Pneumonia is an invasion of the lower respiratory tract, below the larynx by pathogens either by inhalation, aspiration, respiratory epithelium invasion, or hematogenous spread.

Barriers to pneumonia infection include: anatomical structures of nasal hairs, turbinates, epiglottis, cilia, and humoral and cellular immunities.

Breaching these barriers results in infection, either by fomite/droplet spread (mostly viruses) or nasopharyngeal colonization (mostly bacterial), results in inflammation and injury or death of surrounding epithelium and alveoli. 

Such breaching is accompanied by a migration of inflammatory cells to the site of infection, causing an exudative process, which in turn impairs oxygenation.

Chest x-ray the best diagnostic test.

Among the most common causes of hospitalizations and a leading cause of infectious death.

More than 1.5 million adults are hospitalized in the US annually for pneumonia.

Respiratory pathogen encounter defense mechanisms of the respiratory system before reaching the alveoli and include: mucus trapping, mucociliary clearance, coughing and swallowing.

Pathogens can reach the alveoli by means of microaspiration, inhalation, macroaspiration, or hematogenous spread.

Microaspiration is the primary path for microorganisms into the lungs, and macro aspiration may lead to aspiration pneumonia.

The alveolar macrophages the primary defense mechanism in the lung.

If pathogens can overcome the alveolar defense mechanisms, they multiply and cause local tissue damage and stimulate macrophages to  produce cytokines and chemokines that trigger a local inflammatory response.

Cytokine spilled over into the blood stream produces a systemic inflammatory response.

It is the local and systemic inflammatory response that constitutes the physiological response to lung infection, and explains most signs and symptoms as well as laboratory, imaging abnormalities of pneumonia.

Aspiration of oropharyngeal bacterial pathogens to the lower respiratory tract is one of the most important risk factors for pneumonia.

Sixth leading cause of death and tends to be more prevalent in the elderly.

Affects 1.2% of population each year.

Overall mortality rates are generally less than 1%. 

Pneumonia predominantly affects men, with the greatest risk during infancy and in older age.

Pneumonia may lead to intense pulmonary and systemic inflammation, which result in impaired gas exchange, sepsis, and organ failure.

There are four stages of lobar pneumonia.

The first stage occurs within 24 hours and is characterized by alveolar edema and vascular congestion.

Both bacteria and neutrophils are present.

Red hepatization is the second stage, and it has the consistency of the liver.

The stage is characterized by neutrophils, red blood cells, and desquamated epithelial cells.

Fibrin deposits in the alveoli are common.

The third stage of gray hepatization stage occurs 2-3 days later, and the lung appears dark brown.

There is an accumulation of hemosiderin and hemolysis of red cells.

The fourth stage is the resolution stage, where the cellula infiltrates is resorbed, and the pulmonary architecture is restored.

If the healing is not ideal, then it may lead to parapneumonic effusions and pleural adhesions.

For hospitalized patients mortality risk varies from 4-23% and is associated with a more diverse group of pathogens than in the outpatient setting.

Patients who recover from pneumonia experience long-term mortality at a substantially higher rate than an age and sex matched controls, primarily due to comorbidities.

Most common medical diagnosis responsible for hospitalizations in the US.

The leading cause of death in children, with most fatal cases occurring in individuals younger than five years in developing countries.

In children, poverty is thought to be the primary cause of pneumonia, as a result of malnutrition and limited access to medical care.

Death rate 30 per 100,000 episodes.

Leading cause of infectious disease-related mortality in the U.S.

Leading cause of severe hospital acquired infections.

The second most common type of infection acquired in the hospital, behind UTI, with 40% of cases being postoperative.

Categorized according to location of onset: Community-acquired pneumonia and hospital-acquired pneumonia on the 2 major categories.

Hospital acquired pneumonia is divided into ventilator-acquired and non-ventilator acquired pneumonia.

5-10 cases of hospital-acquired pneumonia diagnosed in every 1000 patient admissions to a hospital.

The incidence and severity of pneumonia is influenced by the underlying predisposing conditions.

In most cases radiologic clearing of infiltrates occurs within 1 to 3 months.

Patients on immunosuppressive agents have an incidence of complicated pneumonia 12 times that of a normal host.

Pediatric pneumonia diagnosis evaluated by analyzing patient characteristics such as age, immune status, epidemiological exposures, and chest x-ray appearance.

Viruses are the most common causes of pneumonia in infants and young children.

Factors associated with death: male gender, ((diabetes)) mellitus, neoplastic disease, neurologic disease, tachypnea, hypotension, hypothermia, leukopenia, bacteremia, and multilobar infiltrates.

Patients with impairments in swallowing and cough reflexes have an increased risk for the development of pneumonia.

Mortality from infections correlates with decreased IgG levels, cutaneous anergy, and lymphopenia.

Aspiration is possibly the single most important risk factor for pneumonia in old people.

Estimated to occur in about one-third of patients with stroke.

Less than 10% of patients hospitalized with clinical pneumonia have positive blood cultures for bacterial disease.

Routine blood cultures in pneumonia have extremely low yield and usefulness, irrespective of the severity and risk of disease (Zhang D.).

Histopathological evaluation and immunochemistry testing and PCR methods can detect many more bacterial lung infections at autopsy than can usual clinical methods of diagnosis.

A follow-up chest x-ray is recommended four-eight weeks after the treatment of pneumonia, to exclude underlying malignant neoplasms that may have been predisposed to by post obstructive pneumonia.

The recommendation for post-pneumonia chest x-ray a couple of months after treatment is particularly relevant for patients with higher risk for lung cancer, such as the elderly smokers.

The incidence of new lung cancer is low after pneumonia: approximately 1% within 90 days and 2% over five years-suggesting routine chest x-rays after pneumonia for detecting lung cancer is not warranted although patients over the age of 50 years and smokers should be targeted for such follow-up (Tang KL et al).

9.2% incidence of new lung cancer after pneumonia with five-year follow-up (Mortensen EM et al).

Hospitalization with pneumonia in older adults is associated with subsequent increase in risk of cardiovascular disease.

The risk of cardiovascular disease after pneumonia in hospitalized patients is 4 fold in the first 30 days, and progressively declines during the first year, and remains approximately 1.5 fold higher in subsequent years (Corales-Moedina VF et al).

Among patients with severe community acquired pneumonia, being treated in an ICU, those who received hydrocortisone, had a lower risk of death by day 28 than those who received placebo.



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