Pneumonia

Video summary

Pneumonia is an infection of the lungs that results in air sacs being filled with fluid. It may be caused by bacteria, viruses, or fungal infections. Pneumonia is either community-acquired, meaning a person got sick outside of a hospital or healthcare setting; or hospital-acquired, which is when a person gets it when they are already hospitalized for something else. Common symptoms of pneumonia can include fever, cough, chest pain, shortness of breath, and difficulty breathing. The severity of symptoms can range from mild to severe. Treatment for pneumonia typically involves drugs to kill the invading pathogen and supportive care such as oxygen therapy.

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With pneumonia, pneumon comes from the Greek word meaning lungs, while the suffix ea refers to a condition. So pneumonia literally means a lung condition.
It's an infectious condition that occurs when a pathogen invades the lungs and triggers inflammation, causing breathing difficulties.
When you take a breath, the air travels through your nose or mouth down the trachea. From here, it moves into the primary bronchi, which branch into smaller secondary bronchi, then tertiary bronchi, and finally into the bronchioles.
Bronchioles lead directly to tiny alveoli, where the gas exchange occurs. Now the walls of the airways consist of smooth muscle cells and elastic tissue that help the airways open and return to their shape as we breathe.
These walls are lined with epithelial cells which have tiny brush-like projections called cilia and goblet cells which produce sticky mucus.
The mucus traps dust, bacteria, and other unwanted particles, and the cilia move together in coordinated waves, pushing the mucus and trapped particles toward the throat, so we can clear them out of the lungs.
This system is called the mucociliary escalator. This way we can either swallow or cough out foreign particles that end up in our airways.
But even with this clearing system, some particles still make it down to the alveoli. That's where alveolar macrophages come in.
They patrol the air sacs like security guards, engulfing and destroying any bacteria or particles that slip through. Now pneumonia occurs when the pathogen overcomes these defensive mechanisms and begins multiplying in a warm, moist environment.
Based on the type of pathogen involved, pneumonia is classified as bacterial, viral, and fungal pneumonia. Bacterial pneumonia often occurs after a viral respiratory infection, which weakens the immune defenses.
With the body's guard down, bacteria floating in the air enter the lungs and seize the opportunity. In the alveoli, local immune cells struggle to fight bacteria off, so the body sends reinforcements to battle the infection.
These reinforcements are white blood cells, especially neutrophils, which release inflammatory mediators to destroy the pathogen, but these mediators come at a cost.
They make the lung blood vessels more permeable. As inflammation sets in, fluid begins to leak from vessels into the alveoli, filling them with thick, cloudy fluid.
This fluid carries white blood cells, proteins, and bacteria, forming what is known as an inflammatory exudate. The infected lung tissue then stiffens and solidifies, turning soft and spongy lung tissue into firm and fluid-filled tissue.
As the infection progresses, lung tissue damage follows one of two main patterns. Alright, let's start with lobar pneumonia, which affects an entire lobe and progresses through 4 stages.
The first is congestion, where blood vessels dilate and alveoli fill with fluid containing some neutrophils and many bacteria.
These initial changes make the lobe red, wet, and heavy. Next comes red hepatization, where the alveoli become packed with dense exudate composed of neutrophils, red blood cells, and fibrin.
This transforms the affected lobe into red firm tissue with no airspaces, giving it the appearance of liver tissue, hence the name hepatization.
The third stage is gray hepatization, which occurs as the red blood cells break down, leaving mainly neutrophils and fibrin, turning the lobe grayish.
The final stage is resolution, where enzymes break down the inflammatory exudate into granular viscous debris. Later, this material is resorbed by the body, ingested by macrophages, expelled as sputum, or organized into scar tissue by fibroblasts.
The second pattern is bronchopneumonia, which can affect a single lobe, but more often involves patchy areas of consolidation across multiple lobes, especially the lower ones, because mucus and pus tend to accumulate in gravity-dependent regions.
These patches are poorly defined, slightly raised, and have a granular texture ranging in color from gray red to yellow.
Inside, the bronchi, bronchioles, and the nearby alveoli are packed with thick, neutrophil-rich exudate. Regardless of the pattern, several serious complications can result from bacterial infection.
Severe inflammation can cause tissue necrosis, destroying lung tissue and forming abscesses. Next, the inflammation can spread to the pleural cavity, causing inflammation of the pleura, known as pleuritis.
And if bacteria invade the pleural space, pus can accumulate, resulting in emyema. Finally, bacteria can enter the bloodstream and cause sepsis.
Now the specific bacteria involved depend largely on where the infections acquired. Community acquired pneumonia, which develops outside of healthcare settings, typically involves Streptococcus pneumonia, along with Haemophilus influenza and Moriella catalis, which are common in individuals with COPD.
There are also atypical pathogens, which include Legionella pneumophila, chlamydia pneumonia, Mycoplasma pneumonia, and Coxiella burnettii.
On the other hand, hospital acquired pneumonia happens in hospitals and usually involves tough bacteria that can hang around on surfaces and equipment.
Common pathogens include methicillin resistant Staphylococcus aureus or MRSA, and gram-negative bacteria like Klebsiella pneumonia, Cerracea marcessan, Escherichia coli, and Pseudomonas.
Moreover, pseudomonas can thrive in the moist environment of mechanical ventilators, causing a specific subtype of hospital-acquired pneumonia called ventilator associated pneumonia.
But sometimes the trouble isn't bacteria at all, it's a virus. When a virus reaches the lungs, it invades the epithelial cells lining the airways, causing local damage and triggering non-purulent inflammation.
Instead of filling the air sacs with neutrophils and pus, the immune system sends lymphocytes and macrophages to fight the invader in the alveolar walls.
As the inflammation progresses, these walls, which represent the lung interstitium, become swollen. That's why this type of pneumonia is called interstitial pneumonia.
Affected areas appear congested with a red-blue color and can be patchy or involve entire lobes, either on one or both sides.
In adults, the most common pathogens include influenza A and B. In children, think of respiratory syncytial virus, parainfluenza, human metapneumovirus, and adenovirus.
Finally, in both adults and children, don't forget SARS-COVID-2. On top of that, viruses damage the cilia, causing mucus to build up, creating a perfect environment for bacteria to thrive.
That's why viral pneumonia often sets the stage for bacterial pneumonia. Now switching gears to another type of bacterial pneumonia.
Called aspiration pneumonia. During sleep, we may aspirate tiny amounts of mouth or throat secretions into our lungs, but the body clears them out.
Things get serious when someone with an impaired gag or swallow reflex aspirates a much larger amount of mouth secretions, sometimes including gastric contents.
This can happen in heavily sedated or unconscious individuals, but also in healthy individuals during repeated vomiting.
The acidic gastric fluid causes chemical injury to a lung tissue, while bacteria from the mouth trigger infection. The infection is typically polymicrobial.
It involves anaerobes that reside in the mouth, such as bacteroides and fusobacterium, and aerobes like streptococcus pneumonia.
The combination of chemical lung injury and infection leads to necrotizing inflammation that destroys lung tissue, often leaving abscesses behind.
Finally, there's fungal pneumonia caused by dimorphic fungi. In healthy individuals, these fungi usually affect just the lungs.
However, in immunocompromised individuals, fungi can spread from the lungs to other organs, including the liver, spleen, and bone marrow, which is known as disseminated disease.
In lungs, infected areas are packed with macrophages loaded with fungi, forming small granulomas containing multinucleated giant cells.
Later, these granulomas develop central necrosis, fibrosis, and even calcification, creating a pattern that mimics tuberculosis.
The three main fungi each have distinctive appearances. Histoplasma capsulatum is a small round yeast up to 5 micrometers.
Blastomyces dermatitis is larger, 5 to 25 micrometers in diameter, with broad-based budding. Incoccidioides imitis is the biggest, 20 to 60 micrometers in diameter, with thick-walled, non-budding spheres containing endospores.
Now symptoms of pneumonia usually come suddenly and can range from mild to life threatening. As the body starts to fight the infection, systemic symptoms appear like fever, chills, fatigue, and muscle aches.
Soon after, as the lungs try to clear out mucus, pus, and debris, A productive cough kicks in, bringing thick yellow or green sputum.
Because the infected areas of the lungs cannot exchange oxygen properly, breathing is often rapid and shallow. In more serious cases, individuals might use accessory muscles to help with breathing.
Finally, if the inflammation reaches the pleura, it can cause pleuritic chest pain, which feels sharp and gets worse with deep breaths or coughing.
Next up is the diagnosis. On physical examination, lung auscultation reveals crackles as air moves through fluid-filled alveoli, sounding like Velcro being pulled apart.
In areas of consolidation, bronchial breath sounds can be heard, which is when breath sounds become louder and harsher because sound travels better through the solid area of inflammatory exudae than through air.
Similarly, this increased transmission through the consolidation causes egophony, where the patient says A, but the examiner hears E, as well as increased tactile fremitus, where vibrations can be felt over the patient's chest as they speak.
Finally, you'll find dullness to percussion over the area of consolidation. Next, a chest X-ray helps identify lobar pneumonia, bronchopneumonia, and interstitial pneumonia.
In bacterial pneumonia, lab results show high neutrophil count, while in viral pneumonia, high lymphocyte count. Also, to differentiate the two, you can measure procalcitonin, which is typically elevated in bacterial infections.
Treatment focuses on clearing the infection with antibiotics and helping the person breathe while they recover. Mild cases usually respond well to a single antibiotic like azithromycin or amoxicillin, while more severe cases may require dual therapy with azithromycin and ceftriaxone.
All right, as a quick recap. Pneumonia is an infection that occurs when pathogens invade the alveoli and overcome the lungs' natural defenses, triggering inflammation and causing breathing difficulties.
Bacteria usually cause lobar pneumonia and bronchopneumonia, while viruses lead to interstitial pneumonia. Aspiration pneumonia happens when a person aspirates mouth and gastric content, while fungal pneumonia mainly affects people with weakened immune