Pneumonia is any disease that causes inflammation of the lung tissue, particularly the air sacs (alveoli) of the lung. It is a fairly common respiratory disease that can be life threatening. Most cases of pneumonia are due to infectious causes, with bacterial pneumonia being the most common, followed by viral and fungal pneumonia. However, other non-infectious causes of pneumonia like mechanical or chemical injury can be equally devastating if left untreated.
Pneumonia is commonly seen after respiratory tract infections, especially in older patients or young children. People with chronic debilitating conditions and systemic infections like HIV are at risk of acquiring pneumonia, often with life threatening consequences.
In pneumonia, a lobe, segment or the entire lung may be affected. It results in significant damage to the epithelial lining of the air sacs (alveoli) known as diffuse alveolar damage (DAD). This affects gas exchange and eventually lead to death unless the appropriate treatment is commenced.
Sometimes only a small part of the lung tissue is affected but there is significant inflammation and damage of the distal parts of the respiratory tract. This is known as bronchopneumonia and is often a complication acute bronchitis, tracheitis or influenza.
Interstitial pneumonia is a type of pneumonia where the septa that separates air sacs (alveoli) is affected rather than the alveolar lining itself. It is often triggered by infections, chemicals and other factors yet the cause seems to be associated with immune disorders and impaired healing mechanisms. In many cases of interstitial pneumonia, there is no known trigger – idiopathic.
- Infectious, non-infectious, chemical
- Lobar, bronchopneumonia, interstitial
- Community acquired (CAP), health care-acquired (HCAP), hospital-acquired (HAP), ventilator-acquired (VAP), nursing home-acquired (NHAP)
- Bacterial, viral, fungal, atypical
How does pneumonia occur?
Most cases of pneumonia are due to infectious causes i.e. pathogenic microorganisms, particularly bacteria. The pathogen usually reaches the lung tissue through the respiratory tract when it is inhaled. However, some cases of pneumonia are due to pathogens that reach the lung via the bloodstream or an infection of a neighboring site.
The pathogen causes injury of the alveolar epithelial cells and capillaries of the alveolar septa. The microvascular injury increases vessel permeability thereby allowing proteins, blood cells and fluid from the blood to empty into the interstitial spaces causing alveolar edema (swelling). This then leaks into the alveoli.
As the infection spreads to neighboring alveoli, the same process occurs resulting in the entire lung or a part of it becoming congested with cells, fluid and cellular debris (microorganism, blood cells and lung parenchymal cells). Once large areas of the lung is congested in this manner, it is said to be consolidated.
During the course of pneumonia, the lung’s gas exchange function is impaired. The edema and exudate reduces the surface area of he lung and hampers gas exchange between the air and blood in the lung, although there is normal blood flow to the lung. This causes a drop in oxygen within the blood (hypoxemia) with a simultaneous elevation in blood carbon dioxide (hypercapnia). Although the healthy lung will still be functioning as normal, the combined oxygen saturation is still below normal.
If left untreated or if the body is unable to cope with the disease process, the condition will spread to affect an entire lung or even both lungs. Inability to respond to treatment may lead to death. However, with proper treatment and in healthy patients, the consolidated exudate is essentially ‘cleaned’ out of the lung. This may occur by digestion and resorption, consumption by macrophages, coughing up the exudate or it undergoes fibrosis.
- Bacterial Pneumonia. Emedicine
Article reviewed by Dr. Greg. Last updated on September 30, 2010