Pneumonia

• Definition: infection of the lung parenchyma — alveoli and distal airways — producing an inflammatory exudate that consolidates air spaces and impairs gas exchange. This distinguishes it from bronchitis (airway mucosa only, no infiltrate) and from pulmonary edema (transudate, no pathogen).

Why it matters

• Mortality burden: pneumonia remains the leading infectious cause of death in the United States and one of the most frequent reasons for adult hospitalization; deaths cluster in older adults, nursing-home residents, and the immunocompromised.
• Time-sensitive: untreated bacterial pneumonia progresses to bacteremia, septic shock, and ARDS. Boards test the disposition decision (outpatient vs ward vs ICU) as often as the antibiotic choice.
• Antibiotic stewardship: a substantial minority of adult CAP is viral, and empiric over-treatment drives resistance and Clostridioides difficile infection.

Epidemiology worth recalling

• Bimodal age distribution: incidence is highest at the extremes of life — infants/young children and adults over 65 — reflecting immature and senescent immunity plus weaker cough and mucociliary clearance.
• Setting defines the flora: community-acquired pneumonia is dominated by S. pneumoniae, respiratory viruses, and atypicals; hospital-acquired (≥48 h after admission) and ventilator-associated pneumonia shift toward Pseudomonas aeruginosa, MRSA, and enteric gram-negatives. The ATS/IDSA guideline eliminated the old "healthcare-associated pneumonia" (HCAP) category because it over-treated patients without improving outcomes.
• Seasonality: bacterial CAP peaks in winter, often following influenza or other viral infection that denudes respiratory epithelium — post-influenza pneumonia is classically S. pneumoniae or S. aureus.
• Prevention: ACIP recommends pneumococcal conjugate vaccination for older adults and for younger adults with chronic heart, lung, liver, or kidney disease, diabetes, asplenia, or immunocompromise, plus annual influenza vaccination for essentially everyone ≥6 months.

Routes of entry (group causes by mechanism)

• Micro-aspiration of oropharyngeal flora: the dominant mechanism for typical bacterial CAP. Explains S. pneumoniae, H. influenzae, Moraxella catarrhalis, and, with gross aspiration, oral anaerobes.
• Inhalation of aerosolized organisms: small-particle spread favors organisms that survive in water or air — Legionella pneumophila (cooling towers, hot tubs, decorative fountains), Mycoplasma pneumoniae, Chlamydia psittaci (birds), Coxiella burnetii (parturient livestock), Mycobacterium tuberculosis, and respiratory viruses.
• Hematogenous seeding: right-sided endocarditis or infected catheters throw septic emboli producing multiple peripheral nodules, often cavitating — classically S. aureus.
• Contiguous spread / direct inoculation: from a subdiaphragmatic abscess or via an endotracheal tube, which bypasses glottic and mucociliary defenses entirely (VAP).

Non-modifiable risk factors

• Age extremes: blunted cough reflex, weak mucociliary escalator, immunosenescence.
• Structural lung disease: COPD, bronchiectasis, cystic fibrosis — impaired clearance and biofilm; CF and bronchiectasis are the classic Pseudomonas setups.
• Immune defects: asplenia or sickle cell disease (loss of opsonized-encapsulated-organism clearance → S. pneumoniae, H. influenzae), hypogammaglobulinemia, HIV with low CD4 (Pneumocystis jirovecii), neutropenia, transplant immunosuppression.
• Neurologic impairment: stroke, dementia, Parkinson disease, seizure disorder — dysphagia and depressed airway protection.

Modifiable risk factors (the ones stems plant deliberately)

• Cigarette smoking: paralyzes cilia and impairs alveolar macrophages; the single most important modifiable risk factor and a specific risk for Legionella.
• Alcohol use disorder and sedative/opioid use: obtund consciousness → macroaspiration; alcohol also impairs neutrophil function (Klebsiella, anaerobes).
• Poor dentition/periodontal disease: raises anaerobic inoculum in aspirated secretions.
• Acid suppression (PPIs): gastric bacterial overgrowth.
• Recent antibiotics or hospitalization: selects resistant gram-negatives; the ATS/IDSA 2019 approach uses prior respiratory isolation of MRSA or Pseudomonas and recent parenteral antibiotics — not the retired HCAP label — to trigger broad coverage.
• Missed vaccination: influenza, COVID-19, and pneumococcal vaccines per ACIP.

• Step 1 — inoculum overwhelms defenses: organisms reach the alveolus after defeating the mucociliary escalator, IgA, surfactant proteins A/D, and alveolar macrophages. Disease requires either a large inoculum, a virulent organism (pneumococcal capsule resists phagocytosis), or a defective host.
• Step 2 — cytokine amplification: macrophage recognition of pathogen-associated molecular patterns (lipoteichoic acid, LPS) via toll-like receptors releases TNF-α, IL-1, IL-6, and IL-8. IL-1/TNF act on the hypothalamus (PGE2) → fever, and on the marrow → neutrophilia with left shift. IL-8 recruits neutrophils; IL-6 drives CRP and procalcitonin.
• Step 3 — alveolar flooding: capillary leak fills alveoli with fibrin-rich exudate, neutrophils, and erythrocytes. The classic four stages of lobar pneumonia are congestion → red hepatization (RBCs and fibrin, liver-like texture, the source of rusty sputum) → gray hepatization (RBC breakdown, persistent fibrinosuppurative exudate) → resolution (macrophage clearance, usually without scarring since the alveolar framework is preserved).
• Step 4 — gas exchange failure: consolidated alveoli are perfused but not ventilated, creating an intrapulmonary shunt. Hypoxic pulmonary vasoconstriction only partly compensates, so hypoxemia responds incompletely to supplemental oxygen and the A–a gradient widens. Tachypnea blows off CO2 → early respiratory alkalosis; hypercapnia appears late, with fatigue or underlying COPD.
• Step 5 — the physical exam follows the physics: fluid-filled but patent alveoli transmit high-frequency sound better than air, giving bronchial breath sounds, egophony, whispered pectoriloquy, and increased tactile fremitus; loss of air gives dullness to percussion. Reopening of collapsed small airways in inspiration produces late inspiratory crackles. Extension to the visceral pleura irritates somatic parietal pleural nerves → pleuritic chest pain and a splinted, shallow breathing pattern.
• Why atypicals look different: Mycoplasma (no cell wall — hence intrinsic beta-lactam resistance), Chlamydia, Legionella, and viruses replicate within or upon epithelium and elicit a mononuclear interstitial infiltrate rather than alveolar consolidation — patchy CXR changes with a near-normal exam and dry cough.

Typical bacterial syndrome (think S. pneumoniae)

• Abrupt fever with rigors: pyrogenic cytokines reset the hypothalamic set point; a single shaking chill is classically pneumococcal.
• Productive cough: purulent or rusty sputum from red hepatization; volume reflects alveolar exudate.
• Pleuritic chest pain: parietal pleural inflammation; worsens with deep inspiration and cough.
• Dyspnea and tachypnea: shunt physiology; respiratory rate is the earliest and most reliable vital-sign abnormality.
• Focal exam: dullness, increased fremitus, bronchial breath sounds, egophony, and late inspiratory crackles over one lobe.

Atypical syndrome

• Insidious prodrome: low-grade fever, headache, myalgias, sore throat, then a dry, hacking cough persisting for weeks.
• Exam–imaging mismatch: scattered crackles at most despite bilateral patchy interstitial infiltrates.
• Extrapulmonary clues: Legionella with high fever, relative bradycardia, diarrhea, confusion, and hyponatremia; Mycoplasma in a young adult in a dormitory or military barracks, with bullous myringitis, cold-agglutinin hemolysis, or erythema multiforme.

Demographic/exposure hooks stems use

• Alcohol use disorder: Klebsiella pneumoniae with currant-jelly sputum and an upper-lobe bulging fissure.
• Post-influenza deterioration: S. aureus (including MRSA), often necrotizing.
• COPD: H. influenzae or Moraxella.
• Recent hotel stay, cruise, or air-conditioning exposure: Legionella.
• Bird exposure: Chlamydia psittaci; farm animals/parturient sheep: Coxiella burnetii; southwestern US dust: Coccidioides; Ohio/Mississippi River valley caves or bird droppings: Histoplasma.
• Witnessed aspiration, seizure, or poor dentition: anaerobes, foul-smelling sputum, right lower lobe (or posterior segment of the RUL if supine).

Presentation traps

• Older adults: may present with confusion, falls, anorexia, or decompensated heart failure and be afebrile — a normal temperature does not exclude pneumonia.
• Children: grunting, nasal flaring, retractions, and abdominal pain from lower-lobe diaphragmatic irritation.

Step 1 — imaging establishes the diagnosis

• Chest radiograph (PA and lateral) is the initial test; ATS/IDSA defines CAP as compatible symptoms plus a demonstrable infiltrate. Lobar consolidation with air bronchograms suggests typical bacteria; bilateral patchy interstitial opacities suggest viral/atypical; cavitation suggests anaerobes, S. aureus, Klebsiella, or TB.
• CT chest is more sensitive and is the next step when the radiograph is normal but suspicion is high (early disease, neutropenia, dehydration) or when abscess/empyema is suspected. Lung ultrasound is a reasonable bedside alternative.

Step 2 — assess severity, because it dictates everything else

• CURB-65: Confusion, Urea (BUN) elevation, Respiratory rate ≥30, Blood pressure low (SBP <90 or DBP ≤60), age ≥65 — score 0–1 generally outpatient, 2 ward, ≥3 consider ICU.
• Pneumonia Severity Index (PSI/PORT): the ATS/IDSA 2019 guideline prefers PSI over CURB-65 for the admission decision because it is better validated at identifying low-risk patients.
• ATS/IDSA severe CAP criteria: one major criterion (mechanical ventilation, or septic shock requiring vasopressors) or three or more minor criteria (including RR ≥30, PaO2/FiO2 ≤250, multilobar infiltrates, confusion, uremia, leukopenia, thrombocytopenia, hypothermia, hypotension requiring fluids) → ICU-level care.

Step 3 — microbiology, targeted rather than reflexive

• Outpatient CAP: routine sputum and blood cultures are not recommended.
• **Severe CAP, or empiric MRSA/*Pseudomonas* coverage, or prior isolation of those organisms**: obtain blood cultures and a pre-antibiotic sputum Gram stain/culture (adequate specimen: many neutrophils, few squamous cells). Add MRSA nares PCR — its high negative predictive value permits early de-escalation.
• Urinary antigens: pneumococcal and Legionella antigen in severe CAP or suggestive epidemiology; Legionella urinary antigen detects serogroup 1 only and remains positive after antibiotics.
• Influenza and SARS-CoV-2 PCR during circulating seasons; *Mycoplasma*/*Chlamydia* via multiplex PCR.
• Procalcitonin should not be used to withhold initial antibiotics when imaging and clinical picture support pneumonia; it is a de-escalation aid.
• Thoracentesis with Light's criteria for effusions layering more than ~1 cm; pH <7.20, glucose low, or organisms on Gram stain mark a complicated effusion needing drainage.

Immediate stabilization

• Oxygen and airway support: titrate to an SpO2 in the low-to-mid 90s; escalate to high-flow nasal cannula or intubation for refractory hypoxemia. Shunt physiology means hypoxemia may not correct with nasal cannula alone.
• Sepsis care: blood cultures then prompt antibiotics, balanced crystalloid for hypotension, vasopressors (norepinephrine first-line) if fluid-refractory. Antibiotics should be given as soon as pneumonia is diagnosed — do not delay for microbiology.

First-line empiric therapy (ATS/IDSA 2019 CAP guideline)

• Outpatient, healthy, no comorbidities: amoxicillin, doxycycline, or a macrolide (azithromycin) where local pneumococcal macrolide resistance is low.
• Outpatient with comorbidities (heart, lung, liver, kidney disease, diabetes, alcohol use, malignancy, asplenia): a beta-lactam (amoxicillin-clavulanate or cefpodoxime) plus a macrolide or doxycycline, or a respiratory fluoroquinolone (levofloxacin, moxifloxacin) as monotherapy.
• Inpatient, non-severe: beta-lactam (ceftriaxone or ampicillin-sulbactam) plus a macrolide, or a respiratory fluoroquinolone alone.
• Severe/ICU: beta-lactam plus either a macrolide or a fluoroquinolone — monotherapy with a fluoroquinolone is not adequate here.

Escalation and targeted coverage

• MRSA: vancomycin or linezolid, added only for prior respiratory MRSA isolation, recent hospitalization with parenteral antibiotics, or severe necrotizing/post-influenza disease. Vancomycin is dosed to a 24-hour AUC targeting AUC/MIC 400–600 per the 2020 IDSA/ASHP consensus; the old 15–20 mcg/mL trough goal is retired.
• Pseudomonas: antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, or meropenem), guided by prior isolation and the local antibiogram — the basis of IDSA/ATS 2016 HAP/VAP therapy.
• Aspiration with abscess or empyema: add anaerobic coverage (ampicillin-sulbactam or a carbapenem); the 2019 guideline advises against routine anaerobic coverage for simple suspected aspiration pneumonitis.
• Influenza-positive: oseltamivir regardless of illness duration in hospitalized patients, with antibacterials continued.

Definitive/procedural

• Chest tube or image-guided drainage for empyema or complicated parapneumonic effusion; intrapleural fibrinolytic plus DNase for loculations; VATS decortication if drainage fails.

Duration and what to avoid

• Minimum 5 days, and no stopping until clinical stability (afebrile, stable vitals, normal mentation, tolerating oral intake).
• Not recommended: routine corticosteroids in non-severe CAP; routine follow-up chest radiographs in clinically improving patients; the retired HCAP category to justify blanket broad-spectrum therapy; doxycycline and fluoroquinolones in pregnancy and generally in children.

Pleural and parenchymal

• Parapneumonic effusion → empyema: cytokine-driven capillary leak into the pleural space; bacterial invasion converts it to frank pus. Signal: persistent fever on appropriate antibiotics with a layering effusion; pleural pH <7.20, low glucose, or positive Gram stain. Requires drainage — an emergency for source control, not more antibiotics alone.
• Lung abscess: liquefactive necrosis from anaerobes, S. aureus, or Klebsiella. Signal: air-fluid level within a cavity plus foul-smelling sputum; treat with prolonged antibiotics, drainage if refractory.
• Necrotizing pneumonia: toxin-mediated tissue destruction (Panton-Valentine leukocidin MRSA); may cause bronchopleural fistula.
• Post-obstructive pneumonia: recurrent consolidation in the same lobe in a smoker should prompt evaluation for endobronchial malignancy.

Systemic

• Sepsis and septic shock: cytokine-driven vasodilation and capillary leak — emergency; rising lactate and hypotension despite fluids.
• ARDS: diffuse alveolar damage with bilateral infiltrates and refractory hypoxemia not explained by heart failure — emergency, requires lung-protective low-tidal-volume ventilation.
• Metastatic infection: pneumococcal bacteremia seeding meninges, joints, or heart valves; Austrian syndrome is the triad of pneumococcal pneumonia, meningitis, and endocarditis.
• Hyponatremia: SIADH, classically with Legionella.
• Decompensation of comorbidity: pneumonia precipitates ACS, heart failure, and AF in older adults.

Treatment-related

• C. difficile colitis: antibiotic disruption of colonic flora; watery diarrhea and leukocytosis, especially after fluoroquinolones or clindamycin.
• QT prolongation and torsades: macrolides and fluoroquinolones block IKr — check for hypokalemia and other QT-prolonging drugs.
• Fluoroquinolone class toxicity: tendinopathy/rupture, peripheral neuropathy, CNS effects, and aortic aneurysm/dissection risk (FDA warnings).
• Vancomycin nephrotoxicity: risk rises with high AUC exposure and concomitant piperacillin-tazobactam — another reason for AUC-guided dosing.
• Linezolid: myelosuppression and serotonin syndrome with SSRIs.

• The single best next step in a patient with cough, fever, and focal crackles is a chest radiograph — pneumonia is a clinical-plus-radiographic diagnosis, and a normal film in a high-suspicion patient warrants CT rather than empiric reassurance.
• Exam–imaging mismatch means atypical: a young adult with weeks of dry cough, headache, and bilateral patchy infiltrates but an unimpressive exam is Mycoplasma until proven otherwise; beta-lactams fail because Mycoplasma has no cell wall. Treat with a macrolide or doxycycline.
• **Hyponatremia + diarrhea + confusion + high fever = *Legionella*. Best next test is the urinary antigen** (serogroup 1); it stays positive after antibiotics have started.
• Severity score drives disposition, not antibiotic potency: CURB-65 or, as ATS/IDSA 2019 prefers, the PSI. Severe CAP (one major or ≥3 minor criteria) requires a beta-lactam plus a macrolide or fluoroquinolone — never fluoroquinolone monotherapy in the ICU.
• The association examiners love: post-influenza pneumonia with rapid deterioration and cavitation = Staphylococcus aureus, including MRSA; add vancomycin (AUC-guided, AUC/MIC 400–600) or linezolid.
• Aspiration localizes by gravity: right lower lobe if upright, posterior segment of the right upper lobe or superior segment of the RLL if supine — the right main bronchus is wider and more vertical.
• Persistent fever on day 3–4 of appropriate antibiotics is not "antibiotic failure" first — image for a complicated parapneumonic effusion or empyema and drain it; this is the source-control emergency.
• Common distractors to avoid: do not resurrect "HCAP" to justify blanket MRSA/*Pseudomonas* coverage (ATS/IDSA retired it — use prior isolation and local risk factors); do not use procalcitonin to withhold initial antibiotics; do not order routine follow-up chest films in a patient who is improving; and do not treat every consolidation as bacterial when a viral panel and clinical course say otherwise.

• CAP (Community-Acquired Pneumonia) most common; HAP (Hospital-Acquired) and VAP (Ventilator-Associated) have different organisms
• Streptococcus pneumoniae is most common bacterial cause of CAP overall
• Chest X-ray shows lobar consolidation (bacterial) vs interstitial infiltrates (viral/atypical)
• Atypical pneumonias (Mycoplasma, Chlamydia, Legionella) classically present with minimal findings on exam despite significant CXR changes
• CURB-65 or PSI score determines severity and inpatient vs outpatient management

• Assuming all pneumonia is bacterial: ~20% of CAP is viral; don't over-treat with antibiotics; consider respiratory viruses, Mycoplasma, Chlamydia
• Missing atypical presentations: Atypical organisms have exam findings that seem minimal relative to CXR severity—high clinical suspicion needed
• Incorrect empiric coverage: Don't forget atypical coverage (macrolide or fluoroquinolone) in CAP; Mycoplasma and Chlamydia are beta-lactam resistant