COMMUNITY ACQUIRED PNEUMONIA

Causative Organisms of Pneumonia

The most common causes of CAP include:

Bacterial Causes:

• Streptococcus pneumoniae: The most common bacterial cause of CAP.
• Haemophilus influenzae: Often seen in patients with underlying lung diseases like COPD.
• Moraxella catarrhalis: Also common in patients with COPD.
• Staphylococcus aureus: Including Methicillin-resistant Staphylococcus aureus (MRSA), particularly post-influenza.
• Atypical bacteria:
  • Mycoplasma pneumoniae: Known to cause “walking pneumonia” in younger patients.
  • Legionella pneumophila: Causes Legionnaires’ disease, associated with contaminated water sources.
  • Chlamydophila pneumoniae: Often causes mild pneumonia.
• Klebsiella pneumoniae: Common in alcoholics and patients with diabetes.

Viral Causes:

• Influenza A and B viruses
• Respiratory Syncytial Virus (RSV)
• Human metapneumovirus
• Adenovirus
• Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2, responsible for COVID-19)

Fungal Causes (less common but can occur in immunocompromised patients):

• Histoplasma capsulatum: Associated with soil rich in bird or bat droppings.
• Coccidioides immitis: Found in desert regions of the southwestern U.S.
• Cryptococcus neoformans: Primarily affects immunocompromised patients.

Risk Factors for Pneumonia

1. Age

• Elderly individuals (≥65 years): Reduced immune function and presence of chronic conditions increase the risk.
• Infants and young children (<2 years): Immature immune systems make them more susceptible to infections.

2. Chronic Medical Conditions

• Chronic Obstructive Pulmonary Disease (COPD): Lung damage and reduced clearance of pathogens increase susceptibility.
• Asthma: Increased inflammation of airways can predispose patients to respiratory infections.
• Heart disease (e.g., congestive heart failure): Poor circulation and weakened lung function increase pneumonia risk.
• Diabetes mellitus: Impaired immune responses increase the likelihood of infections.
• Chronic liver disease: Liver dysfunction can impair immune responses.
• Chronic kidney disease: Patients undergoing dialysis or with reduced kidney function are at higher risk.
• Cerebrovascular disease (e.g., stroke): Increases the risk of aspiration pneumonia due to impaired swallowing.

3. Immunocompromised States

• HIV/AIDS: A weakened immune system predisposes patients to opportunistic infections (e.g., Pneumocystis jirovecii).
• Cancer patients: Chemotherapy and radiation therapy weaken the immune system.
• Organ transplant recipients: Use of immunosuppressive medications increases the risk of infections.
• Autoimmune diseases: Immunosuppressive therapy for diseases like lupus or rheumatoid arthritis heightens susceptibility.

4. Lifestyle Factors

• Smoking: Tobacco smoke damages the lungs and impairs the immune response, increasing vulnerability to bacterial and viral infections.
• Alcohol abuse: Excessive alcohol intake suppresses the immune system, impairs the cough reflex, and increases the risk of aspiration.
• Malnutrition: Poor nutritional status weakens the immune system and reduces the body’s ability to fight infections.

5. Environmental and Occupational Exposures

• Exposure to pollutants: Air pollution, exposure to toxic chemicals, and occupational exposure (e.g., in industrial settings) can damage the lungs and increase the risk of pneumonia.
• Living in crowded or institutional settings: Nursing homes, shelters, or prisons increase exposure to infectious pathogens.

6. Hospitalization or Recent Surgery

• Recent hospitalization: Increases the risk of hospital-acquired pneumonia (HAP), particularly if mechanical ventilation or invasive devices are used.
• Mechanical ventilation: Intubation or use of ventilators can introduce pathogens into the lungs, increasing the risk of ventilator-associated pneumonia (VAP).
• Surgery: Especially involving the thoracic or abdominal region, this can impair breathing and promote infection.

7. Aspiration Risk

• Dysphagia: Difficulty swallowing, which may occur in stroke, dementia, or neurological conditions, can lead to aspiration pneumonia.
• Gastroesophageal reflux disease (GERD): Increases the risk of gastric contents entering the lungs.
• Reduced consciousness: Conditions such as alcohol intoxication, drug overdose, or head injury can impair protective airway reflexes, increasing the risk of aspiration.

8. Viral Respiratory Infections

• Influenza: Influenza infection can lead to secondary bacterial pneumonia, often caused by Streptococcus pneumoniae or Staphylococcus aureus.
• COVID-19: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can lead to viral pneumonia and increase the risk of secondary bacterial infections.

9. Poor Oral Hygiene

• Poor oral hygiene can lead to colonization of bacteria in the oropharynx, which can be aspirated into the lungs, particularly in vulnerable populations like the elderly or intubated patients.

10. Other Factors

• Reduced mobility: Bedridden or immobile patients are at higher risk due to decreased lung expansion and impaired clearance of respiratory secretions.
• Cold weather: Winter months are associated with higher incidences of pneumonia due to increased viral infections and people staying indoors, facilitating the spread of pathogens.

Investigations for Pneumonia

1. Clinical Evaluation

Before performing specific investigations, a thorough clinical history and physical examination are essential:

• History: Focus on symptoms such as cough, fever, sputum production, pleuritic chest pain, and dyspnea.
• Physical Examination: Findings such as crackles (rales), bronchial breath sounds, decreased breath sounds, dullness to percussion, or egophony may indicate pneumonia.

2. Laboratory Investigations

a. Complete Blood Count (CBC)

• Purpose: Assess for leukocytosis, which is commonly seen in bacterial pneumonia. Leukopenia may indicate a severe infection or sepsis.
• Findings: Elevated white blood cell (WBC) count with a left shift (increased neutrophils and band cells) in bacterial infections; normal or slightly elevated WBC count in viral infections.

b. C-Reactive Protein (CRP) and Procalcitonin

• Purpose: These are inflammatory markers used to assess the severity of infection and differentiate between bacterial and viral causes.
• Findings: Elevated CRP and procalcitonin levels are often seen in bacterial pneumonia and may help guide decisions on initiating or stopping antibiotic therapy.

c. Blood Cultures

• Purpose: Identify bacteremia and the causative pathogen, especially in severe or hospitalized patients.
• Indications: Recommended in patients with severe pneumonia, immunocompromised status, or suspected sepsis.
• Findings: Positive cultures indicate bacteremia and may help tailor antibiotic therapy.

d. Sputum Gram Stain and Culture

• Purpose: Identify the causative organism and guide antibiotic therapy.
• Indications: Useful in patients with productive cough and suspected bacterial pneumonia.
• Findings: Gram stain can offer a rapid clue to the causative organism (e.g., Gram-positive diplococci for Streptococcus pneumoniae, Gram-negative rods for Klebsiella or Pseudomonas).
• Limitations: Sample quality is important; samples with >25 squamous epithelial cells per low-power field are likely contaminated.

e. Legionella and Pneumococcal Urinary Antigen Tests

• Purpose: Detect antigens of Legionella pneumophila and Streptococcus pneumoniae in urine, which may not be detectable by sputum culture.
• Indications: Suspected atypical pneumonia (Legionella), especially in severe or immunocompromised patients.
• Findings: Positive tests are highly specific and can guide immediate targeted therapy.

f. Viral PCR (Polymerase Chain Reaction)

• Purpose: Detect viral pathogens such as influenza, respiratory syncytial virus (RSV), and SARS-CoV-2.
• Indications: Suspected viral pneumonia, especially in the setting of a respiratory virus outbreak or pandemic (e.g., COVID-19).
• Findings: Positive PCR confirms the presence of viral pneumonia.

g. Arterial Blood Gas (ABG)

• Purpose: Assess oxygenation and acid-base balance in patients with severe pneumonia.
• Indications: Useful in patients with respiratory distress, hypoxemia, or acidosis.
• Findings: Hypoxemia (low partial pressure of oxygen) and possible respiratory alkalosis or acidosis in severe cases.

3. Radiological Investigations

a. Chest X-ray (CXR)

• Purpose: The most commonly used imaging tool to confirm pneumonia and assess the extent of the infection.
• Findings:
  • Lobar pneumonia: A well-defined consolidation affecting a single lobe (commonly seen in Streptococcus pneumoniae infections).
  • Bronchopneumonia: Patchy, diffuse opacities affecting multiple lobes (associated with Staphylococcus aureus, Klebsiella, and Pseudomonas infections).
  • Interstitial pneumonia: Reticular, ground-glass opacities (often seen in viral or atypical pneumonia).
  • Cavitation: May indicate necrotizing pneumonia (Staphylococcus aureus, Klebsiella, Mycobacterium tuberculosis).

b. Computed Tomography (CT) of the Chest

• Purpose: Provides more detailed imaging and is used when there is diagnostic uncertainty or to assess complications like abscesses, empyema, or pulmonary embolism.
• Indications: Severe or complicated pneumonia, or cases where chest X-ray findings are inconclusive.
• Findings: Better visualization of parenchymal abnormalities such as consolidations, interstitial patterns, cavities, and pleural effusions.

4. Additional Diagnostic Procedures

a. Pleural Fluid Analysis (Thoracentesis)

• Purpose: Analyze pleural fluid in patients with pneumonia-associated pleural effusion to diagnose empyema or parapneumonic effusion.
• Indications: Pleural effusion on imaging or clinical suspicion of empyema.
• Findings: High white blood cell count, low glucose, low pH, and positive Gram stain or culture suggest empyema.

b. Bronchoscopy with Bronchoalveolar Lavage (BAL)

• Purpose: Used to obtain lower respiratory tract samples in intubated patients or those with immunosuppression, when sputum samples are difficult to obtain or non-diagnostic.
• Indications: Severe pneumonia, suspicion of atypical or fungal infections, or failure to respond to empiric therapy.
• Findings: Can help identify pathogens and tailor antimicrobial treatment.

c. Pulse Oximetry

• Purpose: A non-invasive method to monitor oxygen saturation in patients with pneumonia.
• Indications: Useful in all cases of pneumonia to assess the need for oxygen therapy or hospitalization.
• Findings: Reduced oxygen saturation (<94%) suggests hypoxemia and may indicate the need for supplemental oxygen.

Management of Pneumonia

1. General Approach to Management

1.1. Assessment of Severity The severity of pneumonia is commonly assessed using scoring systems such as:

• CURB-65:

  • C: Confusion (new onset)
  • U: Urea >7 mmol/L
  • R: Respiratory rate ≥30 breaths/min
  • B: Blood pressure (systolic <90 mmHg or diastolic ≤60 mmHg)
  • 65: Age ≥65 years
  • Score: 0-1 (low risk, outpatient), 2 (moderate risk, consider hospitalization), 3+ (high risk, hospitalization/ICU care).

• PSI (Pneumonia Severity Index): A more comprehensive tool for assessing the need for hospitalization based on age, comorbidities, vital signs, and laboratory findings.

1.2. Site of Care Decisions

• Outpatient treatment: For mild pneumonia (CURB-65 score of 0-1 or PSI class I-II), without significant comorbidities or complications.
• Hospitalization: Moderate to severe cases (CURB-65 score of 2 or PSI class III-V), or when comorbid conditions or social circumstances preclude outpatient care.
• Intensive care unit (ICU): For patients with severe pneumonia (CURB-65 score ≥3) or features of sepsis and respiratory failure.

1.3. Supportive Care

• Oxygen therapy: For patients with hypoxemia (SpO2 <90% or PaO2 <60 mmHg).
• Intravenous fluids: To ensure hydration in hospitalized patients.
• Mechanical ventilation: For patients with respiratory failure or acute respiratory distress syndrome (ARDS).
• Antipyretics: For fever management (e.g., acetaminophen).

2. Antibiotic Therapy

The choice of antibiotics depends on the type of pneumonia (community-acquired, hospital-acquired, or ventilator-associated), severity, and local resistance patterns. The following tables summarize the recommended antibiotics for different types of pneumonia.

Table 1: Antibiotics for Community-Acquired Pneumonia (CAP)

Patient Type	First-Line Antibiotics	Alternative/Second-Line Antibiotics
Outpatient (no comorbidities)	– Amoxicillin (1 g TID)	– Doxycycline (100 mg BID)
	– Doxycycline (100 mg BID)	– Azithromycin (500 mg day 1, then 250 mg/day for 4 days)
Outpatient (with comorbidities)	– Amoxicillin-clavulanate (875 mg/125 mg BID) + Azithromycin or Doxycycline	– Levofloxacin (750 mg daily) or Moxifloxacin (400 mg daily)
	– Cefuroxime (500 mg BID) + Azithromycin
Inpatient (non-severe)	– Ceftriaxone (1-2 g daily) + Azithromycin (500 mg daily)	– Levofloxacin or Moxifloxacin
Inpatient (severe, ICU)	– Ceftriaxone + Azithromycin	– Levofloxacin + Ceftriaxone or Piperacillin-tazobactam

Note: Macrolides (e.g., azithromycin) and doxycycline are effective against atypical pathogens like Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella.

3. Duration of Antibiotic Therapy

• CAP: Typically 5-7 days for outpatient therapy, 7-10 days for hospitalized patients

4. Antibiotic De-escalation

Once culture results are available, the antibiotic regimen should be narrowed (de-escalation) to target the specific pathogen. This helps prevent antibiotic resistance and minimizes side effects.

5. Management of Complications

• Pleural effusion/empyema: If pleural fluid is present, thoracentesis or chest tube drainage may be required.
• Sepsis: Management according to sepsis protocols, including aggressive fluid resuscitation and vasopressors if necessary.
• Acute respiratory distress syndrome (ARDS): Mechanical ventilation with low tidal volume ventilation and positive end-expiratory pressure (PEEP) may be necessary.

Bibliography 

Books

1. Mandell, L. A., Bennett, J. E., & Dolin, R. (2019). Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (9th ed.). Elsevier. This textbook provides a detailed overview of the etiology, pathophysiology, and management of pneumonia.

2. Cunha, B. A. (Ed.). (2010). Pneumonia Essentials. Physicians’ Press. A concise guide focusing on the clinical aspects and treatment of pneumonia in various patient populations.

3. Murray, J. F., & Nadel, J. A. (2016). Textbook of Respiratory Medicine (6th ed.). Elsevier Saunders. A comprehensive reference covering the respiratory system, including an in-depth section on pneumonia.

Journal Articles

4. Metlay, J. P., Waterer, G. W., Long, A. C., et al. (2019). “Diagnosis and Treatment of Adults with Community-Acquired Pneumonia: An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America.” American Journal of Respiratory and Critical Care Medicine, 200(7), e45-e67. DOI: 10.1164/rccm.201908-1581ST. This guideline provides evidence-based recommendations for the diagnosis and treatment of community-acquired pneumonia (CAP).

5. Torres, A., Niederman, M. S., Chastre, J., et al. (2017). “International ERS/ESICM/ESCMID/ALAT Guidelines for the Management of Hospital-Acquired Pneumonia and Ventilator-Associated Pneumonia.” European Respiratory Journal, 50(3), 1700582. DOI: 10.1183/13993003.00582-2017. A comprehensive set of guidelines addressing the management of hospital-acquired and ventilator-associated pneumonia.

6. Jain, S., Self, W. H., Wunderink, R. G., et al. (2015). “Community-Acquired Pneumonia Requiring Hospitalization among U.S. Adults.” New England Journal of Medicine, 373(5), 415-427. DOI: 10.1056/NEJMoa1500245. This study provides a detailed analysis of the incidence and etiology of pneumonia requiring hospitalization in the United States.

Guidelines and Reports

7. National Institute for Health and Care Excellence (NICE). (2019). “Pneumonia (community-acquired): antimicrobial prescribing.” Available at: https://www.nice.org.uk/guidance/ng138. A set of guidelines from NICE focused on the antimicrobial management of community-acquired pneumonia.

8. World Health Organization (WHO). (2014). “Revised WHO Classification and Treatment of Pneumonia in Children at Health Facilities: Evidence Summaries.” Available at: https://www.who.int/maternal_child_adolescent/documents/child-pneumonia-treatment/en/. This document outlines WHO recommendations for the treatment of pneumonia in children.

Online Resources

9. UpToDate. “Overview of Community-Acquired Pneumonia in Adults.” Available at: https://www.uptodate.com/contents/community-acquired-pneumonia-in-adults. A comprehensive resource for clinicians that covers the etiology, diagnosis, and treatment of pneumonia with regularly updated information.

10. Centers for Disease Control and Prevention (CDC). “Pneumonia.” Available at: https://www.cdc.gov/pneumonia/index.html. A reliable source of information for clinicians and the public on pneumonia, including prevention and treatment strategies.

11. American Thoracic Society. “Pneumonia and Respiratory Infections.” Available at: https://www.thoracic.org/patients/patient-resources/resources/pneumonia.pdf. An informative guide for patients and healthcare providers on understanding and managing pneumonia.