Ventilator-Associated Pneumonia

Abstract

The author discusses the pathophysiology of ventilator-associated pneumonia, risk factors for this type of pneumonia, and strategies that may prevent the disease.Pneumonia is the second most common nosocomial infection in the United States and is a leading cause of death due to hospital-acquired infections.1 Ventilator-associated pneumonia (VAP) is a form of nosocomial pneumonia that occurs in patients receiving mechanical ventilation for longer than 48 hours.2 The incidence of VAP is 22.8% in patients receiving mechanical ventilation,3 and patients receiving ventilatory support account for 86% of the cases of nosocomial pneumonia.4 Furthermore, the risk for pneumonia increases 3- to 10-fold in patients receiving mechanical ventilation.5VAP is associated with increases in morbidity and mortality, hospital length of stay, and costs. The mortality rate attributable to VAP is 27% and has been as high as 43% when the causative agent was antibiotic resistant.6 Length of stay in the intensive care unit is increased by 5 to 7 days3 and hospital length of stay 2- to 3-fold in patients with VAP.2 The cost of VAP is estimated to be an additional $40000 per hospital admission per patient with the disease1 and an estimated $1.2 billion per year.7Interventions to prevent VAP begin at the time of intubation and should be continued until extubation. With the extreme shortage of nurses and the resultant increase in the number of less experienced nurses in the intensive care unit, education on the prevention of VAP is essential, because the occurrence of nosocomial infections is directly related to the adequacy of staff.8 Nurses need to understand the pathophysiology of VAP, risk factors for this type of pneumonia, and strategies that may prevent the disease.The onset of VAP can be divided into 2 types: early and late. Early-onset VAP occurs 48 to 96 hours after intubation and is associated with antibiotic-susceptible organisms. Late-onset VAP occurs more than 96 hours after intubation and is associated with antibiotic-resistant organisms2 (Table 1). Interventions to prevent VAP should begin at the time of, or if possible, before intubation. The pathophysiology of VAP involves 2 main processes: colonization of the respiratory and digestive tracts and microaspiration of secretions of the upper and lower parts of the airway.9Colonization of bacteria refers to the presence of bacteria without an active host response.10 Bacterial colonization of the lungs can be due to spread of organisms from many different sources, including the oropharynx, sinus cavities, nares, dental plaque, gastrointestinal tract, patient-to-patient contact, and the ventilator circuit.10 Inhalation of colonized bacteria from any of these sources can cause an active host response and, ultimately, VAP.The presence of an endotracheal tube provides a direct route for colonized bacteria to enter the lower respiratory tract. Upper airway and oral secretions can pool above the cuff of an endotracheal tube and line the tube, forming a biofilm. Starting as early as 12 hours after intubation, the biofilm contains large amounts of bacteria that can be disseminated into the lungs by ventilator-induced breaths.10–12 In addition, the biofilm may become dislodged by instillation of saline into the endotracheal tube, suctioning, coughing, or repositioning of the endotracheal tube.12 Endotracheal tubes cause an abnormal interruption between the upper airway and the trachea, bypassing the structures in the upper airway and providing bacteria a direct route into the lower airway.10 Because the upper airway is bypassed, a decrease occurs in the body’s ability to filter and humidify air.12 In addition, the cough reflex is often eliminated and/or decreased by the presence of an endotracheal tube,2 and mucociliary clearance can be impaired because of mucosal injury during intubation.13 An endotracheal tube provides a place for bacteria to bind in the trachea, a situation that further increases production and secretion of mucus.13 The impairment of these natural host defense mechanisms increases the likelihood of bacterial colonization and subsequent aspiration of the colonized organisms.Aspiration of gastric contents is another potential cause of VAP, because the stomach serves as a reservoir for bacteria. Most patients receiving mechanical ventilation have a nasogastric or an orogastric tube in place for enteral feedings and administration of medications or for gastric decompression. The presence of a nasogastric or an orogastric tube interrupts the gastroesophageal sphincter, leading to increased gastrointestinal reflux and providing a route for bacteria to translocate to the oropharynx and colonize the upper airway. Enteral feedings increase both gastric pH and gastric volume, increasing the risk of both bacterial colonization and aspiration.14Because every patient who is intubated and receiving ventilatory support is at risk for VAP, making an accurate diagnosis of this disease and starting treatment is critical. Diagnosing VAP remains difficult and controversial. The diagnosis can be made on the basis of radiographic findings, clinical findings, results of microbiological tests of sputum, or invasive testing such as bronchoscopy.15 The diagnosis is most often based on visualization of a new or progressive infiltrate on chest radiographs.16 However, findings on chest radiographs are not reproducible and should not be used alone for the diagnosis of VAP.13 Other causes of pulmonary infiltrates visualized on chest radiographs of patients receiving mechanical ventilation include atelectasis, aspiration, pulmonary embolism, pulmonary edema, alveolar hemorrhage, pulmonary infarction, and acute respiratory distress syndrome. The likelihood of VAP increases if a patient has clinical signs and symptoms such as fever, leukocytosis, and purulent sputum in addition to abnormal findings on chest radiographs.16 The results of microbiological tests of sputum specimens obtained by either invasive or noninvasive methods are not sufficient for the diagnosis of VAP, but culture and sensitivity results can be helpful for choosing an antibiotic.17Although any patient with an endotracheal tube in place for more than 48 hours is at risk for VAP, certain patients are at higher risk. The risk factors for VAP can be divided into 3 categories: host related, device related, and personnel related (Table 2). Host-related risk factors include preexisting conditions such as immunosuppression, chronic obstructive lung disease, and acute respiratory distress syndrome. Other host-related factors include patients’ body positioning, level of consciousness, number of intubations, and medications, including sedative agents and antibiotics. In one study,18 bacterial contamination of endotracheal secretions was higher in patients in the supine position than in patients in the semirecumbent position. Whether due to a pathophysiological process, medication, or injury, decreased level of consciousness resulting in the loss of the cough and gag reflexes contributes to the risk of aspiration and therefore increased risk for VAP.19 Reintubation and subsequent aspiration can increase the likelihood of VAP 6-fold.20Device-related risk factors include the endotracheal tube, the ventilator circuit, and the presence of a nasogastric or an orogastric tube. Secretions pool above the cuff of an endotracheal tube, and low cuff pressures can lead to microaspiration and/or leakage of bacteria around the cuff into the trachea.14 As mentioned earlier, nasogastric and orogastric tubes disrupt the gastroesophageal sphincter, leading to reflux and an increased risk for VAP. The question of whether placement of nasogastric or orogastric tubes distal to the pylorus decreases the risk of aspiration and VAP remains unanswered. The results of studies on the relationship between use of small-bore feeding tubes and the incidence of VAP have been inconclusive. The Centers for Disease Control and Prevention makes no recommendations about routine use of postpyloric feeding tubes or small-bore feeding tubes, because these issues remain controversial and further research is needed.1Improper hand washing resulting in the cross-contamination of patients is the biggest personnel-related risk factor for VAP. Patients who are intubated and receiving mechanical ventilation often need interventions such as suctioning or manipulation of the ventilator circuit. These interventions increase the likelihood of cross-contamination between patients if healthcare staff do not use proper hand-washing techniques. Failure to wash hands and change gloves between contaminated patients has been associated with an increased incidence of VAP.21 In addition, failure to wear proper personal protec-resistant organisms have been identified increases the risk of cross-contamination between patients.1Although VAP has multiple risk factors, many nursing interventions can reduce the incidence of this disease (Table 3). Prevention of pneumonia, both in and outside of the hospital, begins with vaccination (Figure 1).22 Nurses are the first line of defense in preventing bacterial colonization of the oropharynx and the gastrointestinal tract. Meticulous hand washing for 10 seconds should be performed before and after all contact with patients.1 In addition, gloves should be worn when contact with oral or endotracheal secretions is possible. Strategically placing a sign on a patient’s door to remind health-care workers to wash their hands and wear gloves is an easy and cost-effective measure that can help minimize transmission of bacteria between patients. The use of protective gowns is not recommended as routine practice, but gowns should be used when antibiotic-resistant pathogens have been isolated and identified.1Oral decontamination, by reducing the amount of bacteria within a patient’s oral cavity, can be accomplished by both mechanical and pharmacological interventions. Mechanical interventions include tooth brushing and rinsing of the oral cavity to remove dental plaque; pharmacological interventions involve the use of antimicrobial agents.23 Bacteria in dental plaque can be removed by brushing the teeth and thoroughly suctioning secretions from the mouth. Both of these interventions decrease the likelihood of colonization of the oropharynx.Pharmacological interventions include twice-a-day use of chlorhexidine oral rinse. In a study24 of patients undergoing cardiac surgery, use of chlorhexidine decreased the incidence of VAP by decreasing colonization. In another study,7 use every 6 hours of a solution containing gentamicin, colistin, and vancomycin decreased the incidence of VAP by 16% and resulted in an estimated cost savings of $13000 for every case of VAP prevented. Unfortunately, except for patients undergoing cardiac surgery, no evidence-based protocols for oral care have been tested and confirmed to decrease the incidence of VAP in patients receiving mechanical ventilation.Bacterial colonization of the stomach can lead to aspiration and colonization of the respiratory tract. Most patients receiving mechanical ventilation are given stress ulcer prophylaxis, often with medications that increase the gastric pH. A study25 in the 1980s indicated that pathogens multiply in an alkaline gastric environment. In the largest study26 on the risk for VAP with stress ulcer prophylaxis, ranitidine, an H2-receptor blocker, significantly reduced the risk of clinically important bleeding without increasing the risk of VAP or mortality. In another large study,27 mechanical ventilation for greater than 48 hours was associated with a 16-fold increase in the risk for gastrointestinal bleeding. In research28 conducted in a pediatric intensive care unit, VAP rates did not differ between patients receiving ranitidine, omeprazole, or sucralfate for stress ulcer prophylaxis. Unfortunately, as the use of proton pump inhibitors for stress ulcer prophylaxis has increased, few studies have been conducted in adults to address whether the incidence of VAP is affected by the use of these inhibitors. In summary, according to the studies done so far, stress ulcer prophylaxis does not play a significant role in the development of VAP but may prevent the serious complication of gastrointestinal bleeding.Mucus in the airways can become stagnant and serve as a medium for bacterial growth. Maintenance of aseptic technique when performing endotracheal suctioning is essential to prevent contamination of the airways. No difference has been found in the incidence of VAP with open versus closed suction systems.29 When a closed system is used, the suction catheter should be rinsed free of secretions away from the patient.Furthermore, saline lavage of endotracheal tubes before suctioning dislodges bacteria from the endotracheal tube into the lower airways, increasing the risk for VAP.30 Saline lavage has long been considered a means to liquefy secretions and prevent plugs of mucus in endotracheal tubes. However, in one study,31 saline instillation did not thin secretions; rather it reduced the amount of oxygen that reached the lungs and increased blood pressure, heart rate, intracranial pressure, and the risk for VAP. Maintaining adequate hydration, ensuring proper humidification of the ventilatory circuit, and using nebulizer or mucolytic agents can help decrease the viscosity of secretions and eliminate the need for saline lavage.30,31 Prophylactic use of systemic antibiotics does not decrease the incidence of VAP and when the agents are used inappropriately, antibiotic resistance can develop.32Routine turning of patients a minimum of every 2 hours can increase pulmonary drainage and decrease the risk for VAP. Use of beds capable of continuous lateral rotation can decrease the incidence of pneumonia but do not decrease mortality or duration of mechanical ventilation.33 These beds are costly and are not necessary for routine use in the prevention of VAP,1 although the use of specialty beds may be cost-effective and therapeutic for patients with poor oxygenation or impaired wound healing.Colonization of the ventilator circuit can also play a role in the development of VAP. Daily changes of the ventilator circuit do not seem to decrease the incidence of VAP.34 The Centers for Disease Control and Prevention does not recommend changing the ventilator circuit more than once every 48 hours,1 and research35 has indicated that changing the ventilator circuit as infrequently as once a week does not increase the risk for VAP. It is recommended that the ventilator circuit be changed when visibly soiled.1 Many investigators have compared the impact of heat and moisture exchangers on the incidence of VAP with the impact of heated humidifiers. The results were inconclusive as to which form of humidity is associated with a higher incidence of VAP.In addition to strategies to prevent colonization, strategies to prevent aspiration can also be used to decrease the risk for VAP. Because the presence of an endotracheal tube predisposes patients to VAP, patients should be assessed on a daily basis for potential weaning and extubation from mechanical ventilation. Several methods of assessing readiness for extubation exist. These include T-piece trials, weaning intermittent mandatory ventilation, and pressure-support ventilation.36Positioning patients in a semi-recumbent position with the head of the bed elevated 30° to 45° prevents reflux and aspiration of bacteria from the stomach into the airways. Simply elevating the head of the bed 30° can decrease VAP by 34%.37 Impaired gastric emptying can lead to overdistention, or increased gastric residual volume, of the stomach and the potential for regurgitation and aspiration. Minimizing the use of narcotic agents can help prevent aspiration of gastric and/or oral contents.2 Decreasing use of narcotic and/or sedative agents in the intensive care unit must be done cautiously, because pain can limit deep breathing and impair oxygenation. Daily interruptions of continuous sedative infusions can shorten the duration of mechanical ventilation by more than 2 days and length of stay in the intensive care unit by 3.5 days.38 Monitoring gastric residual volumes and administering agents to increase gastric motility have been suggested as ways to prevent gastric overdistention.2 Although the effectiveness of these interventions in reducing VAP has not been tested in clinical trials, it is reasonable to avoid gastric overdistention in an attempt to prevent aspiration.Because secretions tend to pool above the cuffs of endotracheal tubes, the oropharynx should be thoroughly suctioned to prevent aspiration of the pooled secretions before an endotracheal tube is replaced. Pressure in the cuff should be measured and should be maintained at no less than 20 cm H2O.39 Maintaining adequate cuff pressures decreases the likelihood that secretions will leak around the cuff or be aspirated. Use of tubes with ports for continuous subglottic suctioning can decrease the incidence of VAP by 50%,40 and costs per episode of VAP have been reduced as much as $18000 when endotracheal tubes with continuous subglottic suctioning are used rather than traditional endotracheal tubes.41Studies are also being conducted with endotracheal tubes coated with silver nitrate. Use of urinary catheters coated with silver nitrate has been associated with a decreased incidence of urinary tract infections; therefore, it is thought that endotracheal tubes coated with silver nitrate might decrease the incidence of VAP. It is hypothesized that silver nitrate interferes with the ability of bacteria to line the endotracheal tube and form a biofilm. In a study11 of dogs receiving mechanical ventilation, endotracheal tubes coated with silver nitrate were less likely to become colonized with bacteria than were traditional endotracheal tubes. Further studies are needed to determine the cost-effectiveness of these coated tubes in preventing VAP.The effects of VAP on morbidity, mortality, length of hospital stay, and cost are immense. Education plays a key role in the management of patients with VAP. Use of self-study education modules on the nursing care of patients at risk for VAP can decrease the rate of this type of pneumonia, the number of days of mechanical ventilation, and the cost of the disease.8Healthcare systems can also play a role in preventing VAP by employing outcome managers to provide a more comprehensive approach to VAP prevention. In one study,42 the duration of mechanical ventilation, hospital and intensive care unit lengths of stay, and mortality decreased when an outcome manager was used. Outcome managers can be responsible for ensuring that protocols to prevent VAP and other complications of intubation and mechanical ventilation are developed and are being followed appropriately. The use of ventilator pathways and/or protocols with preprinted order sets (Figure 2) can also lead to improved outcomes for patients. A simple, yet cost-effective way to ensure compliance with elevating the head of the bed is random daily audits. The rationale for why a patient cannot have the head of the bed elevated should be documented.3 Standardized orders or pathways can be a friendly reminder to healthcare providers about the importance of interventions to prevent VAP.VAP, although often preventable, has a large impact on morbidity and mortality. Together with other health-care providers, nurses play a key role in preventing VAP. Many of the interventions are part of routine nursing care. Education for all healthcare providers should focus on the risk factors for VAP and on preventive measures. In order to further decrease the incidence of VAP, protocols and monitoring tools must be developed. VAP is not a new diagnosis, but education and research on the prevention of this life-threatening problem are ongoing.