Pneumonia

Pneumonia is infection of the lung parenchyma, which causes inflammation and fluid accumulation in the alveoli. Infection occurs when impaired local or systemic defenses allow the infiltration and proliferation of bacterial, viral, or fungal pathogens. – In some patients, multiple pathogens are present, and bacterial pneumonia secondary to viral infection is common. Pneumonia is the number one infectious cause of death; it can lead to acute respiratory distress syndrome and/or sepsis. Pneumonia is significant cause of childhood illness: – In children under 5 years old, Respiratory Syncytial Virus is the most common cause. – In children older than 5, bacteria, including Streptococcus pneumoniae, Mycoplasma pneumoniae, and Chlamydia pneumoniae. Pneumonia symptoms are often classified as "typical" vs "atypical," and are sometimes associated with specific pathogen types; however, overlap and inconsistencies make clear-cut categories difficult. "Typical" pneumonia is characterized by acute onset of fever, malaise, productive or non-productive cough, dyspnea, and chest pain. "Atypical" pneumonia is sometimes referred to as "walking" pneumonia because symptoms are milder and onset is gradual; for example, fever is absent and respiratory symptoms are less severe. Pulmonary crackles, egophony ("EEE" sounds like "AAA" during auscultation), bronchial breath sounds; Tachycardia and tachypnea are also common. Be aware that pneumonia may present differently by age; for example, elderly patients with pneumonia may experience confusion, whereas gastrointestinal symptoms are more likely in children. On chest x-rays we'll see consolidations from edema. We can test for the presence of microbial pathogens (however, be aware that causative pathogens are often not identified in pneumonia patients). 1st Step: Failure of local or systemic defense mechanisms. – Examples:

• Inhibition of microbial phagocytosis by resident alveolar macrophages, which allows invasive pathogens to proliferate.

Macrophage inhibition can be caused by both alcohol and cigarette smoke, among other things.

• Systemic immune deficiencies associated with HIV, cancer, immunosuppressant drugs, etc. are also vulnerable to pneumonia.
• Local mechanical failures allow accumulation of pathogens and fluids. For example, repressed cough reflex, which can be due to neuromuscular disorders, coma, drugs, etc.
• Another example of mechanical failure is mucociliary impairment in the tracheobronchial tree, which can be caused by cigarette smoking, genetic defects, viral infection, etc.

2nd Step: Defense system failures allow pathogens to infiltrate the lungs and proliferate. 3rd Step: This triggers the immune system. 4th Step: Infiltration of neutrophils and other pro-inflammatory mediators produces alveolar and interstitial inflammation. 5th Step: If inflammation goes unchecked, damage produces leaky alveolar walls and pulmonary capillaries. 6th Step: As a result, the alveoli fill with fluid, pro-inflammatory molecules, blood cells, and pathogens, and gas exchange is impaired (notice that we see a similar pathophysiologic process in acute respiratory distress syndrome). 7th Step: Gas exchange is impaired. Pneumonia types can be described and categorized by histopathological patterns, acquired settings, and pathogenic causes. Lobar pneumonia is characterized by continuous inflammation in a lobe, lobes, or the entire lung. Four key stages of bacterial lobar pneumonia: 1. Congestion: the lungs are heavy, red, and "baggy" from engorged vessels and edema, with blood and bacteria in the alveoli. 2. Red hepatization: lungs are pink with red blood cells, neutrophils, and fibrin – they are dry and firm, like the liver (hence, "hepatization"). 3. Gray hepatization: Results from lysed red blood cells and fibrinopurulent exudate. 4. Resolution phase occurs when cellular debris and fibrin are removed from the lung tissue; alveolar macrophages predominate. Bronchopneumonia is characterized by infection that begins in the bronchi and spreads as patchy, bilateral inflammation. Interstitial pneumonia Next, let's learn about pneumonia types based on pathogen acquisition: Community Acquired, Hospital Acquired, and Ventilator Acquired. We'll list the pathogens most commonly associated with settings, which can be helpful in determining treatment course (see the links in our notes for pathogen details). Be aware that the category "Health Care Associated Pneumonia" was previously used to describe non-hospitalized patients who had significant exposure to health care settings and were therefore believed to be at increased risk of pneumonia from multi-drug-resistant organisms; however, we now know that these patients are not at higher risk of MDR pathogens, and this category is no longer used. Community-acquired pneumonia is pneumonia acquired in any non-hospital setting, including assisted living and rehabilitation centers. Typical bacterial pneumonia pathogens: Streptococcal pneumoniae, which accounts for approximately 15% of pneumonia cases in the U.S. and 30% worldwide; indicate that the pneumococcal vaccine protects against this form of pneumonia. Haemophilus influenzae is another common bacterial culprit. Methicillin-Resistant Staphylococcus aureus in an uncommon source of community acquired pneumonia, but produces severe infection. Atypical bacterial pneumonia pathogens: Mycoplasma pneumoniae, Legionella, and Chlamydia pneumoniae; indicate that these pathogens are often associated with interstitial pneumonia. "Atypical" bacteria are difficult to identify using standard bacterial identification methods (unlike the Gram positive/Gram negative typical bacteria). Fungal causes of pneumonia include: Histoplasma capsulatum and Coccidioides immitis. Viral pneumonia is increasingly common, even in immunocompetent individuals. Influenza A and B, Rhinoviruses, Parainfluenza viruses, Adenoviruses, and Respiratory Syncytial Virus. Recall that RSV is a common cause of pneumonia in children under 5 years old. Hospital-acquired pneumonia is defined as pneumonia acquired 48 hours or more after in-patient admission, and includes post-operative infection. Common causes: Methicillin-sensitive and methicillin-resistant Staphylococcus aureus Streptococcus pneumoniae, Pseudomonas aeruginosa, and several enteric bacteria, including Klebsiella species, E. coli, and Enterobacter species. Recall that Pseudomonas aeruginosa is difficult to treat due to increasing antibiotic resistance. Ventilator-associated pneumonia is defined as pneumonia acquired 48 hours or more after endotracheal intubation; common culprits include Pseudomonas aeruginosa and Methicillin-sensitive and methicillin-resistant Staphylococcus aureus. Aspiration pneumonia is the result of aspirated gastric secretions or fluids from the upper respiratory tract. Common culprits include those associated with community-acquired and hospital-acquired pneumonia; oral anaerobes can also cause pneumonia. Immunocompromised patients with pneumonia often have the "atypical" presentation. Common pathogens include community-acquired pathogens and the following opportunistic pathogens: Pneumocystis jirovecii Cytomegalovirus Aspergillus Candida Cryptococcus.