IM3.1-3 | Pneumonia Foundations — Summary & Reflection

KEY TAKEAWAYS

Community-acquired pneumonia (CAP) is acquired outside hospital or within 48 hours of admission (not hospitalised or in long-term care for ≥90 days). Hospital-acquired pneumonia (HAP) develops ≥48 hours after admission; VAP is a subcategory developing ≥48–72 hours post-intubation. Aspiration pneumonia involves bacterial infection (anaerobes, oral flora) from aspirated oropharyngeal secretions — distinguished from chemical aspiration pneumonitis by the presence of bacterial infection and the need for antibiotics.

Key CAP pathogens: S. pneumoniae (lobar consolidation, rust-coloured sputum, most common), H. influenzae (COPD patients, bronchopneumonia), Klebsiella (diabetics, currant-jelly sputum, abscess, bulging fissure), S. aureus (post-influenza, necrotising), Mycoplasma (young adults, dry cough, walking pneumonia, extrapulmonary features), Legionella (water-source aerosols, hyponatraemia, severe, urinary antigen). HAP/VAP: Pseudomonas, Klebsiella ESBL, Acinetobacter, MRSA.

Host immune modifiers: HIV CD4 <200 → Pneumocystis jirovecii (PCP, bilateral perihilar ground-glass, elevated LDH, disproportionate hypoxia); HIV + India → always consider TB co-infection; Neutropenia → Invasive Aspergillosis (halo sign, voriconazole); Diabetes → Klebsiella, bacteraemic pneumococcal.

Pathogenesis sequence (lobar pneumonia): congestion → red hepatisation → grey hepatisation → resolution. Complications: parapneumonic effusion/empyema, lung abscess, bacteraemia, ARDS, AKI, septic shock.

The cardinal rule: classify the syndrome before choosing an antibiotic. The classification determines the empirical pathogen spectrum and the antibiotic class required.

REFLECT

Return to the two patients in the opening hook — Mr. Krishnamurthy with his lobar CAP and the post-surgical patient with VAP. You now understand not only what is happening in each alveolus, but why the organisms responsible are so different, why the host vulnerabilities matter, and why the pathological stage determines the clinical presentation. Consider this: in your clinical postings, you will be called to review patients with fever and a cough in the ward or emergency room. Before you order a single test, ask yourself the three foundational questions: was this acquired in the community or the hospital, what is the host's immune status, and are there any specific clinical clues (sputum character, radiological distribution, extrapulmonary features) that point to a specific pathogen? The clinician who asks these questions in the first five minutes of the encounter will choose the right antibiotic before the culture returns — and that head start saves lives. How would your clinical approach differ if Mr. Krishnamurthy had a CD4 count of 120 cells/mm³ rather than being immunocompetent?