Pathophysiology
Clinical meaning
Pneumonia disrupts gas exchange by filling alveoli with inflammatory exudate (neutrophils, bacteria, fluid, fibrin), creating intrapulmonary shunt physiology. Ventilation-perfusion (V/Q) mismatch occurs when perfused alveoli are not adequately ventilated due to consolidation or atelectasis. The resulting hypoxemia is initially responsive to supplemental oxygen unless massive consolidation creates a true shunt (V/Q = 0). The A-a gradient (PAO2 - PaO2) widens in pneumonia (normal < 15 mmHg in young adults, increases with age). Community-acquired pneumonia (CAP) is most commonly caused by Streptococcus pneumoniae, followed by Haemophilus influenzae, Mycoplasma pneumoniae, and respiratory viruses. Hospital-acquired pneumonia (HAP) involves more resistant organisms including MRSA, Pseudomonas aeruginosa, and Klebsiella pneumoniae. The clinician must calculate severity scores (CRB-65 or PSI/PORT) to guide site-of-care decisions.