Definition of Severe Community-Acquired Pneumonia (CAP)
Severe CAP is defined as the presence of either one major criterion (need for mechanical ventilation or septic shock) or two of three minor criteria (systolic BP ≤90 mm Hg, multilobar disease, PaO2/FiO2 ratio ≤250). 1
Major Criteria for Severe CAP
- Need for mechanical ventilation 1
- Septic shock or need for vasopressors for ≥4 hours 1
- Increase in the size of infiltrates by ≥50% within 48 hours 1
- Acute renal failure (urine output ≤80 ml in 4 hours or serum creatinine ≥2 mg/dl in the absence of chronic renal failure) 1
Minor Criteria for Severe CAP
- Respiratory rate ≥30/min 1
- PaO2/FiO2 ratio ≤250 1
- Bilateral pneumonia or multilobar pneumonia 1
- Systolic BP ≤90 mm Hg 1
- Diastolic BP ≤60 mm Hg 1
Clinical Significance of Severe CAP
- Severe CAP represents approximately 10% of hospitalized CAP cases 2
- Mortality rates in severe CAP range from 21% to 54% 2
- Patients with severe CAP have a distinct spectrum of etiologic agents compared to non-severe CAP 1
- Early recognition of severe CAP is critical for initiating prompt therapy directed at likely pathogens, which can reduce mortality 1
Evolution of Severe CAP Definition
The definition of severe CAP has evolved over time. The original American Thoracic Society (ATS) guidelines identified nine criteria for severe illness, with the presence of any one criterion defining severe CAP. However, this approach was found to be overly sensitive and not specific, with studies showing that 65-68% of all admitted patients would be classified as having "severe CAP" using this definition 1.
A more recent and specific definition was developed that divides the criteria into major and minor categories. This approach has demonstrated better accuracy in predicting ICU admission need with:
- Sensitivity: 78%
- Specificity: 94%
- Positive predictive value: 75%
- Negative predictive value: 95% 1
Additional Indicators of Severe Illness
Other findings that may suggest severe illness but have not been formally tested to define ICU admission need include:
- Confusion 1
- BUN ≥19.6 mg/dl 1
- Presence of focal chest signs 3
- Home oxygen therapy 3
- Altered mental status 3
Progression of Severe CAP
Severe CAP is a progressive disease that can evolve from a local infection to systemic involvement with the following spectrum of complications:
- Sepsis
- Severe sepsis
- Septic shock
- Multiple organ dysfunction 1
Approximately 50% of CAP admissions to ICUs are associated with septic shock. The progression is characterized by hypercoagulation, hypotension, alteration of the microcirculation, and ultimately multiple organ dysfunction 1.
Practical Application
When evaluating a patient with CAP, the presence of these criteria should guide decisions regarding:
- Need for ICU admission 1
- Selection of appropriate antimicrobial therapy 1
- Implementation of early resuscitation measures 4
Early recognition and intervention in severe CAP cases is crucial for improving outcomes and reducing mortality 1, 4.