Bronchiolitis

Bronchiolitis is an acute lower respiratory tract infection primarily affecting infants under 2 years of age, with peak incidence at 2-6 months. Respiratory syncytial virus (RSV) accounts for 50-80% of cases, with other causative agents including rhinovirus, parainfluenza, human metapneumovirus, and adenovirus. RSV is a single-stranded RNA paramyxovirus that infects the bronchiolar epithelial cells, initiating viral replication within the cytoplasm. The infected epithelial cells undergo necrosis and slough into the bronchiolar lumen, triggering an inflammatory cascade with neutrophilic infiltration, submucosal edema, and increased mucus secretion. The combination of desquamated epithelial debris, inflammatory exudate, and excessive mucus creates intraluminal plugs that obstruct the small airways. Because infant bronchioles are already narrow (approximately 1 mm in diameter), even minor mucosal edema causes disproportionate increases in airway resistance (resistance is inversely proportional to the fourth power of the radius per Poiseuille's law). This results in a ball-valve mechanism: air enters the bronchiole during inspiration (when negative intrathoracic pressure distends the airway) but cannot exit during expiration (when positive pressure compresses the already narrowed airway), leading to progressive air trapping, hyperinflation, and ventilation-perfusion (V/Q)...