Pneumonia: Causes and Types

Pneumonia

• Inflammation of one or both lungs that affects the air sacs, or alveoli, which fill with fluid or pus.
• Leading infectious cause of death in children and adults in the United States.
• Affects people of all ages, with the most severe clinical manifestations in the very young, elderly, and chronically ill.
• Occurs throughout the year, with seasonal variations in the relative incidence of disease.
• Caused by various viral and bacterial pathogens.
• Classified as either community-acquired or hospital-acquired.

Hospital-acquired Pneumonia (HAP)

• Occurs after 48 or more hours of endotracheal intubation.
• Risk of developing HAP increases by 6 to 21 times after intubation.
• Strongest predisposing factor is mechanical ventilation or intubation.
• Often caused by gram-negative aerobic bacilli or Staphylococcus aureus.
• Patients with longer hospital admissions or IV antibiotic use within 90 days preceding HAP are more likely to have multidrug-resistant (MDR) organisms.
• Mortality rate can be as high as 50%.

Ventilator-associated Pneumonia (VAP)

• A subtype of hospital-acquired pneumonia.
• Exacerbated by acid-suppressing drugs like H2-receptor blocking agents and proton pump inhibitors.
• More commonly caused by Staphylococcus aureus (20-30%) with a higher incidence of multidrug resistance.

Pathogenesis

• In healthy individuals, the respiratory host defenses effectively remove respiratory pathogens before infection occurs.
• These defense mechanisms include both innate and adaptive immunity pathways.
• Immunocompromised individuals lack robust defense mechanisms and are more susceptible to pneumonia, such as those with cystic fibrosis or prolonged neutropenia.
• Lung infections can suppress antibacterial activity by impairing alveolar macrophage function and mucociliary clearance, making the lung more vulnerable to secondary bacterial pneumonia.
• Any alteration of the normal lung microbiome by infection or disease can evolve to pneumonia requiring antimicrobial treatment.

Community Acquired Pneumonia (CAP)

• Majority of cases caused by Streptococcus pneumoniae (20-30%)
• Community-acquired or hospital acquired
• Can be caused by bacteria, atypical organisms, or viruses

Pathogenesis

• Respiratory defenses include both innate and adaptive immunity
• Defenses are preserved in healthy individuals
• Immunocompromised people have weakened defenses (e.g., cystic fibrosis, prolonged neutropenia)
• Lung infections can suppress antibacterial activity by:
  • Impairing alveolar macrophage function
  • Decreasing mucociliary clearance

Pathophysiology

• CAP: Onset is outside the hospital or within 48 hours of hospital admission
• Local host defenses of the lower respiratory tract:
  • Mucus lining
  • Cilia
  • Antibodies (IgA, IgM, IgG)
  • Complement
  • Alveolar macrophages
• Aspiration can lead to aspiration pneumonia or chemical (acid) pneumonitis
• Systemic release of TNF-α and interleukins (ILs)-1 and -6 can lead to severe symptoms:
  • Hypotension
  • Organ dysfunction
  • Septic or septic-shock

Clinical Presentation

• Fever, chills, malaise (constitutional symptoms)
• Cough, increased sputum production, dyspnea (respiratory symptoms)
• Abrupt onset
• Rust-colored sputum or hemoptysis
• Pleuritic chest pain
• Dyspnea

Diagnostics

• Sputum Gram stain and culture recommended for hospitalized children who can produce a sample
• Blood cultures recommended for moderate/severe CAP
• Urinary antigen tests available for S. pneumoniae and L. pneumophila, recommended in adults with severe CAP
• High specificity (90-99%) but lower sensitivity (50-80%)
• Can detect pathogen antigen days to weeks after starting antibiotic therapy

Treatment

• Continued monitoring of patient's clinical status and diagnostic data
• Decision to continue, narrow, alter, or discontinue therapy based on data
• CURB-65 and CRB-65 are commonly used severity assessments

Risk Factors for CAP

• Previous hospitalization within the past month
• Use of broad-spectrum antibiotics in the past 7 days
• Severe underlying bronchopulmonary disease
• Malnutrition
• Chronic use of steroids >15 mg/day for at least 2 weeks

Empiric Therapy

• Based on likely pathogens
• No P. aeruginosa risk:
  • Ceftriaxone 2g IV q24h OR Ertapenem 1g IV q24h PLUS Azithromycin 500mg IV daily OR Levofloxacin 750mg IV daily
• Risk for P. aeruginosa:
  • Piperacillin-tazobactam OR Cefepime PLUS Azithromycin 500mg IV daily OR Levofloxacin 750mg IV daily
• High-dose amoxicillin (3 g/day) may be used for penicillin-intermediate strains (MIC = 4 mg/L)

MRSA Coverage for Hospitalized Patients with Severe CAP

• Consider MRSA coverage in the following cases:
  • Requirement for intensive care unit (ICU) admission
  • MRSA prevalence of 20% or greater in the hospital or unit

Specific MRSA Regimen

• Vancomycin or Linezolid are the preferred treatments
• Alternatives include:
  • Quinupristin-dalfopristin
  • Ceftaroline
  • Sulfamethoxazole-trimethoprim
  • Clindamycin

Gram-Negative Pathogens

• Haemophilus influenzae is the most common gram-negative cause of HAP.

Hospital Acquired Pneumonia (HAP)

• Most HAP cases are caused by gram-negative bacilli:
  • P. aeruginosa
  • Enterobacteriaceae
  • S. aureus
• Empiric regimens should include at least one antibiotic with coverage against these pathogens:
  • An antipseudomonal, antistaphylococcal β-lactam (e.g., piperacillin/tazobactam or cefepime)
  • An antipseudomonal, antistaphylococcal fluoroquinolone (e.g., levofloxacin)
• Patients with MDR HAP risk factors (e.g., recent IV antibiotics, structural lung disease) should receive coverage for MDR gram-negative bacilli.

Methicillin-Susceptible Staphylococcus aureus (MSSA)

• Empiric therapy:
  • Nafcillin or oxacillin are the preferred agents
  • Dicloxacillin is preferred in the community setting
  • Cefazolin is an alternative
  • Clindamycin or vancomycin can also be used, but are not preferred

Directed Therapy for Important Gram-Negative Pathogens

• Haemophilus influenzae is the most common gram-negative cause of CAP.
• Targeted therapy based on culture and sensitivity testing.

This quiz covers the essential aspects of pneumonia, including its definition, causes, and classifications. Learn about pneumonia's impact on various age groups and the specifics of hospital-acquired pneumonia. Test your knowledge on this critical health issue affecting millions worldwide.