Pneumonia is an infection of the lung, and can be caused by nearly any class of organism known to cause human infections, including bacteria, viruses, fungi, and parasites. In the United States, pneumonia is the sixth most common disease leading to death, and the most common fatal infection acquired by already hospitalized patients. In developing countries, pneumonia ties with diarrhea as the most common cause of death.
The main function of the respiratory system is to provide oxygen, the most important energy source for the body's cells. Inspired air travels down the respiratory tree to the alveoli, where the oxygen moves out of the alveoli and is sent into circulation throughout the body as part of the red blood cells. The oxygen in the inspired air is exchanged within the alveoli for the body's waste product, carbon dioxide, which leaves the alveoli during expiration.
The normal, healthy human lung is sterile, meaning that there are no normally resident bacteria or viruses (unlike the upper respiratory system and parts of the gastrointestinal system, where bacteria dwell even in a healthy state). There are multiple safeguards along the path of the respiratory system that are designed to keep invading organisms from leading to infection.
The first line of defense includes the hair in the nostrils, which serves as a filter for larger particles. The epiglottis is a trap door of sorts, designed to prevent food and other swallowed substances from entering the larynx and then trachea. Sneezing and coughing, both provoked by the presence of irritants within the respiratory system, help to clear such irritants from the respiratory tract.
Mucous, produced throughout the respiratory system, also serves to trap dust and infectious organisms. Tiny hair-like projections (cilia) from cells lining the respiratory tract beat constantly, moving debris, trapped by mucus, upwards and out of the respiratory tract. This mechanism of protection is referred to as the mucociliary escalator.
Cells lining the respiratory tract produce several types of immune substances which protect against various organisms. Other cells (called macrophages) along the respiratory tract actually ingest and kill invading organisms.
The organisms that cause pneumonia, then, are usually carefully kept from entering the lungs by virtue of these host defenses. However, when an individual encounters a large number of organisms at once, either by inhaling contaminated air droplets, or by aspiration of organisms inhabiting the upper airways, the usual defenses may be overwhelmed and infection may occur.
In addition to exposure to sufficient quantities of causative organisms, certain conditions may predispose an individual to pneumonia. Certainly, the lack of normal anatomical structure could result in an increased risk of pneumonia. For example, there are certain inherited defects of cilia which result in less effective protection. Cigarette smoke, inhaled directly by a smoker or second-hand by an innocent bystander, interferes significantly with ciliary function, as well as inhibiting macrophage function.
Stroke, seizures, alcohol, and various drugs interfere with the function of the epiglottis, leading to a leaky seal on the trap door, with possible contamination by swallowed substances and/or regurgitated stomach contents. Alcohol and drugs also interfere with the normal cough reflex, further decreasing the chance of clearing unwanted debris from the respiratory tract.
Viruses may interfere with ciliary function, allowing themselves or other microorganism invaders, such as bacteria, access to the lower respiratory tract. One of the most important viruses which in recent years has resulted in a huge increase in the incidence of pneumonia is HIV (Human Immunodeficiency Virus), the causative virus in AIDS (Acquired Immunodeficiency Syndrome). Because AIDS results in a general decreased effectiveness of many aspects of the host's immune system, a patient with AIDS is susceptible to all types of pneumonia, including some previously rare parasitic types which would be unable to cause illness in an individual possessing a normal immune system.
The elderly have a less effective mucociliary escalator, as well as changes in their immune system, all of which cause them to be more at risk for the development of pneumonia.
Various chronic conditions predispose to pneumonia, including asthma, cystic fibrosis, neuromuscular diseases which may interfere with the seal of the epiglottis, and esophageal disorders which result in stomach contents passing upwards into the esophagus (increasing the risk of aspiration of those stomach contents with their resident bacteria), as well as diabetes, sickle cell anemia, lymphoma, leukemia, and emphysema.
Pneumonia is one of the most frequent infectious complications of all types of surgeries. Many drugs used during and after surgery may increase the risk of aspiration, impair the cough reflex, and cause a patient to underfill their lungs with air. Pain after surgery also discourages a patient from breathing deeply and coughing effectively.
The list of organisms which can cause pneumonia is very large, and includes nearly every class of infecting organism: viruses, bacteria, bacteria-like organisms, fungi, and parasites (including certain worms). Different organisms are more frequently encountered by different age groups. Furthermore, other characteristics of the host may place an individual at greater risk for infection by particular types of organisms.
Viruses, especially respiratory syncytial virus, parainfluenza and influenza viruses, and adenovirus, cause the majority of pneumonias in young children. Pneumonia in older children and young adults is often caused by the bacteria-like Mycoplasma pneumoniae. Adults are more frequently infected with bacteria (such as Streptococcus pneumoniae, Hemophilus inflenzae, and Staphylococcus aureus).
The parasite Pneumocystis carinii is an extremely important cause of pneumonia in patients with immune problems, such as patients being treated for cancer with chemotherapy, or patients with AIDS. People who have reason to come in contact with bird droppings, such as poultry workers, are at risk for pneumonia caused by the parasite Chlamydia psittaci. A very large, serious outbreak of pneumonia occurred in 1976, when many people attending an American Legion convention were infected by a previously unknown organism (subsequently named Legionella pneumophila) which was traced to air conditioning units in the convention hotel.
Pneumonia is suspected in any patient who presents with fever, cough, chest pain, shortness of breath, and increased respirations (number of breaths per minute). Fever with a shaking chill is even more suspicious, and many patients cough up clumps of mucus (sputum) that may appear streaked with pus or blood. Severe pneumonia results in the signs of oxygen deprivation, including blue appearance of the nail beds (cyanosis).
The invading organism causes symptoms, in part, by provoking an overly exuberant immune response in the lungs. The small blood vessels in the lungs (capillaries) become leaky, and protein-rich fluid seeps into the alveoli. This results in less functional area for oxygen-carbon dioxide exchange. The patient becomes relatively oxygen deprived, while retaining potentially damaging carbon dioxide. The patient breathes faster, in an effort to bring in more oxygen and blow off more carbon dioxide.
Mucus production is increased, and the leaky capillaries may tinge the mucus with blood. Mucus plugs actually further decrease the efficiency of gas exchange in the lung. The alveoli fill further with fluid and debris from the large number of white blood cells being produced to fight the infection.
Consolidation, a feature of bacterial pneumonias, occurs when the alveoli, which are normally hollow air spaces within the lung, instead become solid, due to quantities of fluid and debris.
Viral pneumonias and mycoplasma pneumonias do not result in consolidation. These types of pneumonia primarily infect the walls of the alveoli and the parenchyma of the lung.
Diagnosis is for the most part based on the patient's report of symptoms, combined with examination of the chest. Listening with a stethoscope will reveal abnormal sounds, and tapping on the patient's back (which should yield a resonant sound due to air filling the alveoli) may instead yield a dull thump if the alveoli are filled with fluid and debris.
Laboratory diagnosis can be made of some bacterial pneumonias by staining sputum with special chemicals and looking at it under a microscope. Identification of the specific type of bacteria may require culturing the sputum (using the sputum sample to grow greater numbers of the bacteria in a lab dish).
X-ray examination of the chest may reveal certain abnormal changes associated with pneumonia. Localized shadows obscuring areas of the lung may indicate a bacterial pneumonia, while streaky or patchy appearing changes in the x-ray picture may indicate viral or mycoplasma pneumonia. These changes on x-ray, however, are known to lag in time behind the patient's actual symptoms.
Antibiotics, especially given early in the course of the disease, are very effective against bacterial causes of pneumonia. Erythromycin and tetracycline improve recovery time for symptoms of mycoplasma pneumonia, but do not eradicate the organisms. Amantadine and acyclovir may be helpful against certain viral pneumonias.
Because many bacterial pneumonias occur in patients who are first infected with the influenza virus (the flu), yearly vaccination against influenza can decrease the risk of pneumonia for certain patients, particularly the elderly and people with chronic diseases (such as asthma, cystic fibrosis, other lung or heart diseases, sickle cell disease, diabetes, kidney disease, and forms of cancer). A specific vaccine against Streptococcus pneumoniae is very protective, and should be administered to patients with chronic illnesses. Patients who have decreased immune resistance (due to treatment with chemotherapy for various forms of cancer or due to infection with the AIDS virus), and therefore may be at risk for infection with Pneumocystis carinii, are frequently put on a regular drug regimen of Trimethoprim sulfa and/or inhaled pentamidine to avoid Pneumocystis pneumonia.