1. Clinical Manifestations & Initial Assessment

Early recognition of pneumonia in the ICU relies on combining classic signs with laboratory data and maintaining a high index of suspicion for atypical presentations.

A. Cardinal Signs & Symptoms

• Fever (>38 °C) or hypothermia (<36 °C): Systemic inflammatory response.
• New or increased cough: Often with purulent sputum or increased tracheal secretions in ventilated patients.
• Respiratory distress: Dyspnea, tachypnea (respiratory rate ≥ 30 breaths/min), or pleuritic chest pain.
• Hypoxemia: Worsening gas exchange, evidenced by a PaO₂/FiO₂ ratio < 300 or rising FiO₂ requirements.

B. Laboratory Clues

• White Blood Cell Count: Leukocytosis (> 12 × 10⁹/L) or leukopenia (< 4 × 10⁹/L) can indicate infection.
• Inflammatory Markers: Elevated C-reactive protein (CRP) and procalcitonin (PCT) suggest a bacterial process.

C. Atypical Presentations

• Elderly: May be afebrile and present primarily with confusion, delirium, or functional decline.
• Immunocompromised: Often exhibit a blunted white blood cell response and may have subtle or non-specific radiographic changes.

2. Imaging Modalities

Chest radiography is the first-line test but has significant limitations in the ICU setting; CT provides definitive evaluation in complex or equivocal cases.

A. Chest X-Ray (CXR)

The standard for initial diagnosis, a CXR can identify new infiltrates, consolidation, or pleural effusions. However, its utility can be limited:

• Reduced Sensitivity: Sensitivity is approximately 65–75% compared to CT, especially with portable, supine films common in the ICU.
• Delayed Findings: In early or dehydrated states, infiltrates may not be apparent. A repeat CXR at 24–48 hours may reveal previously occult findings.

B. Chest CT Scan

CT offers superior resolution and is indicated when the diagnosis is uncertain or complications are suspected.

• High Sensitivity: Near-100% sensitivity for detecting consolidation, cavitation (abscess), and pleural complications like empyema.
• Differential Diagnosis: Crucial for differentiating pneumonia from atelectasis, pulmonary edema, hemorrhage, or Acute Respiratory Distress Syndrome (ARDS).
• Key Indications: Immunosuppression, suspected abscess, multilobar disease, or failure to respond to empiric therapy.

3. Microbiological Testing

Selective microbiological tests are crucial for identifying the causative pathogen, which allows for targeted antibiotic therapy and supports antimicrobial stewardship.

4. Respiratory Sampling Techniques

For ventilated patients, obtaining lower respiratory tract samples is key to distinguishing infection from colonization. The choice of technique balances invasiveness with diagnostic accuracy.

5. Classification & Severity Scores

Validated scoring tools are essential to stratify mortality risk, guide site-of-care decisions (ward vs. ICU), and determine the necessary breadth of empiric therapy.

A. Key Scoring Systems

• PSI (Pneumonia Severity Index): A 20-variable score that is excellent for identifying low-risk patients suitable for outpatient care (Classes I-II). Higher classes (IV-V) correlate with high mortality and need for hospitalization.
• CURB-65: A simpler 5-point score (Confusion, Urea >7, RR ≥30, low BP, Age ≥65) for rapid bedside assessment. A score ≥3 suggests severe pneumonia and need for ICU consideration.
• CPIS (Clinical Pulmonary Infection Score): A composite score used adjunctively in VAP, but with limited standalone sensitivity and specificity.
• CDC/NHSN Ventilator-Associated Event (VAE): A surveillance tool that tracks worsening oxygenation (VAC), infection (IVAC), and possible/probable VAP based on objective criteria.

B. IDSA/ATS Criteria for Severe CAP

These criteria are the most widely used standard for determining ICU admission for community-acquired pneumonia.

6. Integrating Data to Guide Empiric Therapy

A rapid, systematic workflow that combines clinical assessment, imaging, microbiology, and severity scores is essential to ensure timely and appropriate empiric antibiotic coverage.