Edema: Definition and Types

Edema Definition

• Edema is an increase of fluid in interstitial tissue spaces or in body cavities.
• It represents an excessive fluid accumulation.

Types of Edema

• Hydrothorax: Fluid buildup in the pleural cavity.
• Hydropericardium: Excessive fluid in the pericardial sac.
• Hydroperitoneum (Ascites): Fluid buildup in the peritoneal cavity.
• Anasarca: Severe, widespread edema throughout the body, resulting in significant subcutaneous swelling.

Edema Formation Mechanism

• Approximately 60% of lean body weight is water.
• Two-thirds of this water is intracellular; the remainder is in the extracellular compartment.

Mechanisms of Edema Formation

• Increased capillary pressure
• Decreased colloid osmotic pressure
• Lymphatic obstruction
• Sodium retention

Factors Affecting Fluid Balance Across Capillary Walls and Interstitial Spaces

• Hydrostatic pressure (HP): Pressure of plasma fluid, pushing fluid out of capillaries into the interstitial spaces. Generally 32mmHg at arterial side and 20mmHg at venous side of capillary bed.
• Plasma oncotic pressure (OP): Pressure of plasma proteins (primarily albumin), drawing fluid back into the capillaries. Normally 25 mmHg in both sides of capillary beds.

Starling Forces

• Capillary hydrostatic pressure (HP) pushes fluid out of the capillaries.
• Capillary oncotic pressure (OP) pulls fluid back into the capillaries.
• Net fluid movement is zero across the capillary bed.
• Excess interstitial fluid is drained by the lymphatic system back into the bloodstream.

Causes of Increased Hydrostatic Pressure (HP)

• Localized: Inflammation, venous obstruction
• Generalized: Right-sided heart failure

Causes of Decreased Oncotic Pressure (OP)

• Decreased plasma protein (albumin) intake
• Decreased protein absorption
• Decreased protein synthesis (e.g., liver cirrhosis)
• Increased protein loss (e.g., nephrotic syndrome, ulcerative colitis, burns)

Edema Classification

• Pathophysiological Mechanism:
  • Transudate (non-inflammatory)
  • Exudate (inflammatory)
• Clinical Finding:
  • Pitting (cardiac, renal, liver)
  • Non-pitting (inflammatory, lymphatic)
• Site:
  • Localized (inflammatory, allergic, venous/lymphatic obstruction, pulmonary edema)
  • Generalized (cardiac, hepatic, renal)

Exudative Edema

• Usually localized edema caused by increased hydrostatic pressure or increased vascular permeability.
• Characterized by exudation of high protein-containing fluid.
• Specific gravity usually is greater than or equal to 1.020

Transudative Edema

• Systemic or localized edema due to increased hydrostatic pressure or decreased plasma osmotic pressure.
• Characterized by a low protein content in the fluid.
• Specific gravity less than 1.020

Cardiac Edema:

• Left-sided heart failure: Localized pulmonary edema due to increased hydrostatic pressure in pulmonary veins.
• Right-sided heart failure (RHF): Increased hydrostatic pressure in systemic veins leading to generalized edema due to reduced venous drainage. RHF leads to a gradient of +5 mmHg.

Renal and Hepatic Edema

• Renal and hepatic diseases: Decreased plasma oncotic pressure.
  • Decreased plasma albumin, therefore decreases the amount of fluid returning to the venous circulation.
  • Edema, as a result.
• Liver disease (cirrhosis): Blocked blood flow through the portal vein increases hydrostatic pressure and causes peritoneal edema (ascites)