Morphologically,lobarpneumoniasevolvethroughfourstages:congestion,redhepatization,grayhepatization,andresolution.
OthercommoncausesofbacterialpneumoniasinthecommunityincludeH. influenzae andM. catarrhalis (bothassociatedwithacuteexacerbationsofCOPD),S. aureus (usuallysecondarytoviralrespiratoryinfections),K. pneumoniae (observedinchronicalcoholics),P. aeruginosa (seeninindividualswithcysticfibrosis,inburnvictims,andinpatientswithneutropenia),and
L.
pneumophila, seenparticularlyinorgantransplantrecipients.
Viral pneumonias arecharacterizedbyrespiratorydistressoutofproportiontotheclinicalandradiologicsigns,andbyinflammationthatispredominantlyconfinedtoalveolarsepta,withgenerallyclearalveoli.
CommoncausesofviralpneumoniaincludeinfluenzaAandB,respiratorysyncytialvirus,humanmetapneumovirus,parainfluenzavirus,andadenovirus.
Nosocomial, or hospital-acquired, pneumonias are defined as pulmonary infections acquired in the course of a hospital stay. These infections not only have an adverse impact on the clinical course of ill patients, but http://ebooksmedicine.net also add considerably to the burgeoning costs of health care. Nosocomial infections are common in hospitalized individuals with severe underlying disease, those who are immunosuppressed, or those on prolonged antibiotic regimens. Patients on mechanical ventilation are a particularly high-risk group, and infections acquired in this setting are given the designation ventilator-associated pneumonia. Gram-negative rods (members of Enterobacteriaceae and Pseudomonas spp.) and S. aureus are the most common isolates; unlike community-acquired pneumonias, S. pneumoniae is not a common pathogen in the hospital setting.
Aspiration pneumonia occurs in debilitated patients or those who aspirate gastric contents while unconscious (e.g., after a stroke) or during repeated vomiting. Those affected have abnormal gag and swallowing reflexes that facilitate aspiration. The resultant pneumonia is partly chemical, due to the extremely irritating effects of the gastric acid, and partly bacterial. Typically, more than one organism is recovered on culture, aerobes being more common than anaerobes (see
Table 13.5 ). Aspiration pneumonia is often necrotizing, pursues a fulminant clinical course, and is a frequent cause of death in individuals predisposed to aspiration. In those who survive, abscess formation is a common complication. Microaspiration, by contrast, occurs in many individuals, especially those with gastroesophageal reflux, and may exacerbate other lung diseases but does not lead to pneumonia.
Lung abscess refers to a localized area of suppurative necrosis within the pulmonary parenchyma, resulting in the formation of one or more large cavities. The causative organism may be introduced into the lung by any of the following mechanisms:
Aspiration of infective material from carious teeth or infected sinuses or tonsils. This may occur during oral surgery, anesthesia, coma, or alcoholic intoxication, and in debilitated patients with depressed cough reflexes.
Aspiration of gastric contents, usually accompanied by infectious organisms originating in the oropharynx.
As a complication of necrotizing bacterial pneumonias, particularly those caused by S. aureus, Streptococcus pyogenes, K. pneumoniae, Pseudomonas spp., and, rarely, type 3 pneumococci. Mycotic infections and bronchiectasis also may lead to lung abscesses.