Preventing Hospital-Acquired and Ventilator-Associated Pneumonia

Introduction

Pneumonia refers to the formation of new lung infiltrates and clinically present with infiltrates of infectious origin, which are associated with the onset of fever, leukocytosis (increased production of white blood cells), purulent sputum (sputum with pus), and decline in oxygenation levels. Ventilator-associated pneumonia develops in patients under ventilation.

What Is Hospital-Acquired Pneumonia?

Hospital-acquired pneumonia is also referred to as nosocomial infection. It refers to the infection in the lower respiratory tract that is developed after two or more days of hospitalization.

What Is Ventilator-Associated Pneumonia?

Ventilator-associated pneumonia refers to pneumonia that presents more than 48 hours after endotracheal intubation.

What Is the Pathophysiology of Hospital Acquired and Ventilator-Associated Pneumonia?

The bacteria enters the lungs through three mechanisms such as inhalation, aspiration, and hematogenous. The basic route by which the organism enters the lower respiratory tract is through the aspiration of oropharyngeal secretions into the trachea.

• Primary Inhalation Pneumonia: It is caused by the inhalation of aerobic gram-negative organisms that colonizes the upper respiratory tract or respiratory support equipment. It is also developed when the microorganisms cross or bypass the normal respiratory defense mechanisms.

• Aspiration Pneumonia: It is caused due to the aspiration of colonized upper respiratory tract secretions. Around 45 percent of healthy individuals aspirate during sleep when compared with critically ill patients who aspirate more frequently. The gram-negative bacteria colonizing the stomach (reservoir for bacteria) can ascend and colonize the upper respiratory tract. The risk increases with the usage of proton pump inhibitors but not with histamine -2 blocking agents.

• Hematogenously Acquired Infections: These arise from a distant source and approach the lungs through the bloodstream.

What Is the Cause of Hospital-Acquired and Ventilator-Associated Pneumonia?

• The aerobic gram-negative bacilli, such as Pseudomonas aeruginosa, are the major pathogens associated with hospital-acquired pneumonia.

• The pathologic mechanism involved in the infection includes destructive effects on the lung tissue.

• Aerobic gram-negative pathogens are classified into two categories:

• Organisms that cause necrotizing pneumonia, blood vessel invasion, microabscess formation, and hemorrhage.

• Non-necrotizing gram-negative bacilli such as Serratia marcescens are responsible for causing nosocomial pneumonia.

What Are the Common Causes of Hospital-Acquired Pneumonia?

The common bacterial microorganisms involved in hospital-acquired pneumonia include:

• Pseudomonas aeruginosa.

• Staphylococcus aureus (includes methicillin-susceptible Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus (MRSA).

• Escherichia coli.

• Klebsiella pneumoniae.

• Non-enterobacteriaceae species such as Serratia marcescens, Stenotrophomonas maltophilia, and Acinetobacter species are less common causes.

• Acinetobacter species are commonly seen in respiratory tract infections and are most commonly seen in critically ill patients in intensive care units.

• Hospital-acquired pneumonia caused by Acinetobacter species or B. cepacia are responsible for causing frequent outbreaks in hospital settings.

• Streptococcus pneumoniae and Haemophilus influenzae are isolated only on the early- onset of hospital-acquired pneumonia infections.

What Are the Organisms Associated With the Hospital-Acquired Pneumonia?

The nosocomial infections that typically affect highly immunocompromised patients are linked to a few less common microorganisms, including:

• Human parainfluenza virus 3 (HPIV - 3).

• Human metapneumovirus.

• Influenza A virus.

• Legionella species.

• Respiratory syncytial virus.

Certain species, such as influenza A, respiratory syncytial virus (RSV), human metapneumovirus, and human parainfluenza virus -3 (HPIV-3), are involved in causing hospital-acquired infections, which are contagious and can be easily transferred among people.

Nosocomial Legionella pneumonia causes infection in the form of outbreaks or clusters.

What Are the Bacteria Associated With the Ventilator-Associated Pneumonia?

Certain organisms are associated with ventilator-associated pneumonia, and these include:

• Stenotrophomonas maltophilia.

• Acinetobacter species.

• Pseudomonas aeruginosa.

• Enterobacteriaceae organisms are more commonly involved in ventilator-associated than in hospital-acquired pneumonia.

What Are the Risk Factors Associated With Hospital-Acquired and Ventilator-Associated Pneumonia?

• Endotracheal intubation is considered to be one of the major risk factors, which is associated with multiple factors, including:

• Prolonged duration of ventilation.

• Abnormal swallowing function.

• Secretions pooled above the endotracheal tube.

• Micro aspiration around the endotracheal tube.

• Other risk factors include previous antibiotic use, malnutrition, and cross-contamination with other patients in the intensive care unit.

What Are the Diagnostic Approaches?

• In the case of suspected pneumonia patients, antibiotic treatment is discontinued if the obtained culture results are below the diagnostic threshold, and that includes:

• Bronchoscopic or Mini - Bronchoalveolar Lavage (BAL) – showing 10,000 colony-forming units (CFU) per milliliter.

• Bronchoscopy Protected Specimen Brush (PSB) –1000 colony forming units (CFU) per milliliter.

• Endotracheal Aspirates - 1,000000 colony forming units.

What Is the Microbiologic Approach to Diagnose Ventilator-Associated Pneumonia?

The following methods are performed to collect a sample from the lower respiratory tract to rule out the diagnosis of ventilator-associated pneumonia:

• Blind Tracheobronchial Aspiration (TBAS)- A small flexible catheter is inserted into the distal trachea.There is no direct lung sample where radiographic data suggests infiltrates. There is more risk of contamination if the endotracheal tube is inserted further, which can have a risk of providing false positive results.

• To prevent upper respiratory tract contamination, a protected specimen brush (PSB) could be introduced using a bronchoscope.

• Bronchoscopy With Bronchoalveolar Lavage- This test helps in collecting the samples of lung segments with radiographic evidence of infiltrates. Certain limitations of the technique include operator skill, contaminations, and the risk of hypoxemia (in severe cases).

Radiographic Findings:

• Chest radiography is particularly useful in ruling out pneumonia progression and acting as a marker of the success of adequate antibiotic treatment.

• Radiographic findings reveal pulmonary consolidation associated with pleural effusion in chest X-ray findings and computed tomography.

Computed Tomography Findings:

• Computed tomography or spiral computed tomography scanning helps in the differentiating of nosocomial pneumonia infections.

• The diagnosis of Methicillin-resistant Staphylococcus or Methicillin-sensitive Staphylococcus aureus depends on the presence of clinical findings such as fever, hypotension, cyanosis, and radiographic findings such as rapid cavitation of infiltrates.

What Are the Treatment Modalities for Hospital-Acquired and Ventilator-Associated Pneumonia?

• For patients with hospital-acquired pneumonia or ventilator-associated pneumonia caused due to Pseudomonas aeruginosa, the antibiotics prescribed are based on the results of antimicrobial testing.

• For patients with hospital-acquired and ventilator-associated pneumonia, a seven-day antimicrobial therapy is recommended.

• Once the final culture and sensitivity results are available, the empirical broad spectrum is converted into narrow and specific coverage.

Conclusion:

Both hospital-acquired and ventilator-associated pneumonia are life-threatening conditions if not treated properly. The use of inhaled antibiotic therapy is limited to cases of ventilator-associated pneumonia induced by gram-negative bacilli that are treated only with Polymyxins or aminoglycosides. The intravenous route of administration is more effective and followed in the required situations.