Microbiology

Potential etiologic agents of VAP include MDR and non-MDR pathogens; the prominence of the various pathogens depends on the length of hospital stay at the time of infection and the presence of other risk factors.

Epidemiology, Pathogenesis, and Clinical Manifestations

Prevalence estimates of VAP are 6-52 cases per 100 pts, with the highest hazard ratio in the first 5 days of mechanical ventilation.

• Three factors important in the pathogenesis of VAP are colonization of the oropharynx with pathogenic microorganisms, aspiration of these organisms to the lower respiratory tract, and compromise of normal host defense mechanisms.
• Clinical manifestations are similar to those in other forms of pneumonia.

Diagnosis

Application of clinical criteria consistently results in overdiagnosis of VAP. Use of quantitative cultures to discriminate between colonization and true infection by determining bacterial burden results in less antibiotic use and lower mortality. The more distal in the respiratory tree the diagnostic sampling, the more specific the results.

Etiology and Epidemiology

Bronchiectasis is an irreversible airway dilation that involves the lung in either a focal (due to obstruction) or a diffuse (due to a systemic or infectious process) manner. Bronchiectasis can arise from infectious or noninfectious causes.

• The epidemiology varies greatly with the underlying etiology; in general, the incidence of bronchiectasis increases with age and is higher among women than among men.
• Of pts with bronchiectasis, 25-50% have idiopathic disease.

Pathogenesis

The most widely cited mechanism of infectious bronchiectasis is the “vicious cycle hypothesis,” in which susceptibility to infection and poor mucociliary clearance result in microbial colonization of the bronchial tree. Proposed mechanisms for noninfectious bronchiectasis include immune-mediated reactions that damage the bronchial wall and parenchymal distortion as a result of lung fibrosis (e.g., postradiation fibrosis or idiopathic pulmonary fibrosis).

Clinical Manifestations

Presenting pts typically have a persistent productive cough with ongoing production of thick, tenacious sputum.

• Physical examination usually reveals crackles and wheezing on lung auscultation and occasionally reveals digital clubbing.
• Acute exacerbations are associated with increased production of purulent sputum.

Diagnosis

The diagnosis of bronchiectasis is based on clinical presentation with consistent CXR findings, such as parallel “tram tracks,” a “signet-ring sign” (a cross-sectional area of the airway with a diameter at least 1.5 times that of the adjacent vessel), lack of bronchial tapering, bronchial wall thickening, or cysts emanating from the bronchial wall.

Microbiology

Lung abscess-necrosis and cavitation of the lung following microbial infection-can be categorized as primary (∼80% of cases) or secondary; alternatively, it can be categorized as acute (<4-6 weeks in duration) or chronic (∼40% of cases).

• Primary lung abscesses usually arise from aspiration in the absence of an underlying pulmonary or systemic condition, are often polymicrobial (primarily including anaerobic organisms and microaerophilic streptococci), and occur preferentially in dependent segments (posterior upper and superior lower lobes) of the right lung.
• Secondary lung abscesses arise in the setting of an underlying condition (e.g., a postobstructive process, an immunocompromising condition) and can be due to a number of different organisms, among which P. aeruginosa and other gram-negative rods are most common.

Clinical Manifestations

Initial presentation of lung abscess may be similar to that of pneumonia.

• Anaerobic lung abscesses may have a more chronic and indolent presentation, with night sweats, fatigue, and anemia; in addition, pts may have discolored phlegm and foul-tasting or foul-smelling sputum.
• Pts with lung abscesses due to non-anaerobic organisms (e.g., S. aureus) may present with a more fulminant course characterized by high fevers and rapid progression.

Diagnosis

Chest CT is the preferred radiographic study for precise delineation of the lesion.

• It is not clear whether invasive diagnostics (e.g., transtracheal aspiration) to identify an etiologic agent in primary lung abscesses is helpful.
• Sputum and blood cultures, serologic studies for opportunistic pathogens, and-if needed-more invasive methods of sample collection (e.g., bronchoalveolar lavage, CT-guided percutaneous aspiration) are recommended for secondary lung abscesses or when empirical therapy fails.