Pathophysiology
Clinical meaning
Antibiotic selection for respiratory infections requires understanding of the most likely pathogens based on clinical syndrome, patient factors, and local resistance patterns. CAP: S. pneumoniae (most common), H. influenzae, M. pneumoniae, C. pneumoniae, Legionella. AECOPD: H. influenzae, M. catarrhalis, S. pneumoniae. HAP/VAP: MRSA, P. aeruginosa, Klebsiella, Acinetobacter. The clinician must consider pharmacokinetic-pharmacodynamic (PK-PD) principles: beta-lactams are time-dependent (efficacy depends on time above MIC), aminoglycosides and fluoroquinolones are concentration-dependent (efficacy depends on Cmax/MIC ratio). Antibiotic stewardship principles include using the narrowest effective spectrum, shortest effective duration, de-escalation based on culture results, and avoiding unnecessary antibiotics for viral infections.