Pneumonia is an acute infection of the lung parenchyma that often impairs gas exchange. Pneumonia can be classified in several ways. Based on microbiological etiology, it may be viral, bacterial, fungal, protozoal, mycobacterial, mycoplasmal, or rickettsial in origin.
Based on location, pneumonia may be classified as bronchopneumonia, lobular pneumonia, or lobar pneumonia. Bronchopneumonia involves distal airways and alveoli; lobular pneumonia, part of a lobe; and lobar pneumonia, an entire lobe.
The infection is also classified as one of three types - primary, secondary, or aspiration pneumonia. Primary pneumonia results directly from inhalation or aspiration of a pathogen, such as bacteria or a virus; it includes pneumococcal and viral pneumonia. Secondary pneumonia may follow initial lung damage from a noxious chemical or other insult (superinfection) or may result from hematogenous spread of bacteria from a distant area. Aspiration pneumonia results from inhalation of foreign matter, such as vomitus or food particles, into the bronchi. It's more likely to occur in elderly or debilitated patients, those receiving nasogastric tube feedings, and those with an impaired gag reflex, poor oral hygiene, or a decreased level of consciousness.
Pneumonia occurs in both sexes and at all ages. More than 3 million cases of pneumonia occur annually in the United States. The infection carries a good prognosis for patients with normal lungs and adequate immune systems. In debilitated patients, however, bacterial pneumonia ranks as the leading cause of death. Pneumonia is also the leading cause of death from infectious disease in the United States.
Causes and pathophysiology
In bacterial pneumonia, which can occur in any part of the lungs, an infection initially triggers alveolar inflammation and edema. Capillaries become engorged with blood, causing stasis. As the alveolocapillary membrane breaks down, alveoli fill with blood and exudate, resulting in atelectasis. In severe bacterial infections, the lungs assume a heavy, liverlike appearance, as in adult respiratory distress syndrome (ARDS).
Viral infection, which typically causes diffuse pneumonia, first attacks bronchiolar epithelial cells, causing interstitial inflammation and desquamation. It then spreads to the alveoli, which fill with blood and fluid. In advanced infection, a hyaline membrane may form. As with bacterial infection, severe viral infection may clinically resemble ARDS.
In aspiration pneumonia, aspiration of gastric juices or hydrocarbons triggers similar inflammatory changes and also inactivates surfactant over a large area. Decreased surfactant leads to alveolar collapse. Acidic gastric juices may directly damage the airways and alveoli. Particles with the aspirated gastric juices may obstruct the airways and reduce airflow, which, in turn, leads to secondary bacterial pneumonia.
Certain predisposing factors increase the risk of pneumonia. For bacterial and viral pneumonia, these include chronic illness and debilitation, cancer (particularly lung cancer), abdominal and thoracic surgery, atelectasis, common colds or other viral respiratory infections, chronic respiratory disease (chronic obstructive pulmonary disease, asthma, bronchiectasis, cystic fibrosis), influenza, smoking, malnutrition, alcoholism, sickle cell disease, tracheostomy, exposureto noxious gases, aspiration, and immunosuppressive therapy.
Signs and Symptoms
Symptoms of pneumonia caused by bacteria in otherwise healthy people younger than 65 usually come on suddenly. They often start during or after an upper respiratory infection, such as influenza or a cold, and may include:
Chest X-rays disclose infiltrates, confirming the diagnosis.
Sputum specimen for Gram stain and culture and sensitivity tests shows acute inflammatory cells.
White blood cell count indicates leukocytosis in bacterial pneumonia and a normal or low count in viral or mycoplasmal pneumonia. Blood cultures reflect bacteremia and help to determine the causative organism.
Arterial blood gas (ABG) levels vary depending on the severity of pneumonia and the underlying lung state. Bronchoscopy or transtracheal aspiration allows me collection of material for culture. Pleural fluid culture may also be obtained.
Pulse oximetry may show a reduced level of arterial oxygen saturation.
The patient needs antimicrobial therapy based on the causative agent. Therapy should be reevaluated early in the course of treatment.
Supportive measures include humidified oxygen therapy for hypoxia, bronchodilator therapy, antitussives, mechanical ventilation for respiratory failure, a high-calorie diet and adequate fluid intake, bed rest, and an analgesic to relieve pleuritic chest pain. A patient with severe pneumonia on mechanical ventilation may need positive end-expiratory pressure to maintain adequate oxygenation.
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