key: cord-0037806-4hgskfng
authors: Lorente, Leonardo
title: Respiratory Filters and Ventilator-Associated Pneumonia: Composition, Efficacy Tests and Advantages and Disadvantages
date: 2011-05-25
journal: Humidification in the Intensive Care Unit
DOI: 10.1007/978-3-642-02974-5_20
sha: 8b822052e7d4d1c9014b0603eefc8306d17262b5
doc_id: 37806
cord_uid: 4hgskfng

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).

1. Mechanical fi ltration is determined by several aspects. First, the size of the fi lter's pores causes the organisms to be retained on a relatively large fi lter surface. Second, the provision of nonlinear irregular pores determines the course of airfl ow and causes an increase in inertial force that traps the microorganisms within the mesh. The pore size can allow interception of organisms larger than 1 m m and thanks to the nonlinear arrangement of the pores increases their ability to fi lter microorganisms larger than 0.5 m m. This principle has the limitation that the mesh of tiny pores has high resistance to airfl ow. 2. The electrostatic fi lter is produced by the fi bers of the internal components of the fi lter being subjected to an electric fi eld. Bacteria and viruses also have a surface electric charge, either positive or negative, and remain trapped in the dipole electric fi elds of the fi lter screen. 3. The bactericidal fi lter is made by impregnating the fi lter material with bactericidal agents. Their action allows the growth of bacteria within the fi lter. For this purpose, antiseptic substances such as chlorhexidine acetate have been used. They are not recommended because they can dissolve in the condensate circuit and reach the tracheobronchial tract.

Microbial fi ltration effi ciency of the fi lters is assessed by challenge with a microbial aerosol [ 1 ] . An aerosol with a microorganism and a known concentration is generated, then the aerosol is passed through the fi lter and the concentration examined after passing through the fi lter. Filter effi ciency is analyzed by an aerosol with different organisms; a comparison is made of the concentration of microorganisms in the gas applied to the fi lter and the effl uent gas after it has passed through the fi lter. The fi ltration effi ciency is evaluated for bacteria and viruses. Bacterial fi ltration of tiny Pseudomonas or Serratia marcescens , which have a diameter of 0.3 m m, was evaluated. Viral fi ltration effi ciency was evaluated with the hepatitis C virus, which has a diameter of 0.03 m m.

Many experimental studies have verifi ed the ability of antimicrobial fi lters to prevent the passage of microorganisms. Some fi lters examined in the fi ltration effi ciency tests in vitro reached values of bacterial fi ltration effi ciency greater than 99.999% [ 1 ] .

Respiratory fi lters are inserted in the breathing circuit with a conical socket of 15 mm diameter (in the area of the patient) and a conical plug of 22 mm diameter (in the area of the respirator). This prevents the disconnection of the breathing circuit, which would endanger the patient's life.

The functions of microbial fi lters vary depending on their location in the breathing circuit: (1) In the inspiratory limb, they can prevent antegrade infection of the patient by the respirator; (2) in the expiratory limb, they can prevent the retrograde infection of the patient by the respirator; (3) interposed between the "Y" part and the endotracheal tube, they can have both functions.

Antimicrobial fi lters involve some undesirable effects [ 10 ] : (1) increased resistance to inspiratory airfl ow, (2) increased resistance to expiratory airfl ow and (3) an increase in the dead space of the breathing circuit. 1. The antimicrobial fi lters cause an increase in expiratory fl ow resistance, which can promote air trapping within the patient's lungs. Pulmonary air trapping can have different implications: (a) hemodynamic deterioration, (b) risk of pneumothorax and (c) impaired gas exchange. Pulmonary air trapping leads to increased intrathoracic pressure, which causes the venous return, and therefore can decrease cardiac output and blood pressure. One of the mechanisms of the production of pneumothorax is increased intrathoracic pressure, and this increase appears with air trapping. Moreover, this can also cause air trapping impaired gas exchange because of changes in ventilation/perfusion of the lung, and therefore lead to the development of hypoxemia and/or hypercapnia. This effect could appear when the fi lter is interposed between the "Y" part and the endotracheal tube or is located in the expiratory limb (immediately before the expiratory valve of the respirator). 2. Respiratory fi lters produce an increase in inspiratory fl ow resistance, which may have implications for the patient and the respirator. This increase in inspiratory fl ow resistance increases the work of breathing of the patient to initiate inspiration and may hinder weaning from mechanical ventilation. Besides, this increase in inspiratory fl ow resistance also increases the work of breathing for inspiration, and positive pressure can damage the mechanism of the respirator. This effect can appear when the fi lter is interposed between the "Y" part and the endotracheal tube or is located in the inspiratory limb (immediately after the ventilator inspiratory valve). 3. The bacterial fi lters generate an increase in dead space because the air space does not participate in gas exchange and can lead to hypoventilation and the development of hypoxemia and/or hypercapnia. This effect can appear when the fi lter is interposed between the "Y" part and the endotracheal tube.

The different breathing circuits used, based on the location of respiratory fi lters in the circuit, have different advantages and disadvantages. In a breathing circuit with one fi lter, the fi lter is interposed between the "Y" part and the endotracheal tube. The advantage of a fi lter circuit with one fi lter is that the initial economic cost is lower (because there is only one fi lter). The disadvantages of using one fi lter are the increased dead space in the circuit and that the fi lter has to be changed often as it gets contaminated by patient secretions from coughing. In a breathing circuits with two fi lters, one is placed in the inspiratory limb (immediately after the ventilator inspiratory valve) and another in the expiratory limb (immediately before the expiratory valve of the respirator).

The advantages of circuits with two fi lters are that there is no increasing dead space and no risk of having to change fi lters because of contamination by patient secretions. The disadvantage of using two fi lters is that the initial breathing circuit is more expensive (because there are two fi lters).

The issue of whether contaminated ventilators and anesthesia machines are the origin of nosocomial pneumonia is controversial, with some data implicating them [2] [3] [4] and others not [5] [6] [7] [8] [9] .

Reports from 1952 to 1972 on several outbreaks of respiratory infections attributed the contamination to anesthesia machines [2] [3] [4] . However, none of the reports presented a bacteriological demonstration of a cause-and-effect relationship; however, the study by Tinne et al. reported that the same isolate of Pseudomonas aeruginosa responsible for an outbreak of postoperative pneumonia was cultured from the corrugated tubing of an anesthesia machine and from Ambu bags [ 3 ] .

Contrarily, several studies have shown no contamination of the patient by the anesthesia machine and vice versa [5] [6] [7] [8] [9] . In some studies [ 5, 6 ] of anesthetized patients with and without respiratory infection, samples were taken from several sites of the anesthesia machine and breathing circuits before and after anesthesia, and no differences were found in the contamination of the anesthesia machine and breathing circuits in either patient group. Other studies [6] [7] [8] [9] have simulated the contamination of an anesthesia machine by intentional contamination of the expiratory limb of the breathing circuit with an inoculum of an organism, after the sterilization of the anesthesia machine and the entire respiratory circuit, and with continued contamination of the anesthesia machine and breathing circuit inspiratory limb. The authors suggest that the absence of contamination of the anesthesia machine and breathing circuit inspiratory limb is because microorganisms cannot live in the breathing circuits, because the circulating gas is cold and dry (characteristic of medicinal gases), which hinders the survival of microorganisms.

In an attempt to prevent ventilator-associated pneumonia by contamination of respirators and anesthesia machines, inserting respiratory fi lters in the breathing circuits has been proposed. Some authors have suggested that respiratory fi lters could reduce the incidence of respiratory infections associated with mechanical ventilation because of a reduction in the incidence of infections acquired by exogenous pathogenesis [ 4 ] , i.e., those infections that are caused by microorganisms that do not colonize the oropharynx at the time of diagnosis. This decrease in exogenous respiratory infection processes could be due to the fact that microbial fi lters in respiratory circuits could reduce the risk of exogenous microorganisms reaching the patient antegradely from the inspiratory valve of the respirator or retrogradely from the exhalation valve of the respirator.

However, in clinical studies respiratory fi lters have failed to reduce the incidence of ventilator-associated pneumonia in patients on anesthesia machines [ 11, 12 ] and in critically ill patients [ 13 ] . In 1981, Garibaldi et al. [ 11 ] examined 520 patients on anesthesia breathing circuits with fi lters (inspiratory and expiratory) or without fi lters, and found no difference in the cumulative incidence of ventilator-associated pneumonia (16.7% vs. 18.3%). In 1981, Feeley et al. [ 12 ] studied 293 anesthetized patients, a group with a fi lter circuit in the inspiratory limb and one without fi lters, and no differences in the cumulative incidence of ventilator-associated pneumonia between the two groups (2.2% vs. 2.5%) was found. In one study carried out by our team, 230 critically ill patients were randomized to receive mechanical ventilation with and without respiratory fi lters. We did not fi nd signifi cant differences between patients with and without respiratory fi lters in the percentage of patients who developed VAP (24.56% vs. 21.55%), in the incidence of VAP per 1000 days of mechanical ventilation (17.41 vs. 16 .26 without BF) or in the incidence of exogenous VAP per 1000 days of mechanical ventilation (2.40 vs. 1.74) [ 13 ] .

In the guidelines of the Centers for Disease Control and Prevention (CDC) for the prevention of VAP published in 2004 [ 14 ] , no recommendation was made for or against the use of respiratory fi lters in breathing circuits of respirators, either with hot water humidifi ers or with heat and moisture exchangers, or in breathing circuits of anesthesia machines, because there is insuffi cient evidence or consensus on their effectiveness.

The guidelines of the Canadian Critical Care Society published in 2008 did not recommend using respiratory fi lters [ 15 ] .

The guidelines of British Society for Antimicrobial Chemotherapy published in 2008 recommended the use of expiratory fi lters for patients suffering from highly communicable infections (e.g., human coronavirus) and who require mechanical ventilation to reduce the contamination of ventilator circuits (although they do not reduce the VAP risk) [ 16 ] .

In the guidelines published in 2008 by the Society for Healthcare Epidemiology of America/Infectious Diseases Society of America (SHEA/IDSA) [ 17 ] and in those published by two different European working groups in 2009 [ 18 ] and 2010 [ 19 ] , there were no reviews of the issue of preventing VAP.

The CDC guidelines for preventing the transmission of Mycobacterium tuberculosis recommend the use of respiratory fi lters in patients with suspected or confi rmed bacillary pulmonary tuberculosis undergoing mechanical ventilation [ 20 ] .

Outbreaks of VAP were associated with the contamination of anesthesia machines from 1952 to 1972; however, none of the reports presented a bacteriological demonstration of a cause-and-effect relationship.

Bacterial fi lters have been interposed in respiratory circuits to avoid VAP caused by contamination of ventilators and anesthesia machines.

The use of respiratory fi lters has not decreased the incidence of VAP in patients using anesthesia machines and in critically ill patients.

Besides, respiratory fi lters could have some undesirable effects, such as an increase of resistance to inspiratory airfl ow, increase of resistance to expiratory airfl ow and increase of dead space in the breathing circuit.

The use of respiratory fi lters is not routinely necessary; however, they should be used in patients with suspected or confi rmed highly communicable respiratory infections (such as bacillary pulmonary tuberculosis) and who require mechanical ventilation.

Heat and moisture exchangers and breathing fi lters

Disease transmission by ineffi ciently sanitized anesthetizing apparatus

Cross infection by Pseudomonas aeruginosa as a hazard of intensive surgery

Infection by anaesthetic apparatus

Anaesthetic machines and cross infections

The anesthesia machine and circle system are not likely to be sources of bacterial contamination

Bacterial fi lters: Are they necessary on anesthetic machines?

Anesthetic equipment as a source of infection

Risk of cross infection from inhalation anesthetic equipment

Increase in resistance of in-line breathing fi lters in humidifi ed air

Failure of bacterial fi lters to reduce the incidence of pneumonia after inhalation anesthesia

Sterile anesthesia breathing circuits do not prevent postoperative pulmonary infection

Bacterial fi lters in respiratory circuits: An unnecessary cost?

Guidelines for prevention of healthcare-associated pneumonia

VAP Guidelines Committee and the Canadian Critical Care Trials Group. Comprehensive evidence-based clinical practice guidelines for ventilator-associated pneumonia: prevention

Guidelines for the management of hospital-acquired pneumonia in the UK: report of the working party on hospital-acquired pneumonia of the British Society for Antimicrobial Chemotherapy

Practice Recommendation of Society for Healthcare Epidemiology of America/ Infectious Diseases Society of America (SHEA/IDSA). Strategies to prevent ventilatorassociated pneumonia in acute care hospitals

European HAP working group. Defi ning, treating and preventing hospital acquired pneumonia: European perspective

VAP Care Bundle Contributors. A European care bundle for prevention of ventilator-associated pneumonia

Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care facilities