia. Overall, there is a relatively constant 1&!TN!150;3% risk per day of developing pneumonia while receiving mechanical ventilation. The sensitivity and specificity of clinical criteria alone for diagnosis of ventilator-associated pneumonias (VAP) is low. Several techniques have been developed to sample and quantitate the lower respiratory tract to improve the diagnostic yield. Gram-negative bacillary pneumonias account for the majority of the VAP. Strategies for prevention of VAP such as use of sucralfate for stress ulcer prophylaxis and selective decontamination of the digestive tract have been the focus of many clinical studies. Cost-effective preventive measures are needed to combat the increasing antimicrobial resistance, growing population of immunocompromised patients and increasing number of mechanically ventilated patients.

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.

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 and viral removal efficiency, heat and moisture exchange properties of four filtration devices

Four devices used for filtration of microorganisms and/or for heating and moistening the ventilated air during mechanical ventilation were evaluated. This evaluation included measurement of bacterial and viral removal efficiency, heat and moisture exchange properties, dead space and air flow resistance. The devices included: Pall BB50T and DAR Sterivent (filtration devices); DAR Hygrobac and Gibeck Humid-vent [heat and moisture exchangers (HMEs)]. The two devices which are primarily conceived as filters, had the highest bacterial and viral removal efficiency (titre reduction of 10(5)-10(6) for bacteria and of 10(4)-10(5) for viruses), while removal efficiencies of the HME devices were lower: titre reduction of 10(4) for bacteria and 10(1)-10(3) for viruses. As expected, heat and moisture output of HMEs was better than that of filters. In mechanical ventilation, dead space and air flow resistance are important properties of devices, which might disturb efficient ventilation. There were only minor differences in dead space and air flow resistance. Resistance to airflow in the HMEs was increased by nebulization of medication (mesna) unlike that of the filters.

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.

Bacterial contamination and the effect of filters in anaesthetic circuits in a simulated patient model

In order to investigate bacterial contamination of anaesthetic breathing circuits and means of prevention of this, six different laboratory experiments were performed. These experiments involved the bacterial contamination of Dräger Narkose Spiromat 650 and Dräger AV-1 circle system circuits and of an isolated soda lime carbon dioxide absorber. The effects of anaesthetic gas, gas flow rate and the incorporation of a hydrophobic membrane heat and moisture exchanging bacterial/viral filter (HMEF) at the patient end of these circuits were investigated. It was found that without a HMEF the whole interior of the anaesthetic circuits became contaminated with bacteria. Components closest to the simulated patient showed the highest levels of contamination. Higher gas flows were associated with decreased levels of circuit contamination, presumably because more bacteria were expelled from the system. Halothane (1 volume %) and soda lime were not found to have any demonstrable bactericidal action. The presence of a HMEF between the simulated patient and the Y-piece prevented any detectable contamination from reaching the circuit. Consequently, the presence of a HMEF provides protection of the anaesthetic circuit as well as other patients, healthcare workers and the environment.

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.

Cross-infection from contaminated anaesthetic equipment. A real hazard?

A definite relationship between the use of contaminated anaesthetic equipment and subsequent pulmonary infection remains to be established. There is however indirect and circumstantial evidence suggesting that cross-infection may occur, and further an increased susceptibility of surgical patients to pulmonary infections has been demonstrated. Decontamination should be recommended before the equipment is re-used. Pasteurisation may prove sufficient and this can be obtained employing a specially designed dish-washing machine.

Bacterial filters - are they necessary on anaesthetic machines?

At the Vancouver General Hospital the effectiveness of the system for decontamination of anaesthetic equipment was evaluated to determine the need for bacterial filters on anaesthetic machines. Two groups of patients were studied. Group I consisted of 33 patients, none of whom had clinical symptoms of respiratory tract disease. Group II consisted of 17 patients who had lower respiratory tract secretions. In the latter group 16 had chronic bronchitis and had cystic fibrosis. Of 550 bacterial cultures taken from the anaesthetic equipment immediately before and after anaesthesia in our 50 patients, only five yielded a growth of non-pathogenic bacteria. The results of this study indicate that bacterial colonization of anaesthetic equipment is of a low order and is adequately controlled by pasteurization even after use in patients with chronic lower respiratory tract disease. The use of bacterial filters does not appear justified if a strict regimen of cleaning and pasteurization is followed.

Contamination and resterilization of the Bain circuit

Fifty Bain circuits were tested for contamination following single patient use. A contamination rate of 8 per cent was found. Two methods of resterilization were examined. Sterilization by activated glutaraldehyde proved unsatisfactory, while ethylene oxide sterilization was found to be an effective method which produced no detectable adverse physical or chemical alterations of the circuits.