A plasma substitute solution is an intravenous fluid designed to replace lost blood plasma volume. This therapy is a cornerstone of fluid resuscitation in clinical settings, particularly for patients experiencing hypovolemia due to trauma, surgery, or severe dehydration. By increasing the fluid volume within the circulatory system, these solutions help to restore blood pressure and ensure vital organ perfusion, though they do not possess the oxygen-carrying capacity of red blood cells.
The Two Major Classes of Plasma Substitutes
Plasma substitute solutions are broadly classified into two main categories: crystalloids and colloids. The choice between them depends on clinical context, cost, and the patient's specific condition.
Crystalloids: First-Line Fluid Replacement
Crystalloids are aqueous solutions of mineral salts and other water-soluble molecules that are commonly used for fluid resuscitation. They are inexpensive and widely available, making them a standard first-line treatment for many patients needing volume replacement.
- Composition: Common examples include normal saline (0.9% sodium chloride) and Lactated Ringer's solution, which contains sodium, potassium, calcium, and lactate.
- Mechanism: Once infused, crystalloids distribute throughout the extracellular fluid space, including the interstitial space. This means a larger volume is required to achieve the desired intravascular volume expansion compared to colloids, and the effect is shorter-lived.
Colloids: Staying in the Bloodstream
Colloids are solutions containing larger molecules that do not readily cross the semipermeable vascular barrier. This property allows them to remain in the intravascular space for a longer period, exerting oncotic pressure to pull fluid into the bloodstream from the interstitial space and effectively expanding blood volume with smaller infused volumes.
- Natural Colloids:
- Human Serum Albumin: A protein produced from human plasma, it is a safe and effective plasma expander. It is often used in plasma exchange procedures and for patients with severe hypoalbuminemia or certain burn injuries. Albumin is pasteurized to inactivate pathogens.
- Fresh Frozen Plasma (FFP): Contains all coagulation factors and is typically used when a patient has a bleeding disorder or requires coagulation factor replacement. Unlike albumin, FFP must be blood type-matched and carries a small risk of transmitting blood-borne pathogens.
- Synthetic Colloids:
- Hydroxyethyl Starch (HES): Derived from amylopectin, HES was widely used for volume expansion but has fallen out of favor in many regions due to associations with adverse effects like acute kidney injury and coagulopathy, especially in critically ill patients.
- Dextrans: Glucose polymers historically used for volume expansion and their antithrombotic properties. However, their use is now limited due to risks including allergic reactions, renal impairment, and interference with blood crossmatching.
- Gelatins: Derived from collagen, these are less potent plasma expanders than others but have a better safety profile regarding coagulation. They are used for short-term volume replacement.
How Plasma Substitutes Work
Intravenous fluids work by altering the balance of forces that govern fluid movement across the capillary walls, known as Starling forces.
- Oncotic Pressure: This is the pressure exerted by large molecules, such as albumin and other colloids, which are too large to easily pass through the vessel walls. Colloids increase the blood's oncotic pressure, drawing fluid from the extravascular (interstitial) space into the intravascular space.
- Hydrostatic Pressure: This is the pressure exerted by the fluid itself on the vessel walls. Infusing any fluid, including crystalloids, increases the hydrostatic pressure inside the blood vessels. Crystalloids predominantly increase the overall extracellular fluid volume, with much of the fluid rapidly shifting to the interstitial space.
Colloids, by remaining in the circulation longer, provide a more sustained and efficient intravascular volume expansion per unit of fluid infused compared to crystalloids. However, the choice of fluid is based on the specific clinical goal, as both fluid types have their distinct roles.
Clinical Applications
Plasma substitute solutions are used to manage a variety of life-threatening conditions involving significant volume loss.
- Trauma and Hemorrhage: In emergency situations with massive blood loss, plasma expanders can be used for rapid volume replacement to stabilize a patient's circulation while preparing for a blood transfusion.
- Severe Burns: Patients with severe burns suffer from significant fluid and protein loss from damaged capillaries. Colloids like albumin are used to replace this lost protein and maintain plasma volume.
- Sepsis and Septic Shock: In septic shock, the inflammatory response increases vascular permeability, leading to fluid leakage from the vessels. Colloids or crystalloids may be used to counteract this and maintain circulatory volume, though some synthetic colloids are not recommended in this setting.
- Surgery: Plasma expanders are used during surgery to manage blood loss, maintain stable hemodynamics, and to achieve controlled hemodilution.
Risks, Side Effects, and Modern Considerations
While essential, plasma substitutes are not without risks, and their use requires careful consideration, especially in critically ill patients. Modern evidence has shifted clinical practice away from certain agents due to safety concerns.
- Allergic Reactions: Anaphylactoid reactions can occur with synthetic colloids like dextrans and gelatins.
- Coagulopathy: Synthetic colloids such as HES and dextrans can interfere with blood clotting, potentially leading to or worsening bleeding.
- Renal Impairment: Some colloids, particularly HES, have been linked to an increased risk of acute kidney injury.
- Volume Overload: Rapid infusion of any plasma substitute can lead to circulatory overload, especially in patients with pre-existing heart or kidney conditions, potentially causing pulmonary edema.
- Metabolic Disturbances: Saline can cause hyperchloremic metabolic acidosis with large volumes, while FFP can lead to citrate toxicity, causing hypocalcemia.
The most notable shift in recent years involves synthetic colloids. Robust studies and meta-analyses have raised serious concerns about the safety of HES solutions, especially in patients with septic shock, linking them to higher rates of acute kidney injury and renal replacement therapy. As a result, guidelines increasingly favor crystalloids or albumin, leading to a decline in HES use for these indications. For additional reading on the controversy surrounding HES, the following research is insightful: [https://pmc.ncbi.nlm.nih.gov/articles/PMC4234780/].
| Feature | Crystalloids (e.g., Normal Saline, Ringer's) | Colloids (e.g., Albumin, HES, Dextran) |
|---|---|---|
| Composition | Small, water-soluble electrolytes and molecules | Large molecules (proteins or polymers) suspended in a crystalloid base |
| Intravascular Half-Life | Short; distributes rapidly into extravascular space | Longer; primarily remains in the intravascular space |
| Volume Needed | Requires large volumes for sustained effect | Requires smaller volumes for equivalent volume expansion |
| Cost | Much less expensive | Significantly more expensive |
| Adverse Effects | Risk of electrolyte imbalance and fluid overload; potential for metabolic acidosis with saline | Risk of allergic reactions, coagulopathy (esp. HES, dextrans), renal impairment (esp. HES) |
| Availability | Widely available | Can have supply chain challenges; FFP requires typing and thawing |
Conclusion
Plasma substitute solutions are a cornerstone of modern emergency medicine and surgical care, enabling rapid restoration of blood volume in critical situations like hypovolemia, shock, and severe burns. The two main types, crystalloids and colloids, offer different advantages and risks based on their composition and effect on blood volume. While crystalloids provide a cost-effective, readily available option for initial fluid replacement, colloids offer a more sustained intravascular volume expansion. However, the use of certain synthetic colloids like HES and dextrans has been limited due to serious safety concerns, with albumin and balanced crystalloids often preferred. The choice of the most appropriate plasma substitute is a complex clinical decision, balancing the urgent need for volume restoration against the unique risks and benefits of each available solution for the individual patient.
