Pneumonia pathophysiology

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Editor(s)-in-Chief: C. Michael Gibson, M.S., M.D. [1] Phone:617-632-7753; Associate Editor(s)-In-Chief: Priyamvada Singh, M.D. [2]

Overview

Bacteria and fungi typically enter the lung with inhalation. Once inside the alveoli, these microbes travel into the spaces between the cells and also between adjacent alveoli through connecting pores. This invasion triggers the immune system response by sending white blood cells responsible for attacking microorganisms (neutrophils) to the lungs resulting in manifestations of pneumonia.

Pathophysiology

Viruses

• Viruses must invade cells in order to reproduce.
• Typically, a virus will reach the lungs by traveling in droplets through the mouth and nose with inhalation.
• There, the virus invades the cells lining the airways and the alveoli.
• This invasion often leads to cell death either by directly killing the virus or by self-destruction through apoptosis.
• Further damage to the lungs occurs when the immune system responds to the infection. White blood cells, in particular lymphocytes, are responsible for activating a variety of chemicals (cytokines) which cause leaking of fluid into the alveoli.
• The combination of cellular destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream.
• In addition to the effects on the lungs, many viruses affect other organs and can lead to illness affecting many different bodily functions.
• Viruses also make the body more susceptible to bacterial infection; for this reason, bacterial pneumonia often complicates viral CAP.

Bacteria

• Bacteria and fungi also typically enter the lung with inhalation, though they can reach the lung through the bloodstream if other parts of the body are infected.
• Often, bacteria live in parts of the upper respiratory tract and are constantly being inhaled into the alveoli.
• Once inside the alveoli, bacteria and fungi travel into the spaces between the cells and also between adjacent alveoli through connecting pores.
• This invasion triggers the immune system to respond by sending white blood cells responsible for attacking microorganisms (neutrophils) to the lungs. The neutrophils engulf and kill the offending organisms but also release cytokines which result in a general activation of the immune system.
• Fever, chills, and fatigue are common in CAP. The neutrophils, bacteria, and fluid leaked from surrounding blood vessels fill the alveoli and result in impaired oxygen transportation.
• Bacteria often travel from the lung into the blood stream and can result in serious illness such as septic shock. Septic shock results in low blood pressure leading to damage in multiple parts of the body including the brain, kidney, and heart.

Fungi

Fungi typically enter the lung with inhalation of their spores, though they can reach the lung through the bloodstream if other parts of the body are infected. Also, fungal pneumonia can be caused by reactivation of a latent infection. Once inside the alveoli, fungi travel into the spaces between the cells and also between adjacent alveoli through connecting pores. This invasion triggers the immune system to respond by sending white blood cells responsible for attacking microorganisms (neutrophils) to the lungs. The neutrophils engulf and kill the offending organisms but also release cytokines which result in a general activation of the immune system. This results in the fever, chills, and fatigue that is commonly seen in bacterial and fungal pneumonia. The neutrophils and fluid leaked from surrounding blood vessels fill the alveoli and result in impaired oxygen transportation.

Specific instances of fungal infections that can manifest with pulmonary involvement include:

• Pneumocystis jirovecii pneumonia
• Histoplasmosis, which has primary pulmonary lesions and hematogenous dissemination
• Coccidioidomycosis, which begins with an often self-limited respiratory infection (also called "Valley fever" or "San Joaquin fever")
• Pulmonary blastomycosis
• Sporotrichosis - primarily a lymphocutaneous disease, but can involve the lungs as well
• Cryptococcosis - contracted through inhalation of soil contaminated with the yeast, it can manifest as a pulmonary infection and as a disseminated one
• Aspergillosis, resulting in invasive pulmonary aspergillosis
• Candidiasis - rarely has pulmonary manifestations in immunocompromised patients.

Parasites

• In general, these parasites enter the body through the skin or by being swallowed.
• Once inside the body, these parasites travel to the lungs, most often through the blood. There, a similar combination of cellular destruction and immune response causes disruption of oxygen transportation.

Aspiration Pneumonia Pathophysiology

The location is often gravity dependent, and depends on the patient position. Generally the right middle and lower lung lobes are the most common sites of infiltrate formation due to the larger caliber and more vertical orientation of the right mainstem bronchus.

Patients who aspirate while standing can have bilateral lower lung lobe infiltrates. The right upper lobe is a common area of consolidation in alcoholics who aspirate in the prone position. Depending on the acidity of the aspirate, a chemical pneumonitis can develop, and bacterial pathogens (particularly anaerobic bacteria) may add to the inflammation.

Microscopic Pathology

The upper panel shows a normal lung under a microscope. The white spaces are alveoli that contain air.Lower panel shows a lung with pneumonia under a microscope. The alveoli are filled with inflammation and debris.

Major points for pathogenesis of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia (DONOT EDIT)

“

Major Points for Pathogenesis 1 Sources of pathogens for HAP include healthcare devices, the environment (air, water, equipment, and fomites), and commonly the transfer of microorganisms between the patient and staff or other patients (Level II) . 2 A number of host- and treatment-related colonization factors, such as the severity of the patient's underlying disease, prior surgery, exposure to antibiotics, other medications, and exposure to invasive respiratory devices and equipment, are important in the pathogenesis of HAP and VAP (Level II). 3 Aspiration of oropharyngeal pathogens, or leakage of secretions containing bacteria around the endotracheal tube cuff, are the primary routes of bacterial entry into the lower respiratory tract (Level II). 4 Inhalation or direct inoculation of pathogens into the lower airway, hematogenous spread from infected intravenous catheters, and bacterial translocation from the gastrointestinal tract lumen are uncommon pathogenic mechanisms (Level II). 5 Infected biofilm in the endotracheal tube, with subsequent embolization to distal airways, may be important in the pathogenesis of VAP (Level III) 6 The stomach and sinuses may be potential reservoirs of nosocomial pathogens that contribute to bacterial colonization of the oropharynx, but their contribution is controversial, may vary by the population at risk, and may be decreasing with the changing natural history and management of HAP (Level II)

”

For Level of evidence and classes click here.

Histopathological Findings in Aspiration Pneumonia

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Histopathological Findings

Lobar pneumonia

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Pneumocystis pneumonia

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Aspiration Pneumonia

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Aspiration pneumonia, infant

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Desquamative interstitial pneumonia

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Legionella pneumonia

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Measles pneumonia

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Abscess, bronchopneumonia

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