What is oncotic pressure?

Understanding Oncotic Pressure

Oncotic pressure is the pressure exerted by proteins in blood plasma that tends to pull water into the circulatory system across semipermeable membranes, serving as a critical factor in maintaining fluid balance between intravascular and extravascular compartments.

Definition and Mechanism

• Oncotic pressure (also called colloid osmotic pressure or COP) is one of the four Starling forces that determine fluid movement across capillary membranes 1
• It is primarily generated by plasma proteins, especially albumin, which cannot freely cross the capillary membrane due to their large molecular size 1
• The normal oncotic pressure in healthy adults is approximately 25 mmHg 2

Physiological Importance

• Oncotic pressure counterbalances hydrostatic pressure in the capillaries, preventing excessive fluid movement into the interstitial space 3
• It plays a crucial role in maintaining intravascular volume and preventing edema formation 1
• When oncotic pressure decreases (as in hypoalbuminemia), the balance shifts toward increased fluid movement out of the vasculature, potentially leading to edema 2

Clinical Significance

• Low oncotic pressure can result in:

  • Peripheral edema in skeletal muscle, skin, subcutaneous tissue, intestine, and heart 2
  • Increased risk of pulmonary edema, particularly when combined with elevated pulmonary capillary hydrostatic pressure 4
  • Compromised tissue perfusion due to reduced effective circulating volume 5

• Conditions associated with decreased oncotic pressure include:

  • Hypoalbuminemia (liver disease, malnutrition, protein-losing enteropathy, nephrotic syndrome) 3
  • Burns (where there is significant protein loss through damaged skin) 6
  • Sepsis (due to capillary leak and altered vascular permeability) 3

Measurement and Clinical Applications

• Direct measurement of oncotic pressure is preferred over calculations based on protein or albumin levels, especially when protein fractionation patterns are abnormal 2
• Monitoring oncotic pressure can guide fluid management decisions, particularly in critically ill patients 5
• In burn patients, oncotic pressure typically decreases to about 51% of normal during the first 24 hours 6

Therapeutic Considerations

• Albumin administration may be considered to increase oncotic pressure in specific clinical scenarios:
  • In burn patients, maintaining plasma albumin concentration around 2.5 ± 0.5 g/100 mL with a plasma oncotic pressure of 20 mmHg may be beneficial 7
  • In severe hypoproteinemia with edema, albumin administration may help mobilize fluid 7
• Caution must be exercised when administering albumin:
  • In dehydrated patients, albumin must be given with or followed by additional fluids due to its hyperoncotic properties 7
  • Rapid administration can precipitate circulatory embarrassment and pulmonary edema 7

Limitations and Considerations

• The effect of oncotic pressure on fluid movement varies between different vascular beds:
  • The lung is relatively less sensitive to changes in oncotic pressure due to efficient lymphatic drainage and high interstitial protein concentration 2
  • When capillary permeability is increased (as in inflammatory conditions), the effect of oncotic pressure on preventing edema is diminished 2
• Other factors affecting fluid balance include hydrostatic pressure, capillary permeability, and lymphatic drainage 1