Pathophysiology of community acquired pneumonia.

Introduction C exposure to contaminated air and frequent aspiration of nasopharyngeal flora make lung parenchyma susceptible to virulent micro-organisms. Most microorganisms reach lower respiratory tract as inhaled and contaminated micro droplets. Complex interactions between virulence and quantum of aspirated or inhaled microorganisms, that arrive at lower respiratory tract, integrity of defence barriers and host immunity status, decide occurrence of pneumonia.1,2 Particles with diameter more than 100 μm precipitate easily and are not inhaled. Particles larger than 10 μm get trapped in nasal secretions. Most particles increase in size due to humidification in trachea and are trapped in major bronchi.3 Particles with diameter less than 5 μm reach the alveoli. Such particles can transport a bacterial inoculum of upto 100 microorganisms depending on bacterial size. Although diameter of most bacteria is 1 μm or more, Mycoplasma, Chlamydophila, and Coxiella are 5 to 100 times smaller. Most Community Acquired Pneumonia (CAP) are bacterial in origin and often follow brief viral upper respiratory tract infection. In upright position lower lobes are best ventilated therefore deposition of inhaled micro organisms is higher in these lobes. Inhalation pneumonia is most often due to microorganisms (a) that can remain suspended in air so as to be transported far away, (b) survive long enough while in transit, (c) have a size less than 5 μm (d) carry a high inoculum, and (e) evade local host defence mechanisms. Infection by intracellular bacteria such as Mycoplasma pneumoniae, Chlamydophila and Coxiella burnetii occurs through contaminated aerosol inhalation route. CAP due to Streptococcus pneumoniae, Haemophilus and gram-negative bacilli occurs through micro aspiration. Some of the important pathophysiologic modes of spread of micro organisms are summarized in Table 1.