The Role of Plasma Substitutes in Fluid Resuscitation
When a patient experiences significant blood loss from trauma, burns, or major surgery, their circulating blood volume can drop to dangerously low levels. The primary goal of fluid resuscitation in these situations is to rapidly restore intravascular volume to maintain blood pressure and ensure that vital organs receive adequate blood flow. While whole blood transfusions are ideal, they are not always immediately available, universally compatible, or free from risks. Plasma substitutes, also known as volume expanders, provide an alternative by offering a solution to rapidly increase plasma volume.
These substances contain large molecules that do not readily diffuse out of the blood vessels, allowing them to remain in the intravascular space for a longer period than simple salt solutions (crystalloids). This property, known as colloid osmotic pressure, is key to their function. By holding fluid in the bloodstream, plasma substitutes help correct hypovolemia (decreased blood volume) and stabilize the patient until definitive treatment can be provided.
Dextran: A Classic Example of a Plasma Substitute
What is an example of a plasma substitute? A widely recognized example is dextran. Dextran is a polysaccharide, a complex carbohydrate made of glucose units, produced by certain bacteria. It is available in different molecular weights, with the most common formulations for clinical use being dextran 40 and dextran 70.
How Dextran Works
- Volume Expansion: Dextran's large molecules and high water-binding capacity allow it to effectively pull fluid from the interstitial space into the intravascular space. For example, dextran 70 can expand plasma volume by 1 to 1.5 times the volume of the infused solution.
- Improved Microcirculation: Dextran 40, a lower-molecular-weight version, is particularly noted for its ability to improve blood flow in the microcirculation by reducing blood viscosity and preventing red blood cell aggregation.
- Anti-thrombotic Effects: It can also interfere with platelet function and enhance fibrinolysis, which can be beneficial in preventing clot formation but also increases the risk of bleeding.
Side Effects and Considerations
Despite its historical use, the application of dextran has declined due to its potential for adverse effects. These risks include:
- Hypersensitivity Reactions: Some patients can experience severe allergic or anaphylactoid reactions, which are triggered by dextran-reactive antibodies.
- Coagulopathy: Its interference with coagulation factors and platelet function can increase bleeding time, especially at higher doses.
- Renal Dysfunction: High-molecular-weight dextrans are eliminated by the kidneys, and impairment of renal function has been reported after infusion.
- Cross-matching Interference: Dextran can interfere with blood cross-matching tests, complicating subsequent blood transfusions.
Other Types of Plasma Substitutes
In addition to dextran, a variety of other substances fall into the category of plasma substitutes. These can be broadly classified as either natural or synthetic colloids.
Synthetic Colloids
- Hydroxyethyl Starch (HES): Derived from amylopectin (starch), HES is another class of synthetic volume expanders. Like dextran, it is available in different molecular weights and has been widely used for fluid resuscitation. However, concerns regarding an increased risk of acute kidney injury and mortality in critically ill patients, particularly those with severe sepsis, have led to a significant decrease in its use.
- Gelatins: These are synthetic colloids derived from animal collagen. They provide a short-lived plasma expansion effect due to rapid renal clearance. Side effects include hypersensitivity reactions and interference with hemostasis.
Natural Colloids
- Albumin: A protein derived from human blood plasma, albumin is a natural colloid used for plasma expansion. It is considered a safer option concerning allergic reactions and is pathogen-inactivated. However, it is more expensive than synthetic alternatives, and some studies question its efficacy compared to crystalloids in certain conditions like traumatic brain injury.
- Fresh Frozen Plasma (FFP): While FFP is a natural human blood product, it functions as a plasma substitute and contains all coagulation factors, immunoglobulins, and albumin. Its use is limited by the need for ABO compatibility, thawing time, and a small risk of pathogen transmission.
Comparison of Common Plasma Substitutes
| Feature | Dextran | Hydroxyethyl Starch (HES) | Albumin | Crystalloids (e.g., Saline) |
|---|---|---|---|---|
| Classification | Synthetic Colloid | Synthetic Colloid | Natural Colloid | Electrolyte Solution |
| Mechanism | Osmotic pressure draws fluid into circulation. | Osmotic pressure draws fluid into circulation. | Maintains plasma oncotic pressure. | Increases total extracellular fluid volume. |
| Onset of Action | Rapid. | Rapid. | Rapid. | Immediate. |
| Duration | Longer than crystalloids. | Longer than crystalloids. | Longer than synthetic colloids. | Short-lived in intravascular space. |
| Cost | Less expensive than albumin. | Less expensive than albumin. | High cost due to human sourcing. | Lowest cost. |
| Key Risks | Anaphylaxis, coagulopathy, renal issues. | Coagulopathy, renal toxicity (AKI). | Hypersensitivity (rare), potential harm in TBI. | Edema, hyperchloremic acidosis with large volumes. |
Considerations for Clinical Use
The choice of a plasma substitute is a complex clinical decision that depends on the patient's specific condition. While colloids like dextran were historically used extensively, clinical practice has evolved with a deeper understanding of their risks. For most routine resuscitation, cheaper and safer crystalloid solutions like lactated Ringer's are often the first line of defense.
Colloids may be reserved for specific situations where they offer a distinct advantage, such as managing profound hypovolemia or maintaining plasma oncotic pressure. However, the use of synthetic colloids like HES has fallen out of favor in many settings due to safety concerns, particularly in the critically ill. Human albumin remains a valid, albeit expensive, option for specific indications. Ultimately, no single plasma substitute is superior in all circumstances, and each carries a unique risk-benefit profile that must be carefully considered by medical professionals.
Conclusion
In summary, a notable example of a plasma substitute is dextran, a synthetic colloid historically used to restore intravascular volume during severe blood loss. While effective, its use has become less common due to potential side effects such as allergic reactions and interference with blood coagulation. Other important plasma substitutes include hydroxyethyl starch and human albumin, each with its own set of advantages, disadvantages, and specific clinical indications. As medical understanding has evolved, the role of these agents in fluid resuscitation has been refined, with a shift towards prioritizing patient safety and leveraging less expensive and less risky options like crystalloids where appropriate. A thorough understanding of the properties and risks associated with each type of plasma substitute is vital for effective and safe patient care.
This content is for informational purposes only and does not constitute medical advice. Consult with a healthcare professional for diagnosis and treatment.
