Pneumonia

Pneumonia is inflammation and consolidation of the lung parenchyma, most commonly caused by infection, in which the alveoli fill with inflammatory exudate and impair gas exchange. It is a leading infectious cause of illness and death worldwide and a central topic in respiratory pathology.

Definition

Pneumonia is an acute inflammation of the lung parenchyma, usually infectious, characterised by exudative filling of the alveolar spaces (consolidation) that interferes with gas exchange.

Scope

This entry describes pneumonia as a pathological process: the patterns of alveolar inflammation, the major aetiological categories, and the conventional classifications by anatomy and by setting of acquisition. It outlines mechanisms and epidemiology for reference and does not provide antibiotic selection, dosing, or individualised treatment guidance.

Core questions

• Which compartment and pattern of lung is involved: lobar consolidation, bronchopneumonia, or interstitial inflammation?
• What is the likely class of causative organism, and how does the setting of acquisition shape it?
• How does alveolar filling produce hypoxaemia and the clinical syndrome?
• How are community-acquired and hospital-acquired pneumonia distinguished pathologically and epidemiologically?

Key concepts

• Alveolar consolidation and exudate
• Lobar pneumonia versus bronchopneumonia
• Community-acquired versus hospital-acquired pneumonia
• Aspiration pneumonia
• Interstitial (atypical) pneumonia
• Causative organisms: bacterial, viral, fungal
• Ventilation-perfusion mismatch from filled alveoli

Mechanisms

Pathogens reach the lower airways by inhalation of aerosols, aspiration of oropharyngeal contents, or haematogenous spread, overwhelming local defences such as the mucociliary escalator and alveolar macrophages. The host inflammatory response floods alveoli with neutrophils, fluid, and fibrin, producing consolidation. Classically, lobar pneumonia fills a contiguous region of a lobe, bronchopneumonia produces patchy consolidation centred on airways, and atypical or interstitial pneumonia involves alveolar walls more than air spaces. Filled or inflamed alveoli are perfused but poorly ventilated, creating ventilation-perfusion mismatch and hypoxaemia. Aspiration introduces oropharyngeal flora and gastric contents into dependent lung regions, as Mandell and Niederman describe.

Clinical relevance

Recognising the pattern and likely causative category of pneumonia underlies how clinicians interpret imaging and laboratory findings, and severity-assessment tools help frame prognosis; professional guidelines such as the ATS/IDSA statement structure adult community-acquired pneumonia care. This entry is descriptive reference material and is not a substitute for guideline-based clinical decision-making.

Epidemiology

Lower respiratory infections, of which pneumonia is the dominant component, are consistently among the leading infectious causes of death globally in Global Burden of Disease analyses, with the heaviest burden at the extremes of age and in low-resource settings. Community-acquired pneumonia is a frequent cause of hospital admission in adults.

History

The lobar consolidation of pneumococcal pneumonia and its stages (congestion, red and grey hepatisation, resolution) were classic descriptions in pre-antibiotic pathology. The introduction of antimicrobials, vaccines against major pathogens, and molecular diagnostics progressively reshaped both the microbiology and outcomes of the disease.

Debates

How useful is classifying pneumonia by site of acquisition?

Categories such as community-acquired, hospital-acquired, and the former 'healthcare-associated' grouping aim to predict likely pathogens, but their boundaries and predictive value have been reassessed as microbiology and antimicrobial resistance evolve.

Seminal works

• prina-2015
• metlay-2019
• mandell-2019

Frequently asked questions

What is the difference between lobar pneumonia and bronchopneumonia?

Lobar pneumonia produces consolidation of a contiguous portion of a lobe, whereas bronchopneumonia produces patchy consolidation centred on the airways; both reflect alveolar filling with inflammatory exudate.

Why does pneumonia cause low blood oxygen?

Alveoli filled with inflammatory exudate are still perfused with blood but cannot be ventilated, creating ventilation-perfusion mismatch that lowers arterial oxygenation.