Pneumonia
An infection that inflames the air sacs in one or both lungs, which may fill with fluid or pus, causing cough, fever, and difficulty breathing.
Emergency Management: Severe hypoxemic respiratory failure (PaO2 < 60 mmHg despite supplemental oxygen) or hypercapnia with acidosis requiring emergent endotracheal intubation and transfer to the ICU for mechanical ventilation.
Pneumonia is an acute infection of the pulmonary parenchyma characterized by inflammation and consolidation of the alveolar spaces. It can be caused by bacteria, viruses, or fungi.
Detailed Overview
When microorganisms evade upper airway defense mechanisms and reach the alveoli, they trigger a massive inflammatory cascade. The resulting influx of neutrophils and exudate fills the alveoli, compromising gas exchange and leading to hypoxemia. Pneumonia is categorized by acquisition setting: Community-Acquired (CAP), Hospital-Acquired (HAP), and Ventilator-Associated (VAP), which dictate the likely pathogens and empiric antibiotic choices. Despite modern antibiotics, it remains a leading cause of infectious death globally.
Epidemiology & Demographics
CAP affects >5 million adults annually in the US. The mortality rate for outpatient CAP is <1%, but jumps to 10-12% for hospitalized patients and >30% for ICU admissions. Highest risk at age <2 or >65.
Etiological Mechanism
CAP: Streptococcus pneumoniae (most common typical), Mycoplasma pneumoniae, Haemophilus influenzae, Respiratory Syncytial Virus, Influenza, SARS-CoV-2. HAP/VAP: Pseudomonas aeruginosa, MRSA, Klebsiella pneumoniae.
Primary Causes
Aspiration of oropharyngeal flora, inhalation of aerosolized infectious droplets, or hematogenous spread from an extrapulmonary source.
Pathogens reach the lower respiratory tract via microaspiration. Alveolar macrophages fail to clear the inoculum, initiating an intense immune response. Cytokines (TNF-alpha, IL-8) recruit neutrophils to the alveoli. The classical stages of lobar pneumonia include: 1) Congestion (days 1-2): prominent vascular engorgement and intra-alveolar fluid. 2) Red hepatization (days 3-4): massive exudation with RBCs, neutrophils, and fibrin filling alveoli (lung looks liver-like). 3) Gray hepatization (days 5-7): RBCs disintegrate, but fibrinosuppurative exudate remains. 4) Resolution (day 8+): enzymatic digestion of the exudate by macrophages. The consolidated alveoli create right-to-left physiologic shunting (V/Q mismatch), leading to hypoxemia.
Diagnostic Criteria & Guidelines
Clinical signs of acute infection (fever, cough) combined with the presence of a new pulmonary infiltrate on chest imaging.
Outpatient CAP (no comorbidities): Amoxicillin 1g PO TID OR Doxycycline 100mg PO BID for 5-7 days. Outpatient CAP (with comorbidities): Amoxicillin/Clavulanate 875/125mg PO BID PLUS Azithromycin 500mg PO day 1, then 250mg days 2-5. Inpatient Ward CAP: Ceftriaxone 1g IV daily PLUS Azithromycin 500mg IV daily.
Second-Line & Adjunctive Therapy
For penicillin-allergic patients or refractory CAP: Respiratory fluoroquinolone (e.g., Levofloxacin 750mg PO/IV daily) for 5-7 days. For HAP/VAP with risk of Pseudomonas/MRSA: Cefepime 2g IV q8h PLUS Vancomycin (trough target 15-20 mcg/mL).
Surgical & Procedural Management
Generally not indicated, unless there is a complication such as a loculated empyema requiring VATS decortication or a large lung abscess failing medical therapy.
Patient Counseling & Advice
Complete the entire course of prescribed antibiotics even if feeling better in 48 hours. Explain that while the fever should break within 3 days, fatigue and a mild cough can persist for up to a month.
Follow-Up & Monitoring Schedule
Re-evaluate clinically in 48-72 hours if no improvement. A follow-up Chest X-ray in 6-8 weeks is recommended for patients >50 years or smokers to ensure resolution and rule out an underlying lung malignancy that was masked by the infection.
Preventive Strategies
Pneumococcal vaccines (PCV15 followed by PPSV23, or PCV20 alone) for adults >65 or those with comorbidities. Annual influenza and updated COVID-19 vaccinations.
Most young, healthy individuals recover completely in 1-2 weeks. Mortality for admitted patients is significant (10%), mostly driven by age, comorbidities, and progression to sepsis.