Harrison 22 edition 131 pnemonia and 132 lung abscess

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Harrison's Principles of Internal Medicine, 22nd Edition (2025)

Chapter 131 - Pneumonia

Lionel A. Mandell, Michael S. Niederman

Definition

Pneumonia is infection of the pulmonary parenchyma. It is classified as:

Community-acquired (CAP)
Hospital-acquired (HAP)
Ventilator-associated (VAP)

The former category "healthcare-associated pneumonia (HCAP)" has been discontinued - it did not reliably predict resistant pathogens and drove overuse of broad-spectrum antibiotics. Individual patient risk assessment is now preferred.

Aspiration pneumonia accounts for 5-15% of CAP cases and is best viewed as a continuum with CAP/HAP, not a separate category.

Pathophysiology

Pneumonia is not simply invasion of a sterile space. The lung has its own microbiota, and pneumonia is an emergent phenomenon from disruption of this homeostasis:

An inflammatory trigger (often viral infection + microaspiration) causes epithelial/endothelial injury
Released cytokines, chemokines, and catecholamines selectively promote growth of specific bacteria (e.g., S. pneumoniae, P. aeruginosa)
A positive feedback loop forms: inflammation → nutrient availability → bacterial overgrowth → more inflammation
If innate/adaptive immunity cannot contain this, full pneumonia develops

Key mediators:

IL-6, TNF → fever
IL-8, G-CSF → neutrophil recruitment
Capillary leakage → alveolar edema (basis of infiltrate on imaging)

Community-Acquired Pneumonia (CAP)

Etiology

Setting	Common Pathogens
Outpatient	S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, respiratory viruses
Inpatient (non-ICU)	S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, viruses, aspiration, Legionella
ICU	S. pneumoniae, S. aureus, Legionella, gram-negative rods, H. influenzae

MRSA and P. aeruginosa are uncommon in typical CAP but must be considered in patients with prior respiratory isolates or recent hospitalization + antibiotics.

Clinical Diagnosis

Requires: compatible symptoms (cough, sputum, fever, dyspnea) + new infiltrate on CXR
Physical exam sensitivity/specificity: only ~58%/67%
CT useful for: cavitation, loculated effusion, post-obstructive pneumonia
Severity scores: PSI (Pneumonia Severity Index) and CURB-65 (Confusion, Urea >7, RR ≥30, BP <90/60, age ≥65) guide site-of-care decisions

Criteria for ICU admission (major criteria):

Septic shock requiring vasopressors
Respiratory failure requiring mechanical ventilation

Minor criteria (≥3 = severe): RR ≥30, PaO2/FiO2 ≤250, multilobar infiltrates, confusion, BUN ≥20, WBC <4,000, platelets <100,000, temp <36°C, hypotension requiring IV fluids.

Etiologic Diagnosis

Sputum Gram stain/culture: recommended only for hospitalized patients, especially severe CAP or risk of MRSA/P. aeruginosa; valid specimen requires >25 neutrophils and <10 squamous cells per LPF
Blood cultures: only 5-14% positive; most common isolate is S. pneumoniae
Urinary antigens: S. pneumoniae (sensitive + specific) and Legionella (detects serogroup 1 only, ~80% of community cases)
Procalcitonin: may help guide antibiotic duration (not initiation)
Multiplex PCR panels: approved for respiratory pathogen detection

Treatment - CAP

Fig. 131-1 Algorithm: Assessment of MRSA/P. aeruginosa risk

Outpatients (no comorbidities):

Amoxicillin alone OR doxycycline (covers atypicals too)

Outpatients (with comorbidities - cardiac, pulmonary, renal, liver, DM, alcoholism, asplenia, malignancy):

Respiratory fluoroquinolone (levofloxacin, moxifloxacin) OR
Beta-lactam + macrolide

Inpatients (non-severe):

Beta-lactam + macrolide OR respiratory fluoroquinolone

Inpatients (severe CAP - ICU):

Beta-lactam + macrolide OR beta-lactam + respiratory fluoroquinolone
If MRSA risk: add vancomycin or linezolid
If P. aeruginosa risk: use anti-pseudomonal beta-lactam

Aspiration pneumonia: Routine anaerobic coverage is unnecessary unless poor dentition, lung abscess, or necrotizing pneumonia is present.

Duration: Typically 5 days for CAP (if clinical stability achieved and afebrile for 48-72h). Procalcitonin-guided protocols may shorten courses further.

Switch from IV to Oral

Criteria: improving cough/dyspnea, afebrile on ≥2 measurements 8h apart, WBC trending down, functioning GI tract, no concern for non-oral pathogens.

Failure to Respond

Consider: wrong diagnosis, resistant organism, unusual pathogen, non-infectious cause (PE, malignancy, vasculitis), empyema, metastatic infection. Repeat imaging and bronchoscopy as needed.

Antimicrobial Resistance

Penicillin-resistant pneumococcus: MIC ≥2 µg/mL; respiratory fluoroquinolones, vancomycin, or linezolid for high-level resistance
Macrolide-resistant M. pneumoniae: up to 95% in China, 5-13% in US/France; due to 23S rRNA domain V mutation
ESBL gram-negatives: use carbapenem

Ventilator-Associated Pneumonia (VAP)

Epidemiology

Affects 6-52% of ventilated patients; ~10% of ICU patients on any given day
Highest hazard ratio in the first 5 days; cumulative rate up to 70% for patients ventilated ≥30 days
COVID-19 has increased VAP frequency after years of decline

Pathogenesis (3 critical factors)

Colonization of oropharynx with pathogens
Aspiration into lower respiratory tract (microaspiration around cuff)
Compromise of host defenses (immunoparalysis in sepsis/trauma)

The endotracheal tube is the most important risk factor: bypasses normal mechanical defenses, promotes biofilm formation on its surface, and microaspiration is worsened by secretions pooling above the cuff.

Clinical Manifestations

Nonspecific: fever, leukocytosis, increased secretions, pulmonary infiltrate, declining oxygenation. New or worsening infiltrate + ≥2 of: fever (>38°C) or hypothermia (<36°C), leukocytosis (>10,000) or leukopenia (<4,000), purulent secretions.

Diagnosis

Clinical Pulmonary Infection Score (CPIS) adds sputum character, oxygenation, CXR, tracheal cultures to quantify likelihood
Quantitative cultures (BAL >10⁴, mini-BAL >10⁴, protected brush >10³ CFU/mL): reduce antibiotic use but do not improve mortality vs. qualitative cultures
Blood cultures usually negative (<15%)

Treatment

Empirical broad-spectrum initially; de-escalate based on cultures at 48-72h
Duration: 7-8 days (even for gram-negatives); short courses reduce resistance without increasing mortality
For P. aeruginosa and Acinetobacter: combination not superior to monotherapy in non-neutropenic patients, but often used empirically

Prevention

Avoid or minimize intubation duration - most effective strategy
Elevate head of bed ≥30° (preferably 45°)
Daily sedation holds + formal weaning protocols
Subglottic secretion drainage via specialized endotracheal tubes
Closed-suction catheter systems
Ventilator circuits changed only when soiled, not on schedule
Hand hygiene emphasized to reduce cross-infection

Hospital-Acquired Pneumonia (HAP)

Similar to VAP but lower frequency of MDR pathogens (better outcomes)
Anaerobes slightly more common (macroaspiration risk)
Diagnosis harder: lower respiratory samples difficult to obtain from non-intubated patients
Blood cultures positive in <15%
Monotherapy feasible more often than in VAP

Chapter 132 - Lung Abscess

Rebecca M. Baron, Beverly W. Baron, Miriam Baron Barshak

Definition

Lung abscess = necrosis and cavitation of lung following microbial infection, usually a single dominant cavity >2 cm.

Classification

Type	Details
Primary (~80%)	Aspiration-related, anaerobic bacteria, no underlying condition
Secondary (~20%)	Underlying condition (post-obstructive, immunocompromise, septic emboli)
Acute	<4-6 weeks duration
Chronic	~40% of cases; >4-6 weeks

Etiology and Microbiology (Table 132-1)

Clinical Setting	Pathogens
Primary (aspiration)	Anaerobes (Peptostreptococcus, Prevotella, Bacteroides, milleri group streptococci), microaerophilic streptococci
Secondary (immunocompromised)	S. aureus, P. aeruginosa, Enterobacteriaceae, Nocardia, Aspergillus, Mucorales, Cryptococcus, Legionella, Rhodococcus equi, P. jirovecii
Septic emboli	S. aureus (endocarditis), Fusobacterium necrophorum (Lemierre's syndrome)
Endemic infections	M. tuberculosis, M. avium, M. kansasii, Coccidioides, Histoplasma capsulatum

Key microbiology points:

Primary abscesses often polymicrobial; no pathogen isolated in up to 40% (termed "nonspecific lung abscess")
Putrid lung abscess (foul-smelling breath/sputum) = essentially diagnostic of anaerobic infection
Right lung more commonly affected due to less angulated right mainstem bronchus
Dependent segments most common location: posterior upper lobes and superior lower lobes

Epidemiology and Risk Factors

Incidence decreased since antibiotic era
Middle-aged men more commonly affected
Major risk factor: aspiration
High-risk patients: altered mental status, alcoholism, drug overdose, seizures, bulbar dysfunction, CVA, neuromuscular disease, esophageal dysmotility/lesions, GERD, recumbent position
Gingivitis/periodontal disease critical: anaerobic colonization of gingival crevices + aspiration risk = abscess formation. Lung abscess is considered extremely rare in edentulous patients.

Historical note: The incidence dropped sharply in the late 1940s when oral surgery changed from seated (no ETT) to supine with cuffed ETT, and again when penicillin was introduced.

Pathology

Lemierre's syndrome: pharyngeal infection (F. necrophorum) → septic thrombophlebitis of internal jugular vein → septic emboli → multiple lung abscesses
Septic emboli from tricuspid valve endocarditis (IVDU + S. aureus)

Clinical Manifestations

Acute presentation: fever, cough, sputum production, chest pain (similar to pneumonia initially)
Chronic/indolent presentation: night sweats, fatigue, weight loss, hemoptysis (especially in secondary abscesses or those with underlying malignancy)
Foul-smelling sputum = anaerobic etiology
Chest exam: may reveal signs of consolidation or pleural effusion
CXR/CT: cavity with air-fluid level is classic

Diagnosis

CXR: cavity with air-fluid level, usually in dependent segments
CT chest: better characterization of cavity, wall thickness, associated findings; helps exclude endobronchial lesion or malignancy
Sputum Gram stain/culture (aerobic + anaerobic)
Bronchoscopy (early): for patients with possible bronchial obstruction (foreign body, tumor), atypical presentation, or immunocompromised hosts
CT-guided needle aspiration: in patients with risk factors for malignancy or atypical features
Induced sputum for AFB: in patients from TB-endemic areas or with HIV risk factors

Approach:

Low malignancy risk + aspiration risk factors → empirical treatment first, further workup if no response
High malignancy risk or immunocompromised → early bronchoscopy + biopsy or CT-guided aspiration

Treatment

Medical therapy (mainstay):

Amoxicillin-clavulanate or clindamycin (historically gold standard for anaerobes; but clindamycin now second-line due to C. diff risk)
Beta-lactam/beta-lactamase inhibitors are first-line for anaerobic coverage
Moxifloxacin has shown efficacy comparable to ampicillin-sulbactam in aspiration pneumonia/primary lung abscess
Duration: prolonged - typically 3-6 weeks or until radiographic resolution (cavity closure or small stable residual lesion)
Secondary abscesses: culture-directed therapy essential

Drainage:

Most primary abscesses drain spontaneously via the tracheobronchial tree
Percutaneous catheter drainage: indicated for abscesses not responding to medical therapy; a systematic review (Lee et al, 2022) supported this approach
Bronchoscopic drainage: useful when associated with obstructing lesion
Surgical resection (lobectomy): reserved for failure of medical/percutaneous therapy, complications (massive hemoptysis, bronchopleural fistula), or when malignancy cannot be excluded

Complications

Pneumatoceles or bronchiectasis (persistent cystic changes, especially with large cavities)
Empyema (extension to pleural space)
Life-threatening hemoptysis
Massive aspiration of abscess contents

Prognosis

Type	Mortality
Primary abscess	As low as 2%
Secondary abscess	Up to 75% (case series)

Poor prognostic factors:

Age >60
Malignancy-related abscess
Aerobic bacteria as cause
Sepsis at presentation
Symptom duration >8 weeks
Abscess size >6 cm

Prevention

Airway protection in at-risk patients
Oral hygiene (treat periodontal disease)
Minimize sedation; elevate head of bed
Antimicrobial prophylaxis in highly immunocompromised patients (BMT, solid organ transplant, advanced HIV)

Source: Harrison's Principles of Internal Medicine, 22nd Edition (2025, McGraw Hill Medical) - Chapters 131 and 132

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