Pneumonia is an infection of the lung parenchyma. Patients may present with any of a combination of cough, fever, pleuritic chest pain, sputum production, shortness of breath, hypoxia, and respiratory distress. Certain clinical presentations are associated with particular infectious agents. For example, the "typical" pneumonia is often described as having a sudden onset of fever, cough with productive sputum, often associated with pleuritic chest pain, and possibly rust-colored sputum. This is the classic description of pneumococcal pneumonia. The "atypical" pneumonia is characterized as having a more insidious onset, with a dry cough, prominent extrapulmonary symptoms such as headache, myalgias, sore throat, and a chest radiograph that appears much worse than the auscultatory findings. This type of presentation usually is attributed to Mycoplasma pneumoniae. Although these characterizations are of some diagnostic value, it is very difficult to reliably distinguish between typical and atypical organisms based on clinical history and physical examination as the cause of a specific patient's pneumonia. Therefore, pneumonias typically classified according to the immune status of the host, the radiographic findings, and the setting in which the infection was acquired, in an attempt to identify the likely causative organism and to guide initial empiric therapy.
Typical community-acquired pneumonia, as opposed to nosocomial or hospital-acquired pneumonia, is most commonly caused by S. pneumoniae, MoraxeUa catarrhalis, or Haemophilus influenzae. Viruses, such as influenza and adenovirus, can also cause pneumonia. Atypical pneumonia, which is characterized by a more insidious onset, a dry cough, muscle aches, headaches, and sore throat, typically is caused by M. pneumoniae. However, other organisms, such as Legionella (typically associated with preceding gastrointestinal symptoms) and Chlamydia pneumoniae, should be considered. In a patient with specific risk factors. Chlamydia psittaci (bird exposure), coc-cidiomycosis (travel to the American southwest), or histoplasmosis (endemic to the Mississippi Valley) may be the cause. In a patient with AIDS or immunosuppression. Pneumocystis carinii should be added to the differential diagnosis. Tuberculosis is a possibility in patients with a history suggestive of exposure or predisposition (e.g.. those with AIDS) to this disease.
Those with hospital- or nursing home-acquired infection, termed nosocomial pneumonia, should have antibiotic coverage for gram-negative organisms. An inability to perform basic hygiene leads to colonization of the oral cavity with enteric gram-negative bacteria such as Escherichia coli. In these cases, the patient's normal defenses against pulmonary infection are often compromised. A patient's level of consciousness may be depressed; he may be intubated or unable to cough efficiently, leading to an increased risk for pulmonary infection.
Once the clinical diagnosis of pneumonia has been made, the next step is to try to risk stratify the patients, to decide which patients can be treated safely as outpatients with oral antibiotics and which require hospitalization. In 1997, Fine and colleagues described a prediction rule that accurately identified those patients with community-acquired pneumonia who had a low risk for death or other adverse outcomes. Patients younger than 50 years without these medical problems or physical examination findings have an approximately 0.1% risk of death and a 5.1% risk of hospitalization over the next 30 days. Patients in the lowest risk class usually can be safely treated as outpatients with oral antibiotics. Patients who are hypoxemic, are hemodynamically unstable, or have multiple risk factors (Table 39-1) have poor prognoses and usually require hospitalization.
Table 39-1 RISK STRATIFICATION OF PNEUMONIA
History consistent with high risk Age >50 years History of cancer Congestive heart failure Cerebrovascular disease Renal or liver disease
Physical findings of high risk Altered mental status Tachycardia (>125 bpm) Tachypnea (>30 breaths per minute) Hypotension (<90 mmHg systolic) Temperature <95°F (35°C) >I04°F (40°C)
Although outpatients usually are diagnosed and empiric treatment is begun based on clinical findings, further diagnostic evaluation is important in hospitalized patients. Chest radiography is important to try to define the cause and extent of the pneumonia and to look for complications, such as parapneumonic effusion or lung abscess. Unless the patient cannot mount an immune response, as in severe neutropenia, or the process is very early, every patient with pneumonia will have a visible pulmonary infiltrate. The pattern of infiltration can yield diagnostic clues.' Diffuse interstitial infiltrates are common in P. carinii pneumonia and viral processes. Conversely, pleural effusions are almost never seen in P. carinii pneumonia. Bilateral apical infiltrate suggests tuberculosis. Appearance of cavitation suggests a necrotizing infection such as Staphylococcus aureus, tuberculosis, or gram-negative organisms such as Klebsiella pneumoniae. Serial chest radiography of inpatients usually is unnecessary, because many weeks arc required for the infiltrate to resolve: serial chest radiography typically is performed if the patient does not show clinical improvement, has a pleural effusion, or has a necrotizing infection.
Microbiologic studies, such as sputum Gram stain and culture, and blood cultures are important to try to identify the specific etiologic agent causing the illness. However, use of sputum Gram stain and culture is limited by the frequent contamination by upper respiratory flora as the specimen is expectorated. However, if the sputum appears purulent and it is minimally contaminated (>25 polymorphonuclear cells and <10 epithelial cells per low-power field), the diagnostic yield is good. Additionally, blood cultures can be
'infection with Streptococcus pneumoniae classically presents with a dense lobar infiltrate, often with an associated parapneumonic effusion.
helpful, because 30-40% of patients with pneumococcal pneumonias are bacteremia Serologic studies can be performed to diagnose patients who are infected with organisms not easily cultured, for example, Legionella, Mycoplasma, or C. pneumoniae.
Finally, fiberoptic bronchoscopy with bronchoalveolar lavage often is performed in seriously ill or immunocompromised patients, or in those patients who are not responding to therapy, to try to obtain a specimen from the lower respiratory tract for routine Gram stain and culture, as well as more sophisticated testing, such as direct fluorescent antibody testing for various organisms, for example, Legionella.
Initially, empiric treatment is based upon the most common organisms given the clinical scenario. For outpatient therapy of community-acquired pneumonia, macrolide antibiotics, such as azithromycin, or antipneumococcal quinoiones. such as gatifloxacin or levofloxacin, are good choices for treatment of S. pneumoniae. Mycoplasma, and other common organisms. Hospitalized patients with community-acquired pneumonia usually are treated with an intravenous third-generation cephalosporin plus a macrolide or with an antipneumococcal quinolone. For immunocompetent patients with hospital-acquired or ventilator-associated pneumonias, the causes include any of the organisms that can cause community-acquired pneumonia, Pseudomonas aeruginosa, or S. aureus, as well as more gram-negative enteric bacteria and oral anaerobes. Accordingly, the initial antibiotic coverage is broader and includes an antipseudomonal /3-lactam, such as piperacillin or cefepime. plus an aminoglycoside. and may include clindamycin.
For immunocompromised patients, such as AIDS patients, the spectrum of potential pathogens is much broader and includes P. carinii pneumonia, unusual bacteria such as Rhodococcus equi, tuberculosis, as well as fungal organisms such as Histoplasma or Cryptococcus. It should be remembered, however, that even in AIDS patients, the most common cause of pneumonia is 5. pneumoniae. If an AIDS patient presents with an acute onset of fever and a productive cough with purulent sputum, pneumococcal pneumonia is the most likely diagnosis.
Two other commonly confused pulmonary syndromes deserve mention at this point. Aspiration pneumonitis is a chemical injury to the lungs caused by aspiration of acidic gastric contents into the lungs. Because of the high acidity, gastric contents are normally sterile, so this is not an infectious process but rather a chemical burn that causes a severe inflammatory response, which is proportional to the volume of the aspirate and the degree of acidity. This inflammatory response can be profound and produce respiratory distress and a pulmonary infiltrate that is apparent within 4-6 hours and typically resolves within 48 hours. Aspiration of gastric contents is most likely to occur in patients with a depressed level of consciousness, such as those under anesthesia or suffering from a drug overdose, or after a seizure.
Aspiration pneumonia, by contrast, is an infectious process caused by inhalation of oropharyngeal secretions that are colonized by bacterial pathogens. It should be noted that many healthy adults frequently aspirate small volumes of oropharyngeal secretions while sleeping (this is the primary way that bacteria gain entry to the lungs), but usually the material is cleared by coughing, ciliary transport, or normal immune defenses so that no clinical infection results. However, any process that increases the volume or bacterial organism burden of the secretion or impairs the normal defense mechanisms can produce clinically apparent pneumonia. This is most commonly seen in elderly patients with dysphagia, such as stroke victims, who may aspirate significant volumes of oral secretions, and those with poor dental care. The affected lobe of the lung depends upon the patient's position: in recumbent patients, the posterior segments of the upper lobes and apical segments of the lower lobes are most common. In contrast to aspiration pneumonitis, where aspiration of vomitus may be witnessed, the aspiration of oral secretions typically is silent and should be suspected when any institutionalized patient with dysphagia presents with respiratory symptoms and pulmonary infiltrate in a dependent segment of the lung.
Antibiotic therapy for aspiration pneumonia is similar to that of other pneumonias, that is, it should cover typical respiratory pathogens such as S. pneumoniae and H. influenzae, as well as gram-negative organisms and oral anaerobes. Treatment for aspiration pneumonitis, because it usually is not infectious, is mainly supportive. Antibiotics are often added if secondary bacterial infection is suspected because of failure to improve within 48 hours, or if the gastric contents are suspected to be colonized because of acid suppression or bowel obstruction.
[39.11 A 65-year-old cigarette smoker with a history of hypertension and mild congestive heart failure presents to the emergency room with worsening cough, fever, and dyspnea at rest. The illness began 1 week ago with fever, muscle aches, abdominal pain, and diarrhea, with nonproductive cough developing later that week and rapidly becoming worse. Therapy for which atypical organism must be considered in this case?
B. Mycoplasma pneumoniae
C. Legionella pneumophila
[39.2] An 85-year-old nursing home resident has dementia such that she requires assistance in all activities of daily life. She has a 3-day history of fever and productive cough. Chest x-ray reveals a right middle lobe consolidation. Which of the following is the most likely description of the mechanism of infection?
A. Aspiration of oral flora
B. Aspiration of gastric contents
C. Inhalation of M. tuberculosis droplet nuclei
[39.3] A 56-year-old man is brought into the emergency room intoxicated with alcohol. He has repeated bouts of emesis and is found choking. Lung examination reveals some crackles in the right lung base. Which of the following is the most appropriate management?
A. Initiate azithromycin
B. Initiate corticosteroid therapy
C. Initiate haloperidol therapy
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