Pneumonia ranks first as the cause of death from infection and ninth as the leading cause of death in general in the United States.
More than 2 million cases of community-acquired pneumonia (CAP) occur each year in the United States, resulting in approximately 10 million physician visits, more than 50,000 deaths, and more than 500,000 hospitalizations, especially among elderly and those with underlying lung disease such as emphysema.
CAP may be viral, bacterial, or fungal in etiology; however, a causative pathogen may not be identified in up to 50% to 60% of patients in spite of extensive laboratory testing.
Etiologic viruses include the influenza viruses, respiratory syncytial virus, adenovirus, parainfluenza virus, herpes simplex virus, human metapneumovirus, and Hantavirus.
The most commonly encountered bacteria include Streptococcus pneumoniae (20% to 60%), Haemophilus influenzae (2% to 31%), Moraxella catarrhalis (2% to 13%), and “atypical bacteria” such as Mycoplasma pneumoniae (13% to 37%), Chlamydia pneumoniae (6% to 17%), and the Legionella species (1% to 16%).
Coinfection with atypical bacterial pathogens is estimated to occur in up to 48% of all patients with CAP.
Enteric gram-negative bacteria are not common etiologies in CAP, yet they may be encountered in particular settings, such as use of alcohol associated with klebsiella pneumoniae, and diabetes mellitus, chronic steroid use, or structural lung disease association with Pseudomonas aeruginosa.
Pneumocystis jiroveci (formerly carinii) and endemic fungi (Cryptococcus neoformans, Histoplasma capsulatum, Blastomyces dermatitiditis, Coccidioides immitis) constitute other etiologic agents that are often dependent on epidemiologic and host factors. The frequency of these pathogens varies with the setting in which the infection was acquired. Variables include the season of the year, geographic location, environmental exposure, and host factors such as age, smoking, alcohol use, and underlying illnesses.
Similarly, atypical zoonotic pathogens such as Chlamydia psittaci, Francisella tularensis, and Coxiella burnetii may cause CAP in specific exposure scenarios, particularly, close contact with psittacine birds (psittacosis), deer or rabbits that are infested with ticks (tularemia), and parturient cat or sheep (Q fever), respectively. These zoonoses will be discussed separately in relevant sections, yet their initial presentation may mimic CAP.
Patients with CAP may present with systemic, nonspecific, pulmonary, or extrapulmonary symptoms.
Systemic and nonspecific symptoms include fever or hypothermia, rigors, sweats, fatigue, and anorexia. Pulmonary symptoms include new cough with or without sputum production or change in color of respiratory secretions in a patient with chronic cough, chest discomfort, or the onset of dyspnea.
Extrapulmonary symptoms may include headache, myalgias, earache, abdominal pain, and diarrhea. Atypical pathogens have been classically linked to extrapulmonary symptoms, for example, diarrhea, abdominal pain, and myalgia with Legionella pneumophila, headache and myalgia with C. pneumoniae, and earache with M. pneumoniae.
Findings on exam include documentation of fever or hypothermia, tachypnea, tachycardia, hypotension, cyanosis, or even septic shock in severe cases with overwhelming infection. Local findings include diminished intensity of breath sounds over affected lung areas with bronchial quality of breath sounds, and crackles may be heard.
Diagnosis is suggested by the clinical features and documented by the presence of new infiltrates on routine chest x-ray.
Determination of the severity of pneumonia is important in order to plan the site of care, further laboratory workup, and treatment. The pneumonia PORT (pneumonia outcomes research team) severity index (PSI), or the modified British Thoracic Society (BTS) criteria are best used for this purpose. The PSI classifies patients in five mortality risk classes and advises outpatient therapy for classes I and II, management in an observational unit or short hospital stay for class III, and inpatient treatment for classes IV and V, respectively (Table 67-1).
The modified BTS criteria identified five indicators of increased mortality including confusion (based on a specific mental test or disorientation to person, place, or time), BUN level >20 mg/dL, respiratory rate ≥30 breaths/minute, low blood pressure (systolic, <90 mm Hg; or diastolic, ≤60 mm Hg), and age ≥65 years with the acronym CURB-65. Patients with CURB-65 score of ≥2 should be admitted to hospital, and those with ≥3 should be managed in ICU setting.
While a chest x-ray or other imaging technique is usually required to confirm the diagnosis of CAP, the low yield and infrequent positive impact on clinical care argue against the use of further tests for patients who will be treated on outpatient basis.
Patients who will be treated in the hospital, especially those with severe pneumonia or comorbidities such as asplenia, chronic liver disease, active alcohol use, leukopenia, lung cavitation, pleural effusion, and patients who had recent travel, would benefit from further testing. Determination of the specific pathogen causing pneumonia in these settings will guide and may alter empiric therapy.
Table 67-1 Point Scoring System for Step 2 of the Prediction
Rule for Assignment to Risk Classes II, III, IV, and V.
Characteristic
Points Assigneda
Demographic factor
Age
Men Age (year)
Women
Age (year) -10
Nursing home resident
+10
Coexisting illnessesb
Neoplastic disease
+30
Liver disease
+20
Congestive heart failure
+10
Cerebrovascular disease
+10
Renal disease
+10
Physical examination findings
Altered mental statusc
+20
Respiratory rate ≥30/minute
+20
Systolic blood pressure ≤90 mm Hg
+20
Temperature <35°C or ≥40°C
+15
Pulse ≥125/minute
+10
Laboratory and radiographic findings
Arterial pH <7.35
+30
Blood urea nitrogen ≥30 mg/dL
+20
Sodium <130 mmol/L
+20
Glucose ≥250 mg/dl (14 mmol/L)
+10
Hematocrit <30%
+10
Partial pressure of arterial oxygen
+10
<60 mm Hgd
Pleural effusion
+10
a A total point score for a given patient is obtained by summing the patient’s age in years (age—10 for women) and the points for each applicable characteristic. The points assigned to each predictor variable were based on coefficients obtained from the logistic regression model used in step 2 of the prediction rule (see the Methods section).
b Neoplastic disease is defined as any cancer except basal or squamous cell cancer of the skin that was active at the time of presentation or diagnosed within 1 year of presentation. Liver disease is defined as a clinical or histologic diagnosis of cirrhosis or another form of chronic liver disease, such as chronic active hepatitis. Congestive heart failure is defined as systolic or diastolic ventricular dysfunction documented by history, physical examination, and chest radiograph, echocardiogram, multiple-gated acquisition scan, or left ventriculogram. Cerebrovascular disease is defined as a clinical diagnosis of stroke or transient ischemic attack or stroke documented by magnetic resonance imaging or computed tomography. Renal disease is defined as a history of chronic renal disease or abnormal blood urea nitrogen and creatinine concentrations documented in the medical record.
c Altered mental status is defined as disorientation with respect to person, place, or time that is not known to be chronic, stupor, or coma.
d In the pneumonia PORT cohort study, an oxygen saturation of <90% percent on pulse oximetry or intubation before admission was also considered abnormal.Related posts:
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