Understanding Plasma Substitutes
Plasma substitutes, often called plasma expanders, are intravenous (IV) solutions used to restore or maintain circulating blood volume. They are particularly crucial in emergency situations involving severe blood loss, trauma, burns, or septic shock where a rapid increase in intravascular volume is necessary. Unlike whole blood or red blood cell transfusions, these substances do not carry oxygen but focus on providing volume expansion to maintain blood pressure and organ perfusion. The two main types are colloids and crystalloids, though the term "plasma substitute" more precisely refers to colloids which contain large molecules that remain within the vascular space longer.
Natural Colloid Plasma Substitutes
Naturally derived colloids come from human blood products and include albumin and fresh frozen plasma (FFP). They are generally safe, though more expensive and less readily available than their synthetic counterparts.
- Human Albumin: This is a natural protein fraction of human plasma, available as a 5% solution. It is pasteurized to inactivate pathogens, has a very low risk of allergic reaction, and doesn't require blood type matching. Albumin exerts high oncotic pressure, effectively drawing fluid into the intravascular space to expand volume. It is useful in plasma exchange procedures and for specific conditions but does not replace the clotting factors found in FFP.
- Fresh Frozen Plasma (FFP): FFP contains all the plasma clotting factors, making it the most physiological option. However, it must be thawed before use, requires ABO blood type matching, and carries a slight risk of allergic reactions and transmitting infectious diseases. It is typically reserved for situations where specific clotting factors are needed, rather than simple volume expansion.
Synthetic Colloid Plasma Substitutes
Synthetic colloids are large-molecular-weight substances suspended in crystalloid solutions. They are a cost-effective alternative to natural colloids but can carry specific risks and potential adverse effects.
- Dextrans: A synthetic colloid derived from glucose polymers, available in formulations like Dextran 40 and Dextran 70.
- They are effective plasma expanders that improve microcirculatory blood flow and inhibit platelet aggregation.
- Potential side effects include allergic reactions, coagulopathy (acquired von Willebrand's syndrome), and renal dysfunction, leading to a decline in their routine use.
- Pretreatment with Dextran 1 can help reduce the risk of anaphylactoid reactions to higher molecular weight dextrans.
- Gelatins: Synthetic colloids made of polypeptides produced from the degradation of bovine collagen.
- They are sterile, pyrogen-free, and available in forms like modified fluid gelatin.
- Their smaller molecular size and rapid renal clearance mean that repeat infusions are often necessary to maintain volume.
- Similar to dextrans, they can interfere with platelet function and have been associated with a higher incidence of anaphylaxis compared to other synthetic colloids.
- Hydroxyethyl Starch (HES): A class of synthetic colloids derived from amylopectin, a starch from maize or potatoes.
- HES solutions are available in different concentrations and molecular weights (e.g., HES 130/0.4, HES 200/0.5), which affects their volume-expanding effect and half-life.
- Concerns over potential nephrotoxicity and increased mortality in critically ill patients have led to recommendations against their use in this population by regulatory bodies in Europe and the US.
Comparison of Major Plasma Substitutes
| Feature | Human Albumin (Natural Colloid) | Synthetic Colloids (Dextran, Gelatin, HES) |
|---|---|---|
| Source | Human blood plasma | Glucose polymers (Dextran), bovine collagen (Gelatin), amylopectin (HES) |
| Mechanism | Exerts oncotic pressure, drawing fluid into blood vessels | Contains large molecules to remain in the intravascular space |
| Cost | More expensive | Generally less expensive |
| Risk of Transmitted Disease | Very low, as it is pasteurized | None, as it is not blood-derived |
| Allergic Reactions | Extremely rare | Possible, varies by type; higher incidence with gelatin and dextran |
| Coagulopathy | Minimal effect on coagulation compared to synthetics | Can interfere with platelet function and other coagulation factors |
| Renal Impact | Minimal in most cases | HES is associated with renal toxicity, especially in critically ill patients |
| Storage | Liquid at room temperature, long shelf life | Long shelf life, various storage conditions |
Clinical Considerations and Modern Usage
The use of plasma substitutes, particularly synthetic colloids, has evolved significantly based on clinical research and safety data. Studies like the CRISTAL trial have shown that colloids do not necessarily offer a superior benefit over cheaper crystalloids in critically ill patients. This, combined with concerns over adverse effects like kidney injury and bleeding associated with HES and dextran, has led to a more cautious and selective approach to their use.
Today, crystalloid solutions such as normal saline or lactated Ringer's are widely preferred for initial fluid resuscitation due to their cost-effectiveness and favorable safety profile. Synthetic colloids are reserved for specific circumstances and patient populations where their properties are clearly indicated. For example, albumin may be preferred in certain burn victims or for plasma exchange, while the use of HES is now largely discouraged in critically ill patients due to safety concerns.
The choice between different plasma substitutes is complex and depends on factors like the patient's underlying condition, severity of volume loss, and presence of coagulopathy or renal impairment. A balanced approach, often starting with crystalloids and considering colloids for specific indications, is the modern standard of care.
Conclusion
In summary, the most common examples of plasma substitutes are broadly categorized as natural and synthetic colloids. Natural options include human albumin and fresh frozen plasma, offering physiological benefits but with higher costs and limited availability. Synthetic colloids, such as dextran, gelatins, and hydroxyethyl starch, are less expensive but have documented risks, including coagulopathy and renal issues, that have limited their widespread use. Modern practice favors using cheaper and safer crystalloid solutions for initial volume resuscitation, reserving colloids for targeted applications. The decision ultimately rests on a careful assessment of the patient's clinical needs and the risk-benefit profile of each option.
World Health Organization: Use of plasma volume substitutes and plasma in developing countries
