Pathogenesis and etiology (causes)
The mechanism of development of pulmonary edema is always associated with an imbalance of four key factors. As a rule, pathogenesis involves an increase in the first three parameters and a decrease in the fourth:
- Increase in hydrostatic intracapillary pressure. The process is localized in the venous part of the lesser circulation. This is caused by acute left ventricular failure, mitral stenosis, pulmonary vein obstruction, and hypervolemia of the lesser circulation.
- Decrease in blood oncotic pressure. Occurs against the background of hypoalbuminemia (protein deficiency). Main etiological factors: nephrotic syndrome, liver diseases, and protein-losing enteropathies.
- Increase in vascular-tissue permeability. The capillaries of the alveolar septum begin to leak excess plasma. A classic example of such a disorder is adult respiratory distress syndrome (ARDS).
- Decreased lymphatic drainage. Obstruction or insufficiency of lymphatic drainage pathways prevents the adequate removal of accumulated fluid from the lung tissue.
Clinical presentation and diagnostics
The symptoms of the syndrome are pronounced and require immediate evaluation. Patients suffer from severe shortness of breath and cough. Orthopnea is characteristic—a forced sitting position that the patient assumes to facilitate breathing. Coughing may produce specific sputum: pink and foamy.
During auscultation, the doctor hears crepitation, localized predominantly in the basal (lower) parts of the lungs.
| Method of examination | Detected changes |
|---|---|
| Functional tests | Decrease in vital capacity (VC) |
| Blood gas analysis | Hypoxemia (decreased $O_2$), hypercapnia (increased $CO_2$) |
| Radiography | Enhanced vascular pattern, vascular dilation, fluid levels (Kerley lines) |
Morphology: macro- and microscopic picture
The post-mortem picture of pulmonary edema consists of distinct macroscopic and microscopic changes.
Macroscopic picture: at autopsy, the lungs appear heavy, moist, and acquire a pronounced red color. Characteristic sign: when pressure is applied to the cut surface of the organ, red foamy fluid flows out abundantly.
Microscopic picture develops in stages:
- First stage: edema fluid begins to accumulate exclusively in the interstitial tissue.
- Second stage: fluid appears directly in the lumen of the alveoli. The mechanism of this transition is associated with a sharp increase in interstitial fluid pressure, which leads to the physical destruction of tight junctions between type I pneumocytes.
- Progression: following the fluid, erythrocytes and other blood cellular elements enter the alveolar lumen.
Outcomes of pulmonary edema
Depending on the severity of the condition, pulmonary edema can have several possible outcomes:
- Complete resolution of the process without any morphological consequences for the tissue.
- Development of interstitial fibrosis (proliferation of connective tissue in the interstitium).
- With chronic progression of the process, sclerosis and hemosiderosis of the lungs are formed. In pathological anatomy, this outcome has the classical name—brown induration of the lungs.
To remember the 4 mechanisms of pathogenesis, the abbreviation GOPL helps: Hydrostatic pressure (increases), Oncotic pressure (decreases), Permeability of capillaries (increases), Lymphatic drainage (decreases).
Class and exam questions
Why does pulmonary edema produce pink foamy sputum?
The foamy structure is formed during breathing, and the pink color appears at the progression stage when erythrocytes and other blood cells begin to enter the alveolar lumen.
How does fluid penetrate the alveoli in the second stage?
Due to a sharp increase in interstitial fluid pressure, the tight junctions between type I pneumocytes are destroyed, and fluid enters the alveolar lumen.
What does chronic pulmonary edema lead to?
Chronic progression of the process leads to the development of sclerosis and hemosiderosis of the lung tissue. Macroscopically, this outcome is called brown induration of the lungs.
What else to study on this topic
- Acute left ventricular failure and mitral stenosis
- Adult respiratory distress syndrome (ARDS)
- Nephrotic syndrome and protein-losing enteropathies
- Brown induration of the lungs: pathogenesis of hemosiderosis
- Radiological signs: Kerley lines
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