Definition and types of accumulated fluid
Edema is a standard pathological process characterized by the accumulation of fluid in the tissues or cavities of the body due to a disturbance of the water-salt balance. According to the nature of the fluid, three types of contents are distinguished.
- Transudate is a non-inflammatory effusion; it is protein-poor, contains no formed elements or pathogens, and forms purely mechanically as fluid escapes the vascular bed.
- Exudate is a product of inflammatory exudation; it is protein-rich (including fibrinogen, which converts to fibrin), contains formed elements, and may include bacteria detected by PCR or culture methods.
- Mucus is a rare variant; it is characteristic of hypothyroidism, where the accumulation of hyaluronic and chondroitin acids forms myxedematous edema.
Classification by localization and prevalence
According to localization, they are divided into:
- Anasarca is edema of the subcutaneous adipose tissue.
- Dropsy is the accumulation of fluid in cavities: ascites (abdominal cavity), hydrothorax (pleural cavity), hydropericardium (pericardial cavity), hydrocephalus (cerebral ventricles), hydrocele (scrotal cavity).
According to prevalence, edemas are divided into local (area of trauma, inflammation, fracture) and general, encompassing all organs and systems in systemic diseases — disturbance of colloid-osmotic pressure, generalized inflammation (sepsis, septic shock), hepatic failure (decreased albumin synthesis), nephrotic syndrome (protein loss in urine).
According to the rate of development, fulminant (seconds — with insect bites), acute (pulmonary edema in acute left ventricular failure against the background of myocardial infarction), and chronic (days and weeks — in nephrotic syndrome) are distinguished.
Frank-Starling mechanism
The understanding of edemas is based on the balance of forces at the capillary level. At the arterial end, hydrostatic pressure is 36–38 mm Hg, oncotic pressure is about 25 mm Hg; the predominance of hydrostatic pressure ensures the filtration of fluid into the interstitium along with oxygen and nutrients. At the venous end, hydrostatic pressure drops to 14–16 mm Hg, and oncotic pressure (25 mm Hg) exceeds it, causing fluid to return to the vessel. Excess (about 2 liters per day) is removed via the lymphatic system. A disruption of any link in this equilibrium triggers edema.
Pathogenetic factors of edema
- Hydrodynamic — increase in venous pressure and/or circulating blood volume; impedes fluid reabsorption at the venous end of the capillary, while the lymphatic system fails to cope.
- Lymphogenic — congenital hypoplasia, compression, obstruction of lymphatic vessels, parasitic infection, spasm of lymphangions, increase in central venous pressure; leads to lymphatic insufficiency.
- Oncotic — hypoproteinemia due to insufficient protein intake, malabsorption, impaired synthesis, or urinary losses; decreased plasma oncotic pressure reduces fluid return into the vascular bed. Interstitial hyperonquia is also possible when albumin escapes through a permeable vascular wall.
- Osmotic — decrease in plasma osmolarity (administration of hypoosmolar solutions, excess ADH) or increase in interstitial osmolarity (sodium accumulation in the focus of inflammation). Sodium always retains water.
- Membranogenic — increase in vascular wall permeability during inflammation: acidosis, the action of histamine, hydrolytic enzymes, endothelial activation with widening of intercellular clefts; characteristic of inflammatory and allergic edemas.
Combination of factors using chronic heart failure as an example
In chronic heart failure, the initiating factor is hydrodynamic: right ventricular failure causes congestion in the venous system and an increase in central venous pressure, leading to lymphatic insufficiency. Venous hyperemia of the liver disrupts albumin synthesis, and the oncotic factor joins in. Activation of the RAAS with sodium and water retention increases circulating blood volume, amplifying the hydrodynamic component. Clinically, this manifests as cold edema of the lower extremities, leaving a slowly disappearing pit upon pressure.
Key points
- Edema is a disturbance of the water-salt balance with the accumulation of fluid in tissues or cavities.
- Transudate is poor in protein, exudate is rich in protein and cells, and mucus is characteristic of hypothyroidism.
- Localization determines terminology: ascites, hydrothorax, hydropericardium, hydrocephalus, hydrocele, anasarca.
- The Frank-Starling mechanism is the balance of hydrostatic and oncotic pressures at the ends of the capillary.
- Six pathogenetic factors: hydrodynamic, lymphogenic, oncotic, osmotic, membranogenic.
- Hypoproteinemia in hepatic failure and nephrotic syndrome is a typical cause of oncotic edemas.
- Inflammatory edema is associated with increased vascular permeability under the action of histamine and acidosis.
- In chronic heart failure, edema forms through a combination of hydrodynamic, lymphogenic, and oncotic factors with the activation of the RAAS.