Pneumonia 

Background 

Pneumonia is an acute infection of the lung parenchyma characterized by inflammation of the alveoli and/or interstitium, resulting in consolidation, impaired gas exchange, and respiratory symptoms. It can be caused by bacteria, viruses, fungi, or parasites. Pneumonia presents clinically with cough, fever, dyspnea, and abnormal lung sounds and is a leading cause of morbidity and mortality worldwide, particularly in the very young, elderly, and immunocompromised. 

II) Classification/Types 

By Origin: 

• Community-acquired pneumonia (CAP): Acquired outside of hospitals or long-term care facilities. 

• Hospital-acquired pneumonia (HAP): Occurs ≥48 hours after hospital admission, not incubating at admission. 

• Ventilator-associated pneumonia (VAP): Develops ≥48 hours after endotracheal intubation. 

• Aspiration pneumonia: Due to inhalation of oropharyngeal or gastric contents into the lower respiratory tract. 

By Etiology: 

• Bacterial: Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, atypicals like Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella spp. 

• Viral: Influenza, RSV, adenovirus, SARS-CoV-2 

• Fungal: Histoplasma, Coccidioides, Pneumocystis jirovecii 

• Parasitic: Rare, typically in immunocompromised hosts 

By Radiographic Pattern: 

• Lobar pneumonia: Consolidation of a single lobe 

• Bronchopneumonia: Patchy infiltrates in multiple lobes 

• Interstitial pneumonia: Diffuse, reticular infiltrates 

Pathophysiology 

Pneumonia begins when pathogens bypass host defenses and reach the alveoli. The immune system responds with alveolar macrophages and neutrophils, leading to local inflammation. The alveoli fill with exudate, resulting in impaired oxygen exchange. Bacterial toxins and host inflammatory mediators contribute to systemic symptoms such as fever and leukocytosis. In severe cases, sepsis, respiratory failure, or abscess formation can occur. 

Epidemiology 

• Pneumonia is a leading cause of death worldwide, especially in children under 5 and adults over 65. 

• Incidence of CAP in the U.S. is ~5–7 cases per 1,000 adults annually. 

• HAP is the second most common nosocomial infection and a major cause of hospital mortality. 

• Streptococcus pneumoniae is the most common cause of CAP across all age groups. 

• Risk increases with age, smoking, chronic disease, and immunosuppression. 

Etiology 

I) Causes 

Bacterial: 

• Streptococcus pneumoniae 

• Haemophilus influenzae 

• Staphylococcus aureus 

• Pseudomonas aeruginosa 

• Mycoplasma pneumoniae 

• Chlamydophila pneumoniae 

• Legionella pneumophila 

Viral: 

• Influenza A/B 

• Respiratory syncytial virus (RSV) 

• Coronavirus (including SARS-CoV-2) 

• Parainfluenza 

• Adenovirus 

Fungal: 

• Pneumocystis jirovecii (HIV patients) 

• Aspergillus spp. 

• Histoplasma capsulatum 

Others: 

• Aspiration of gastric contents (anaerobic organisms) 

II) Risk Factors 

• Advanced age or infancy 

• Chronic lung disease (COPD, asthma) 

• Smoking or alcohol abuse 

• Immunocompromised states (HIV, chemotherapy, steroids) 

• Aspiration risk (neurologic disease, dysphagia) 

• Hospitalization or mechanical ventilation 

Clinical Presentation 

I) History (Symptoms) 

• Cough (productive or dry) 

• Fever, chills 

• Dyspnea or tachypnea 

• Pleuritic chest pain 

• Malaise or fatigue 

• Confusion or altered mental status in elderly 

II) Physical Exam (Signs) 

• Fever and tachycardia 

• Dullness to percussion over consolidated areas 

• Bronchial breath sounds or crackles 

• Egophony and increased tactile fremitus 

• Cyanosis in severe cases 

Differential Diagnosis (DDx) 

• Acute bronchitis 

• Pulmonary embolism 

• Congestive heart failure 

• Lung abscess 

• Tuberculosis 

• COVID-19 

• Acute respiratory distress syndrome (ARDS) 

Diagnostic Tests 

Initial Work-Up 

• Chest X-ray: Confirms diagnosis; look for lobar or patchy infiltrates 

• CBC: Leukocytosis with left shift 

• Blood cultures: Especially in hospitalized or septic patients 

• Sputum Gram stain and culture: Helps identify pathogen 

• Pulse oximetry/ABG: To assess oxygenation 

• CRP/Procalcitonin: Inflammatory markers that may help assess severity 

• COVID-19 and influenza PCR: In appropriate settings 

• Urinary antigen tests: For Legionella and Streptococcus pneumoniae 

• Bronchoscopy with lavage: In ventilated or immunocompromised patients 

Treatment 

I) Acute Management 

Outpatient CAP: 

• Healthy patient: Amoxicillin or doxycycline 

• With comorbidities: Amoxicillin-clavulanate + macrolide OR respiratory fluoroquinolone 

Inpatient (non-ICU): 

• IV ceftriaxone + azithromycin or doxycycline 

• OR monotherapy with respiratory fluoroquinolone (e.g., levofloxacin) 

ICU or severe CAP: 

• Ceftriaxone + azithromycin or levofloxacin ± MRSA/Pseudomonas coverage (vancomycin, piperacillin-tazobactam) 

Aspiration pneumonia: 

• Ampicillin-sulbactam or clindamycin 

Viral pneumonia: 

• Oseltamivir for influenza 

• Supportive care for most viruses 

II) Supportive Therapy 

• Oxygen supplementation 

• Antipyretics and analgesics 

• IV fluids and electrolyte correction 

• Mechanical ventilation if respiratory failure develops 

Medications 

Device Therapy 

• Supplemental oxygen: Nasal cannula or non-rebreather mask 

• Non-invasive ventilation: BiPAP/CPAP for moderate respiratory distress 

• Mechanical ventilation: In cases of respiratory failure or ARDS 

Patient Education, Screening, Vaccines 

• Emphasize medication adherence and full antibiotic course 

• Educate on smoking cessation and good hand hygiene 

• Pneumococcal vaccine (PCV15 or PCV20 + PPSV23) 

• Annual influenza vaccination 

• COVID-19 vaccination and boosters as indicated 

Consults/Referrals 

• Pulmonology: For severe or atypical cases 

• Infectious Disease: For immunocompromised hosts or unclear etiology 

• Critical Care: For ICU-level management 

• Speech Therapy: In aspiration risk patients 

Follow-Up 

Short-Term 

• Reassess 48–72 hours after initiation of antibiotics 

• Monitor for clinical improvement or progression 

• Adjust therapy based on culture results 

Long-Term 

• Follow-up chest imaging in 6–8 weeks if high risk for malignancy 

• Monitor for complications like effusion or abscess 

• Pulmonary rehabilitation if prolonged illness 

Prognosis 

• Most patients with CAP recover with appropriate therapy 

• Mortality <1% in healthy outpatients, but up to 20–30% in ICU patients with HAP/VAP 

• Poor prognostic factors: advanced age, comorbidities, hypoxia, bacteremia, or multilobar involvement 

• Timely antibiotics, vaccination, and addressing comorbidities improve outcomes