# Lecture 9: Pulmonary Infections ## Unit 1.8: Respiratory System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Classify pneumonias by clinical setting and etiology 2. Describe the pathophysiology and clinical features of community-acquired pneumonia 3. Explain the diagnosis and management of bacterial pneumonias 4. Describe atypical pneumonias and their distinguishing features 5. Explain the pathophysiology, diagnosis, and treatment of tuberculosis 6. Describe fungal and viral respiratory infections --- ## Overview of Pulmonary Infections Pulmonary infections represent one of the most common causes of infectious disease morbidity and mortality worldwide. Understanding their classification, microbiology, and management is essential for clinical practice. ### Classification by Clinical Setting The setting in which a pneumonia is acquired provides crucial information about likely pathogens and guides empiric therapy selection. Community-acquired pneumonia (CAP) develops in patients outside healthcare facilities and typically involves different organisms than nosocomial infections. Hospital-acquired pneumonia (HAP) develops at least 48 hours after hospital admission and often involves more resistant organisms due to healthcare exposure. Ventilator-associated pneumonia (VAP) represents a subset of HAP occurring 48 or more hours after endotracheal intubation, with unique risk factors related to bypassing normal airway defenses. Healthcare-associated pneumonia occurs in patients with significant healthcare contact, such as nursing home residents, dialysis patients, or those recently hospitalized, who are at risk for resistant pathogens. ### Classification by Radiographic Pattern Pulmonary infections produce characteristic patterns on chest imaging that reflect underlying pathophysiology. Lobar pneumonia presents as consolidation affecting an entire lobe, typically caused by organisms like Streptococcus pneumoniae that spread through alveolar pores. Bronchopneumonia appears as patchy, multifocal infiltrates centered on airways, commonly seen with Staphylococcus aureus or gram-negative organisms. Interstitial pneumonia produces diffuse reticular or ground-glass infiltrates, characteristic of viral infections and atypical bacteria. ### Host Defense Mechanisms The respiratory tract employs multiple layers of defense against inhaled pathogens. The upper airway provides mechanical filtration through turbulent airflow and particle impaction on nasal mucosa. Mucociliary clearance continuously transports trapped particles upward toward the pharynx. In the lower airways, the cough reflex expels foreign material while resident immune cells survey for pathogens. At the alveolar level, macrophages serve as the primary phagocytic defense, assisted by surfactant proteins with antimicrobial properties. Systemic defenses including complement, antibodies, and recruited inflammatory cells provide backup when local mechanisms are overwhelmed. Panel A: Three chest X-ray patterns of pneumonia classification: lobar pneumonia with dense consolidation filling the right lower lobe, bronchopneumonia with scattered patchy infiltrates bilaterally, and interstitial pneumonia with diffuse bilateral reticular markings. Panel B: Vertical cross-section of the respiratory tract from nasopharynx to alveoli showing defense mechanisms at each level including nasal hair filtration, ciliated epithelium with mucus layer, and cough reflex at the trachea. Panel C: Enlarged alveolar detail showing resident immune defenses including macrophages with pseudopods, surfactant proteins, and complement molecules. Panel D: Directional arrows illustrating mucociliary clearance transporting trapped particles upward toward the pharynx for elimination. --- ## Community-Acquired Pneumonia Community-acquired pneumonia affects 5 to 11 per 1,000 adults annually and remains a leading cause of infection-related hospitalization and death. Approximately 20% of patients require hospital admission, with mortality ranging from 1% in outpatients to 5-15% in hospitalized patients. ### Common Pathogens Streptococcus pneumoniae remains the most frequently identified bacterial cause of CAP, producing classic lobar pneumonia with rusty-colored sputum due to blood mixing with purulent secretions. Haemophilus influenzae commonly causes pneumonia in patients with underlying COPD, as chronic airway inflammation provides favorable conditions for colonization. Moraxella catarrhalis similarly affects COPD patients and the elderly. Staphylococcus aureus pneumonia often follows influenza infection, as viral damage to respiratory epithelium permits bacterial invasion, and characteristically produces cavitary lesions. Klebsiella pneumoniae targets patients with alcoholism, producing a distinctive thick, bloody "currant jelly" sputum and frequently causing lung abscesses. Atypical organisms including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila account for a significant proportion of cases, particularly in younger patients. Viral etiologies including influenza, RSV, and SARS-CoV-2 may cause primary pneumonia or predispose to secondary bacterial infection. ### Pathophysiology Lobar pneumonia progresses through four classic pathological stages. During the congestion phase, edema fluid and bacteria accumulate in alveoli as the inflammatory response begins. Red hepatization follows as the alveolar spaces fill with red blood cells, fibrin, and neutrophils, giving the lung a liver-like consistency. In gray hepatization, red cells degrade while the fibrinopurulent exudate persists, producing a grayish appearance. Resolution occurs as macrophages clear debris and normal alveolar architecture is restored. ### Clinical Presentation Typical bacterial pneumonia presents with abrupt onset of fever, shaking chills, and productive cough with purulent or rusty sputum. Dyspnea develops proportional to the extent of lung involvement. Pleuritic chest pain—sharp and worsened by breathing—indicates pleural inflammation. Physical examination reveals crackles (rales) over the affected area, bronchial breath sounds when consolidation transmits large airway sounds to the periphery, dullness to percussion from fluid and cellular material replacing air, and egophony (E-to-A change) at the border between consolidated and aerated lung. Panel A: Four histological cross-sections of alveoli representing the stages of lobar pneumonia: congestion with pink edema fluid and scattered bacteria, red hepatization with alveoli packed with red cells and fibrin, gray hepatization with degraded cells and persistent fibrin, and resolution with macrophages clearing debris. Panel B: Anterior view of a patient's chest with stethoscope placement and percussion diagram indicating dullness over the consolidated right lower lobe. Panel C: Audio waveform representations of auscultatory findings: crackles as an irregular spiky pattern, bronchial breath sounds as a loud tubular pattern, and egophony showing distorted E-to-A change. Panel D: Comparison of normal alveolar architecture versus consolidated alveoli showing progression through each pathological stage with cellular composition labeled. --- ## CAP Diagnosis and Management ### Diagnostic Evaluation All patients with suspected pneumonia should undergo chest radiography to confirm the diagnosis, assess extent of disease, and identify complications such as effusion or abscess. Hospitalized patients should have blood cultures drawn before antibiotics, though yield is only 5-14%. Sputum culture is helpful when a quality sample can be obtained, defined by fewer than 10 epithelial cells and more than 25 neutrophils per low-power field. Urinary antigen testing for Streptococcus pneumoniae and Legionella pneumophila serogroup 1 provides rapid results in severely ill patients. Procalcitonin levels help distinguish bacterial infection (elevated) from viral causes (low), though clinical judgment remains essential. CT chest is reserved for complicated cases or diagnostic uncertainty. ### Severity Assessment The CURB-65 score guides disposition decisions using five clinical criteria. One point is assigned for each of: Confusion (new disorientation), Urea greater than 7 mmol/L (BUN greater than 20 mg/dL), Respiratory rate 30 or higher, Blood pressure below 90 systolic or 60 or less diastolic, and age 65 years or older. Patients scoring 0-1 can typically be treated as outpatients. A score of 2 indicates increased severity warranting consideration of hospital admission. Scores of 3-5 predict high mortality and require inpatient care, with scores of 4 or higher suggesting ICU-level monitoring. ### Antibiotic Selection Treatment selection depends on the care setting and patient risk factors. For healthy outpatients without comorbidities or recent antibiotic use, amoxicillin monotherapy or a macrolide (azithromycin) provides adequate coverage. Outpatients with comorbidities such as diabetes, heart disease, or chronic lung disease should receive either a respiratory fluoroquinolone (levofloxacin, moxifloxacin) or a beta-lactam combined with a macrolide to cover resistant and atypical organisms. Hospitalized non-ICU patients receive similar combination therapy. ICU patients require more intensive regimens combining a beta-lactam (ceftriaxone, ampicillin-sulbactam, or cefotaxime) with either a macrolide or fluoroquinolone. When MRSA risk factors are present (prior colonization, recent influenza, cavitary disease), vancomycin or linezolid should be added. Pseudomonas coverage with an antipseudomonal beta-lactam is indicated for structural lung disease or severe immunocompromise. Standard treatment duration is 5-7 days for uncomplicated pneumonia, extending to 7-14 days for complicated cases including those with empyema, abscess, or certain pathogens. Panel A: Clinical decision flowchart beginning with chest X-ray for suspected CAP, branching by CURB-65 score into outpatient (0-1), consider admission (2), and inpatient with possible ICU (3-5) pathways. Panel B: Sidebar listing CURB-65 criteria with checkboxes: Confusion, Urea, Respiratory rate, Blood pressure, and age 65 or older. Panel C: Outpatient antibiotic regimens showing amoxicillin or macrolide for healthy patients and fluoroquinolone or beta-lactam plus macrolide for patients with comorbidities. Panel D: Inpatient antibiotic regimens showing beta-lactam plus macrolide or fluoroquinolone, with additional boxes for vancomycin if MRSA risk and antipseudomonal coverage if Pseudomonas risk. --- ## Atypical Pneumonias Atypical pneumonias differ from typical bacterial pneumonia in presentation, radiographic appearance, and treatment. They characteristically present with gradual onset, low-grade fever, dry nonproductive cough, prominent extrapulmonary symptoms, and relatively normal white blood cell counts despite extensive radiographic infiltrates. ### Mycoplasma pneumoniae Mycoplasma pneumonia predominantly affects young adults and often occurs in outbreaks within close communities such as college dormitories or military barracks. The onset is gradual over several days with prominent constitutional symptoms including headache, malaise, and sore throat. The cough is characteristically dry and hacking, sometimes persisting for weeks. Extrapulmonary manifestations are common and diagnostically helpful, including bullous myringitis (hemorrhagic blisters on the tympanic membrane), cold agglutinin-induced hemolytic anemia, and erythema multiforme skin lesions. A classic feature is that the chest X-ray often looks worse than the patient appears clinically, showing patchy interstitial infiltrates that seem out of proportion to mild physical findings. Diagnosis relies on PCR or serologic testing, as the organism cannot be cultured on routine media. Treatment options include macrolides, doxycycline, or respiratory fluoroquinolones. ### Chlamydophila pneumoniae Chlamydophila pneumoniae affects all age groups and causes a prolonged illness characterized by pharyngitis, hoarseness, and persistent cough. The course tends to be subacute, with symptoms lasting weeks if untreated. Treatment is the same as for Mycoplasma. ### Legionella pneumophila Legionella causes a distinctive pneumonia acquired from environmental water sources rather than person-to-person transmission. Outbreaks are associated with contaminated cooling towers, hospital water systems, hot tubs, and decorative fountains. Risk factors include smoking, older age, and immunocompromised states. Unlike other atypical pneumonias, Legionella often causes severe disease with high fevers, prominent gastrointestinal symptoms (diarrhea, nausea), and neurological manifestations including confusion. Laboratory abnormalities include hyponatremia and elevated liver enzymes. Urinary antigen testing detects serogroup 1, which causes most clinical infections, though culture on specialized media remains the gold standard. First-line treatment is a fluoroquinolone or azithromycin. Panel A: Mycoplasma pneumoniae presentation in a young adult with dormitory exposure icon, bullous myringitis on the eardrum, erythema multiforme lesions on the forearm, and chest X-ray showing patchy bilateral infiltrates disproportionate to clinical appearance. Panel B: Chlamydophila pneumoniae presentation in a middle-aged patient with pharyngitis icon and clock indicating prolonged subacute course. Panel C: Legionella pneumophila presentation in an older patient with cooling tower exposure icon, laboratory values showing hyponatremia and elevated liver enzymes, confusion symbols, and gastrointestinal symptom icons. Panel D: Comparison table of typical versus atypical pneumonia features across onset, fever pattern, sputum character, chest X-ray pattern, and white blood cell count. --- ## Tuberculosis Tuberculosis remains a global public health challenge, causing approximately 10 million new cases annually. Understanding its unique pathophysiology is essential for diagnosis and management. ### Epidemiology and Transmission Mycobacterium tuberculosis spreads through airborne droplet nuclei generated when infected individuals cough, sneeze, or speak. These tiny particles remain suspended in air and can be inhaled deep into the respiratory tract. Risk factors for infection include HIV coinfection, which dramatically increases susceptibility, immunosuppressive therapy, crowded living conditions, malnutrition, and healthcare work with inadequate protection. ### Pathophysiology Primary tuberculosis occurs when inhaled bacilli reach the middle or lower lung zones, where they are engulfed by alveolar macrophages. Rather than being killed, the organisms survive and replicate within macrophages. The immune response generates a characteristic granuloma composed of infected macrophages transformed into epithelioid cells, surrounded by lymphocytes, with multinucleated Langhans giant cells formed by macrophage fusion. The combination of the primary lung focus (Ghon focus) with involved draining lymph nodes constitutes the Ghon complex. In most immunocompetent individuals, granulomas successfully contain the infection, which enters a latent state. The tuberculin skin test becomes positive, indicating immune memory without active disease. Reactivation tuberculosis occurs when cell-mediated immunity wanes, allowing dormant organisms to resume replication. Reactivation characteristically affects the lung apices, where high oxygen tension favors mycobacterial growth. The immune response causes tissue destruction and cavity formation, which provides an oxygen-rich environment for massive bacterial proliferation. Patients become highly infectious at this stage. If cell-mediated immunity fails completely, disseminated (miliary) tuberculosis results, with widespread hematogenous spread producing millet-seed-sized granulomas throughout multiple organs. ### Clinical Features and Comparison Primary tuberculosis is often asymptomatic or produces mild, nonspecific symptoms. Chest radiograph may show a middle or lower lobe infiltrate with prominent hilar lymphadenopathy. Cavitation is rare in primary disease. Reactivation tuberculosis presents with chronic constitutional symptoms: night sweats that drench bedclothes, unintentional weight loss, low-grade fevers, and chronic productive cough, often with hemoptysis. Imaging shows upper lobe infiltrates, frequently with cavitary lesions. Lymphadenopathy is minimal in reactivation disease. ### Diagnosis Screening for latent tuberculosis infection uses either the tuberculin skin test (TST/PPD) or interferon-gamma release assays (IGRAs). TST positivity thresholds vary by risk: 5 mm for HIV-infected or immunosuppressed patients and close contacts, 10 mm for high-risk groups such as healthcare workers and immigrants from endemic areas, and 15 mm for low-risk individuals. IGRAs offer greater specificity, particularly in BCG-vaccinated populations. Active tuberculosis diagnosis relies on demonstrating the organism. Sputum smear for acid-fast bacilli provides rapid results but limited sensitivity. Culture remains the gold standard but requires 2-6 weeks. Nucleic acid amplification tests such as GeneXpert provide rapid results and simultaneously detect rifampin resistance, guiding therapy selection. Chest radiograph typically shows upper lobe infiltrates with or without cavitation. Panel A: Primary tuberculosis infection showing inhaled droplet nuclei traveling to the mid-lung zone, macrophages engulfing bacilli, and granuloma formation with central caseous necrosis surrounded by epithelioid cells, Langhans giant cells, and peripheral lymphocytes. Panel B: Reactivation tuberculosis showing an upper lobe cavity with thick wall, magnified view of massive bacterial replication within a fibrous capsule. Panel C: Two chest X-rays comparing primary TB with lower lobe infiltrate and prominent hilar lymphadenopathy versus reactivation TB with apical cavitary infiltrates in the upper lobes. Panel D: Diagnostic modalities including tuberculin skin test with wheal measurement on the forearm, interferon-gamma release assay blood tube, and GeneXpert cartridge device for rapid molecular testing. --- ## Tuberculosis Treatment ### Latent Tuberculosis Treatment of latent tuberculosis infection prevents progression to active disease, which occurs in approximately 5-10% of infected immunocompetent individuals over their lifetime (much higher in HIV coinfection). Several regimens are effective: isoniazid daily for 9 months has the longest track record; rifampin daily for 4 months offers a shorter alternative; and isoniazid plus rifapentine weekly for 3 months (12 doses total) provides the shortest supervised regimen. ### Active Drug-Susceptible Tuberculosis Active tuberculosis requires multidrug therapy to prevent resistance emergence. The standard regimen uses the mnemonic RIPE: Rifampin, Isoniazid, Pyrazinamide, and Ethambutol. Treatment proceeds in two phases: an intensive phase of 2 months with all four drugs eliminates the bulk of organisms and prevents resistance, followed by a continuation phase of 4 months with rifampin and isoniazid to sterilize remaining bacteria. Total treatment duration is 6 months minimum for pulmonary disease, longer for extrapulmonary or drug-resistant cases. ### Drug-Specific Considerations Each medication has characteristic toxicities requiring monitoring. Rifampin induces hepatic cytochrome P450 enzymes, causing numerous drug interactions, and produces harmless orange discoloration of body fluids. Hepatotoxicity is the most serious concern. Isoniazid causes peripheral neuropathy through pyridoxine (vitamin B6) depletion, prevented by B6 supplementation, particularly in malnourished patients, pregnant women, and those with diabetes. Hepatotoxicity increases with age and alcohol use. Pyrazinamide causes hepatotoxicity and elevates uric acid, sometimes precipitating gout. Ethambutol produces dose-dependent optic neuritis; patients require baseline and periodic visual acuity and color vision testing. ### Special Situations Multidrug-resistant TB (MDR-TB) is defined by resistance to both isoniazid and rifampin, requiring prolonged treatment with second-line agents. Extensively drug-resistant TB (XDR-TB) adds resistance to fluoroquinolones and at least one injectable agent, presenting extreme treatment challenges. HIV-TB coinfection requires coordinating antiretroviral and antituberculous therapy while monitoring for immune reconstitution inflammatory syndrome (IRIS). Airborne infection isolation in a negative-pressure room with N95 respirators for healthcare workers is essential until patients achieve at least three consecutive negative sputum smears. Panel A: Treatment timeline showing the intensive phase (months 1-2) with four RIPE tablets color-coded for rifampin, isoniazid, pyrazinamide, and ethambutol, followed by the continuation phase (months 3-6) with rifampin and isoniazid only. Panel B: Drug-specific side effects showing rifampin with orange body fluid discoloration and hepatotoxicity, isoniazid with peripheral neuropathy and vitamin B6 supplementation, pyrazinamide with hepatotoxicity and gout, and ethambutol with optic neuritis and visual acuity chart. Panel C: Infection control measures including a negative-pressure room diagram with directional airflow arrows and an N95 respirator for healthcare workers. Panel D: Sputum monitoring showing three collection containers marked with negative checkmarks indicating the requirement for three consecutive negative smears before discontinuing isolation. --- ## Fungal Pulmonary Infections ### Endemic Mycoses Certain fungi cause infection primarily in specific geographic regions where environmental conditions favor their growth. Histoplasma capsulatum is endemic to the Ohio and Mississippi River valleys, where it grows in soil contaminated with bird or bat droppings. Exposure occurs when disturbing contaminated sites such as caves, old buildings, or chicken coops. Coccidioides immitis inhabits desert soils of the southwestern United States and causes "Valley fever" when spores become airborne during dust storms or excavation. Blastomyces dermatitidis is found in moist soil near waterways in the Great Lakes region and southeastern states. ### Histoplasmosis Most Histoplasma infections are asymptomatic or cause a mild flu-like illness that resolves spontaneously. Acute pulmonary histoplasmosis may produce fever, cough, chest pain, and fatigue lasting several weeks. Chronic pulmonary histoplasmosis resembles tuberculosis, occurring in patients with underlying emphysema and producing progressive cavitary upper lobe disease. Disseminated histoplasmosis occurs in immunocompromised patients, particularly those with advanced HIV, causing hepatosplenomegaly, pancytopenia, and multiorgan involvement. Diagnosis uses urinary and serum antigen testing or tissue culture. Treatment for mild disease is often unnecessary; moderate to severe disease requires itraconazole, while life-threatening illness necessitates amphotericin B. ### Coccidioidomycosis Primary coccidioidomycosis manifests as "Valley fever" 1-3 weeks after exposure, with fever, cough, chest pain, and characteristic arthralgias (desert rheumatism) and erythema nodosum (tender red nodules on the shins). Chest imaging may show nodules, cavities, or hilar adenopathy. Most cases resolve without treatment. Disseminated disease occurs in immunocompromised patients and those of Filipino or African descent, and may involve skin, bones, and meninges. Diagnosis relies on serology; culture is hazardous due to high infectivity of the organism. Treatment with fluconazole is indicated for persistent or severe disease. ### Opportunistic Fungal Infections Immunocompromised patients are susceptible to fungal infections that rarely affect healthy individuals. Aspergillus species cause invasive aspergillosis in severely neutropenic patients and those on high-dose corticosteroids, producing nodular or cavitary lung lesions with the characteristic "halo sign" on CT. In patients with asthma or cystic fibrosis, Aspergillus causes allergic bronchopulmonary aspergillosis (ABPA), an allergic response with bronchiectasis. Cryptococcus neoformans primarily affects HIV-infected individuals with low CD4 counts, typically presenting as meningitis but also causing pulmonary nodules. Pneumocystis jirovecii (formerly P. carinii) causes Pneumocystis pneumonia (PCP) in patients with AIDS when CD4 counts fall below 200 cells/μL, and increasingly in patients on immunosuppressive therapy. The presentation is subacute with gradually progressive dyspnea, dry cough, and fever over days to weeks. Chest imaging shows bilateral diffuse ground-glass infiltrates. Diagnosis requires demonstration of organisms in induced sputum or bronchoalveolar lavage using silver stains. Treatment is trimethoprim-sulfamethoxazole; adjunctive corticosteroids are added when hypoxemia is significant (PaO2 less than 70 mmHg). Primary prophylaxis with TMP-SMX is indicated when CD4 drops below 200. Panel A: Map of the United States with color-coded endemic zones for fungal infections: Ohio-Mississippi River valleys for Histoplasma with cave and bat icons, Southwest for Coccidioides with desert landscape, and Great Lakes/Southeast for Blastomyces with waterway icon. Panel B: Histoplasma features including yeast forms within macrophages, chest X-ray with apical cavity, and hepatosplenomegaly; Coccidioides features including spherules with endospores, erythema nodosum on the shin, and arthralgias. Panel C: Aspergillus with branching septate hyphae and CT showing the halo sign of a nodule surrounded by ground-glass opacity. Panel D: Pneumocystis jirovecii with foamy alveolar exudate on silver stain, CT showing bilateral ground-glass infiltrates, and CD4 count display below 200 cells per microliter. --- ## Viral Respiratory Infections ### Influenza Influenza viruses (types A and B) cause seasonal epidemics during fall and winter months. The infection presents with abrupt onset of high fever, severe myalgias, headache, and nonproductive cough. Unlike common upper respiratory infections, influenza produces significant systemic symptoms and prostration. Complications include primary viral pneumonia with diffuse infiltrates and secondary bacterial pneumonia, often due to Staphylococcus aureus or Streptococcus pneumoniae, occurring several days after initial improvement. Diagnosis uses rapid antigen testing or PCR on nasopharyngeal samples. Antiviral treatment with oseltamivir is most effective when initiated within 48 hours of symptom onset but may benefit high-risk patients even when started later. Annual vaccination remains the primary prevention strategy. ### COVID-19 SARS-CoV-2 infection ranges from asymptomatic to critical illness. Transmission occurs through respiratory droplets and aerosols. The incubation period is typically 2-14 days, with infectious potential beginning before symptoms. Common symptoms include fever, cough, dyspnea, and characteristic anosmia (loss of smell). Severe disease manifests as viral pneumonia progressing to ARDS with bilateral ground-glass opacities and consolidation on imaging. Multiorgan involvement may include cardiac, renal, and thrombotic complications. Treatment depends on severity: antiviral agents for acute infection, corticosteroids for hypoxemic patients, and supportive care including mechanical ventilation for respiratory failure. ### RSV in Adults Respiratory syncytial virus, well-known as a pediatric pathogen, also causes significant disease in elderly and immunocompromised adults. Presentation includes bronchiolitis and pneumonia, with wheezing prominent on examination. Treatment is primarily supportive, with ribavirin reserved for severe cases in high-risk patients. ### Distinguishing Viral from Bacterial Pneumonia Several features help differentiate viral from bacterial pneumonia, though overlap is common. Viral infections typically have gradual onset, low-grade fever, scant sputum production, normal or low white blood cell count, low procalcitonin, and diffuse interstitial infiltrates on imaging. Bacterial pneumonia characteristically presents abruptly with high fever, rigors, purulent sputum, elevated white count with left shift, elevated procalcitonin, and focal lobar or segmental consolidation. However, secondary bacterial infection complicating viral illness is common, and many patients require empiric coverage for both. Panel A: Influenza presentation showing virus particle structure with hemagglutinin and neuraminidase spikes, high fever on thermometer, fall/winter seasonality on calendar, and lungs with bilateral patchy infiltrates. Panel B: COVID-19 presentation showing coronavirus spike protein structure, symptom icons for fever, cough, and anosmia, CT image with bilateral ground-glass opacities and peripheral consolidation, and oxygen saturation monitor. Panel C: RSV presentation showing virus particle, elderly patient silhouette, and lungs with bronchial thickening and wheezing pattern. Panel D: Comparison chart of viral versus bacterial pneumonia features including onset, fever pattern, sputum character, white blood cell count, procalcitonin level, and chest X-ray pattern. --- ## Hospital-Acquired and Ventilator-Associated Pneumonia ### Definitions and Pathogens Hospital-acquired pneumonia develops 48 or more hours after hospital admission in patients who were not intubated at admission. Ventilator-associated pneumonia occurs 48 or more hours after endotracheal intubation. Both conditions involve different pathogens than community-acquired pneumonia due to healthcare exposure and antibiotic selection pressure. Common causative organisms include Pseudomonas aeruginosa, notorious for multidrug resistance and biofilm formation; Staphylococcus aureus including methicillin-resistant strains (MRSA); Klebsiella and Enterobacter species among gram-negative enterics; and Acinetobacter baumannii, increasingly common in ICUs and often extensively drug-resistant. ### Risk Factors and Prevention Intubation bypasses normal upper airway defenses and allows direct bacterial access to the lower respiratory tract. Gastric colonization occurs with acid suppression therapy, and aspiration of gastric contents around the endotracheal tube cuff introduces pathogens. Prior antibiotic exposure selects for resistant organisms. Supine positioning increases aspiration risk. Prevention bundles address these modifiable risk factors. Elevating the head of bed to 30-45 degrees reduces aspiration. Daily sedation vacations permit earlier extubation. Oral care with chlorhexidine reduces bacterial colonization. Subglottic secretion drainage through specialized endotracheal tubes removes pooled secretions above the cuff. Deep venous thrombosis prophylaxis and stress ulcer prophylaxis prevent complications but must be balanced against infection risks. ### Treatment Approach Empiric therapy for HAP/VAP must cover resistant gram-negative organisms including Pseudomonas and MRSA. Initial regimens typically combine an antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, meropenem) with coverage for MRSA (vancomycin or linezolid). Therapy is subsequently narrowed based on culture results. Treatment duration is typically 7 days if clinical response is adequate. Panel A: Cross-section of an intubated patient showing endotracheal tube in place, secretions pooling above the cuff, microaspiration arrows bypassing the cuff, and gastric contents with bacterial colonization in the stomach promoted by supine positioning. Panel B: Key VAP pathogens labeled alongside the endotracheal tube: rod-shaped Pseudomonas, cluster-forming Staphylococcus, and rod-shaped Acinetobacter. Panel C: VAP prevention bundle illustrating head-of-bed elevation to 30 degrees, sedation vacation timing clock, oral care with chlorhexidine, specialized endotracheal tube with subglottic suction port, and DVT prophylaxis stockings. Panel D: Treatment algorithm showing initial empiric broad-spectrum antibiotic coverage narrowing to targeted therapy based on culture and sensitivity results. --- ## Lung Abscess and Empyema ### Lung Abscess A lung abscess is a necrotic cavity within the lung parenchyma containing purulent material. The most common cause is aspiration of oropharyngeal contents, introducing anaerobic bacteria and other oral flora into the lung. Risk factors include conditions predisposing to aspiration: altered consciousness from alcohol intoxication, sedation, or seizures; dysphagia; and poor dentition providing a bacterial reservoir. Necrotizing pneumonia from certain organisms (Klebsiella, Staphylococcus aureus) may also produce abscesses. The typical location following aspiration is the posterior segments of the upper lobes or superior segments of the lower lobes when aspiration occurs in the supine position. Chest radiograph characteristically shows a thick-walled cavity with an air-fluid level, indicating communication with airways. Patients present with indolent symptoms including fever, cough productive of foul-smelling sputum, weight loss, and night sweats over weeks. Treatment requires prolonged antibiotic therapy, typically 4-6 weeks, covering anaerobes and other oral flora. Drainage procedures are reserved for abscesses failing medical management or those larger than 6 cm. ### Empyema Empyema is defined as pus in the pleural space, representing the most serious complication of parapneumonic effusion. As pneumonia develops, inflammation may produce a sympathetic pleural effusion. This parapneumonic effusion progresses through stages. In the exudative stage, fluid is sterile with low cell count and normal pH and glucose, and responds to antibiotics alone. The fibrinopurulent stage is characterized by bacterial invasion, falling pH below 7.20, glucose below 60 mg/dL, and rising LDH above 1000 U/L, with loculations forming from fibrin deposition. This stage requires chest tube drainage. The organizing stage features thick fibrous peel encasing the lung, potentially requiring surgical decortication. Thoracentesis findings distinguish complicated effusions requiring drainage: purulent appearance, positive Gram stain or culture, pH below 7.20, glucose below 60 mg/dL, or loculations on imaging all indicate the need for tube thoracostomy. Antibiotics alone are insufficient once the effusion becomes complicated. Panel A: Lung abscess cross-section showing a thick-walled cavity containing air-fluid level with necrotic debris and inflammatory cells in the cavity wall, and a chest X-ray with classic right lower lobe abscess appearance. Panel B: Risk factor icons for lung abscess including alcohol bottle, seizure symbol, and poor dentition, representing conditions predisposing to aspiration. Panel C: Parapneumonic effusion progression in three sequential images: exudative stage with clear yellow fluid, fibrinopurulent stage with turbid fluid and fibrin loculations, and organizing stage with thick fibrous peel encasing compressed lung. Panel D: Thoracentesis needle sampling pleural fluid with laboratory values displayed (pH 7.0, glucose 30, LDH 1500) alongside treatment icons showing chest tube drainage and surgical instruments for decortication. --- ## Summary Community-acquired pneumonia is most commonly caused by Streptococcus pneumoniae, with severity guiding disposition using CURB-65 scoring and treatment ranging from outpatient monotherapy to ICU-level combination regimens covering resistant and atypical organisms. Atypical pneumonias caused by Mycoplasma, Legionella, and Chlamydophila present with gradual onset, dry cough, and extrapulmonary features, requiring macrolide, fluoroquinolone, or doxycycline therapy. Tuberculosis causes granulomatous infection that may remain latent or reactivate with upper lobe cavitary disease; standard treatment uses the RIPE regimen for 6 months, with each drug having characteristic toxicities requiring monitoring. Latent tuberculosis should be treated in high-risk groups to prevent reactivation. Fungal pulmonary infections include endemic mycoses (Histoplasma, Coccidioides, Blastomyces) distributed geographically, and opportunistic infections (Pneumocystis, Aspergillus) affecting immunocompromised hosts. Viral pneumonias from influenza and SARS-CoV-2 may cause severe disease; antivirals are most effective when given early. Hospital-acquired and ventilator-associated pneumonias involve resistant organisms including Pseudomonas and MRSA, requiring broad-spectrum empiric therapy and prevention bundles to reduce incidence. Lung abscess from aspiration requires prolonged antibiotics, while empyema requires drainage in addition to antibiotics once the effusion becomes complicated. --- ## Key Terms | Term | Definition | |------|------------| | Community-acquired pneumonia | Pneumonia acquired outside healthcare settings | | Atypical pneumonia | Pneumonia with gradual onset and extrapulmonary features | | Tuberculosis | Mycobacterial infection with granulomatous inflammation | | Latent TB | Contained TB infection without active disease | | Ghon complex | Primary TB lesion with associated lymph node involvement | | Empyema | Pus in the pleural space | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*