Community-acquired pneumonia

A full spectrum of microorganisms is responsible for CAP in adults, and patients with certain risk factors are more susceptible to infections by certain groups of microorganisms. Identifying people at risk for infection by these organisms aids in appropriate treatment. Many less-common organisms can cause CAP in adults; these may be determined by identifying specific risk factors, or when treatment for more common causes fails.

Risk factors

Some patients have an underlying problem which increases their risk of infection. Some risk factors are:

• Obstruction - When part of the airway (bronchus) leading to the alveoli is obstructed, the lung cannot eliminate fluid; this can lead to pneumonia. One cause of obstruction, especially in young children, is inhalation of a foreign object such as a marble or toy. The object lodges in a small airway, and pneumonia develops in the obstructed area of the lung. Another cause of obstruction is lung cancer, which can block the flow of air.
• Lung disease - Patients with underlying lung disease are more likely to develop pneumonia. Diseases such as emphysema and habits such as smoking result in more frequent and more severe bouts of pneumonia. In children, recurrent pneumonia may indicate cystic fibrosis or pulmonary sequestration.
• Immune problems - Immune-deficient patients, such as those with HIV/AIDS, are more likely to develop pneumonia. Other immune problems that increase the risk of developing pneumonia range from severe childhood immune deficiencies, such as Wiskott–Aldrich syndrome, to the less severe common variable immunodeficiency.^[10]

Pathophysiology

The symptoms of CAP are the result of lung infection by microorganisms and the response of the immune system to the infection. Mechanisms of infection are different for viruses and other microorganisms.^{[citation needed]}

Viruses

Up to 20 percent of CAP cases can be attributed to viruses.

Typically, a virus enters the lungs through the inhalation of water droplets and invades the cells lining the airways and the alveoli. This leads to cell death; the cells are killed by the virus or they self-destruct. Further lung damage occurs when the immune system responds to the infection. White blood cells, particularly lymphocytes, activate chemicals known as cytokines which cause fluid to leak into the alveoli. The combination of cell destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream. In addition to their effects on the lungs, many viruses affect other organs. Viral infections weaken the immune system, making the body more susceptible to bacterial infection, including bacterial pneumonia.

Bacteria and fungi

Although most cases of bacterial pneumonia are caused by Streptococcus pneumoniae, infections by atypical bacteria such as Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila can also cause CAP. Enteric gram-negative bacteria, such as Escherichia coli and Klebsiella pneumoniae, are a group of bacteria that typically live in the large intestine; contamination of food and water by these bacteria can result in outbreaks of pneumonia. Pseudomonas aeruginosa, an uncommon cause of CAP, is a difficult bacteria to treat.

Bacteria and fungi typically enter the lungs by inhalation of water droplets, although they can reach the lung through the bloodstream if an infection is present. In the alveoli, bacteria and fungi travel into the spaces between cells and adjacent alveoli through connecting pores. The immune system responds by releasing

Patients with symptoms of CAP require evaluation. Diagnosis of pneumonia is made clinically, rather than on the basis of a particular test.[13] Evaluation begins with a physical examination by a health provider, which may reveal fever, an increased respiratory rate (tachypnea), low blood pressure (hypotension), a fast heart rate (tachycardia) and changes in the amount of oxygen in the blood. Palpating the chest as it expands and tapping the chest wall to identify dull, non-resonant areas can identify stiffness and fluid, signs of CAP. Listening to the lungs with a stethoscope (auscultation) can also reveal signs associated with CAP. A lack of normal breath sounds or the presence of crackles can indicate fluid consolidation. Increased vibration of the chest when speaking, known as tactile fremitus, and increased volume of whispered speech during auscultation can also indicate the presence of fluid.^[14]

Several tests can identify the cause of CAP.

Patients who have been vaccinated against streptococcus pneumoniae, health professionals, nursing-home residents and pregnant women should be vaccinated annually against influenza.^[17] During an outbreak, drugs such as amantadine, rimantadine, zanamivir and oseltamivir have been demonstrated to prevent influenza.^[18]

Treatment

Chest X-rays of a CAP patient before (left) and after treatment

CAP is treated with an antibiotic that kills the infecting microorganism; treatment also aims at managing complications. If the causative microorganism is unidentified, which is often the case, the laboratory identifies the most effective antibiotic; this may take several days.

Health professionals consider a person's risk factors for various organisms when choosing an initial antibiotic. Additional consideration is given to the treatment setting; most patients are cured by oral medication, while others must be hospitalized for intravenous therapy or intensive care. Current treatment guidelines recommend a beta lactam, like amoxicillin and a macrolide, like azithromycin or clarithromycin, or a

Ceftriaxone and azithromycin are often used to treat community acquired pneumonia, which usually present with a few days of cough, fever, and shortness of breath. Chest x-ray typically reveals a lobar infiltrate (rather than diffuse).^[19]

Newborns

Most newborn infants with CAP are hospitalized, receiving IV

Children

Treatment of CAP in children depends on the child's age and the severity of illness. Children under five are not usually treated for atypical bacteria. If hospitalization is not required, a seven-day course of

In 2001 the American Thoracic Society, drawing on the work of the British and Canadian Thoracic Societies, established guidelines for the management of adult CAP by dividing patients into four categories based on common organisms:^[23]

• Healthy outpatients without risk factors: This group (the largest) is composed of otherwise-healthy patients without risk factors for DRSP, enteric gram-negative bacteria, pseudomonas or other, less common, causes of CAP. Primary microorganisms are viruses, atypical bacteria, penicillin-sensitive streptococcus pneumoniae and haemophilus influenzae. Recommended drugs are macrolide antibiotics, such as azithromycin or clarithromycin, for seven^[24] to ten days. A shorter course of these antibiotics has been investigated, however, there is not sufficient evidence to make recommendations.^[25]
• Outpatients with underlying illness or risk factors: Although this group does not require hospitalization, they have underlying health problems such as emphysema or heart failure or are at risk for DRSP or enteric gram-negative bacteria. They may be treated with a quinolone active against streptococcus pneumoniae (such as
  β-lactam antibiotic (such as cefpodoxime, cefuroxime, amoxicillin or amoxicillin/clavulanic acid) and a macrolide antibiotic, such as azithromycin or clarithromycin, for seven to ten days.^[26]
• Hospitalized patients without risk for pseudomonas: This group requires intravenous antibiotics, with a quinolone active against streptococcus pneumoniae (such as levofloxacin), a β-lactam antibiotic (such as cefotaxime, ceftriaxone, ampicillin/sulbactam or high-dose ampicillin plus a macrolide antibiotic (such as azithromycin or clarithromycin) for seven to ten days.
• Intensive-care patients at risk for pseudomonas aeruginosa: These patients require antibiotics targeting this difficult-to-eradicate bacterium. One regimen is an intravenous antipseudomonal beta-lactam such as cefepime, imipenem, meropenem or piperacillin/tazobactam, plus an IV antipseudomonal fluoroquinolone such as levofloxacin. Another is an IV antipseudomonal beta-lactam such as cefepime, imipenem, meropenem or piperacillin/tazobactam, plus an aminoglycoside such as gentamicin or tobramycin, plus a macrolide (such as azithromycin) or a nonpseudomonal fluoroquinolone such as ciprofloxacin.

For mild-to-moderate CAP, shorter courses of antibiotics (3–7 days) seem to be sufficient.^[22]

Some patients with CAP will be at increased risk of death despite antimicrobial treatment. A key reason for this is the host's exaggerated inflammatory response. There is a tension between controlling the infection on one hand and minimizing damage to other tissues on the other. Some recent research focuses on immunomodulatory therapy that can modulate the immune response in order to reduce injury to the lung and other affected organs such as the heart. Although the evidence for these agents has not resulted in their routine use, their potential benefits are promising.^[27]

Hospitalization

Some CAP patients require intensive care, with clinical prediction rules such as the pneumonia severity index and CURB-65 guiding the decision whether or not to hospitalize.^[28] Factors increasing the need for hospitalization include:

• Age greater than 65
• Underlying chronic illnesses
• Respiratory rate greater than 30 per minute
• Systolic
  mmHg
• Heart rate greater than 125 per minute
• Temperature below 35 or over 40 °C
• Confusion
• Evidence of infection outside the lung

Laboratory results indicating hospitalization include:

• Arterial oxygen tension less than 60 mm Hg
• Carbon dioxide over 50 mmHg or pH under 7.35 while breathing room air
• Hematocrit under 30 percent
• Creatinine over 1.2 mg/dl or blood urea nitrogen over 20 mg/dl
• White-blood-cell count under 4 × 10^9/L or over 30 × 10^9/L
• Neutrophil count under 1 x 10^9/L

X-ray findings indicating hospitalization include:

• Involvement of more than one lobe of the lung
• Presence of a cavity
• Pleural effusion

Prognosis

The CAP outpatient mortality rate is less than one percent, with fever typically responding within the first two days of therapy, and other symptoms abating in the first week. However, X-rays may remain abnormal for at least a month. Hospitalized patients have an average mortality rate of 12 percent, with the rate rising to 40 percent for patients with bloodstream infections or those who require intensive care.^[29] Factors increasing mortality are identical to those indicating hospitalization.

When CAP does not respond to treatment, this may indicate a previously unknown health problem, a treatment complication, inappropriate antibiotics for the causative organism, a previously unsuspected microorganism (such as

CAP is common worldwide, and is a major cause of death in all age groups. In children, most deaths (over two million a year) occur in the newborn period. According to a World Health Organization estimate, one in three newborn deaths result from pneumonia.^[30] Mortality decreases with age until late adulthood, with the elderly at risk for CAP and its associated mortality.

More CAP cases occur during the winter than at other times of the year. CAP is more common in males than females, and more common in black people than Caucasians.^[31] Patients with underlying illnesses (such as Alzheimer's disease, cystic fibrosis, COPD, tobacco smoking, alcoholism or immune-system problems) have an increased risk of developing pneumonia.^[32]

See also

• Bacterial pneumonia
• Viral pneumonia
• Fungal pneumonia
• Parasitic pneumonia

References