Pneumonia

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What is Pneumonia?

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Pneumonia is a disease of the lung. The lung is composed of large airways (beginning with the trachea and then two main bronchi), which branch out to form smaller airways (segmental bornchi and bronchioles) which finally lead to the alveoli - the small air sacs where gas exchange takes place.

Pneumonia represents an acute lower respiratory tract infection - it affects the lung parenchyma, alveoli and small airways. When the lung tissue becomes infected (whether it is bacteria, viruses, or fungi) there is inflammation of the lung tissue which leads to an exudate filling the alveoli and small airways - which impairs gas exchange.

Who gets Pneumonia?

Pneumonia is a common disease world-wide. 5 million children die of pneumonia each year. The incidence of community-acquired pneumonia is 10-15/1000 person years; the incidence of hospital admission is 1-3/1000 person years. Pneumonia is mostly more common during winter (although Legionella is more common during summer).

Predisposing Factors

Although pneumonia can occur in anyone (everybody can come into contact with an infective carrier), certain patients are at higher risk:

Patients with any chronic lung disease or a structurally abnormal lung are especially prone. In particular - patients that suffer from COPD or an airway-obstructing tumour are at high risk.
Patients that spend long periods of time in hospital are more likely to contract pneumonia, as are immuno-compromised patients.
People who cannot protect their own airway are at a very high risk of contracting aspiration pneumonia (pneumonia due to inhalation of gastric and oropharyngeal contents). This includes people with a lowered conscious state (coma or anaesthesia), oesophageal disease (achalasia or GORD), stroke.
Natural lung defences are impaired by smoking, alcohol, pulmonary oedema or congestion.

Progression
There are several ways of classifying pneumonia. One method is by the origin of the infective organism.
Community-acquired pneumonia is the most frequent type and may be primary or secondary to lung abnormality/disease.
The common organisms responsible are viruses (eg influenza), or certain bacteria.
The so-called "atypical pneumonias" are also classified as community-acquired. This subcategory includes slightly different species who tend to produce a more different clinical picture and are treated differently.
Nosocomial pneumonia is defined as pneumonia contracted more than 48 hours after admission to hospital and is due to more virulent organisms.
Aspiration pneumonia is commonly due to chemical damage and secondary infection by bacteria.

Complications of pneumonia include fibrosis, pleural effusion, lung abscess, empyema, septicaemia, septic shock or respiratory failure.

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