The Pathophysiology of Burn Shock and Fluid Shift
Severe burn injuries, typically those exceeding 20% of the total body surface area (TBSA) in adults, trigger a dramatic and systemic inflammatory response. This response involves the release of inflammatory mediators like histamines and prostaglandins, which cause widespread changes in microvascular function. The most significant change is a rapid increase in capillary permeability, a phenomenon known as “capillary leak”.
This capillary leak allows fluid, electrolytes, and plasma proteins to shift rapidly from the intravascular space (the bloodstream) into the interstitial space (the tissue surrounding the blood vessels). This causes two critical problems:
- Intravascular hypovolemia: A sudden and profound decrease in the volume of blood circulating in the body.
- Massive edema: The accumulation of excess fluid in the tissues, both in burned and unburned areas.
This combination of hypovolemia and edema results in burn shock, a life-threatening condition characterized by decreased cardiac output and inadequate perfusion of vital organs. Without immediate and effective intervention, this can lead to organ failure and death.
The Role of Crystalloids in Resuscitation
Why we give crystalloids in burns is fundamentally about addressing the core problem of volume depletion. Crystalloid solutions, such as Lactated Ringer's, are aqueous solutions containing mineral salts and are isotonic, meaning they have a similar electrolyte concentration to plasma. They are the preferred initial choice for fluid resuscitation for several key reasons:
- Rapid Volume Replacement: They effectively and immediately replace the lost water and electrolytes, helping to restore the depleted intravascular volume and maintain blood pressure.
- Availability and Cost: Crystalloids are inexpensive, readily available, and safe for wide-scale use, making them a practical choice in emergency settings.
- Optimal for Initial Phase: In the initial 24 hours post-burn, capillary permeability is at its maximum. During this period, larger molecules found in colloids (like albumin) would also leak into the interstitium, drawing even more fluid with them and potentially worsening edema. Crystalloids, composed of smaller molecules, are less likely to cause this paradoxical augmentation of third-space fluid during the peak leak phase.
- Metabolic Correction: Lactated Ringer's solution, the most common crystalloid used, contains lactate that is metabolized to bicarbonate. This helps to buffer the metabolic acidosis that often accompanies severe burn injuries and shock.
Comparison of Crystalloids and Colloids
| Feature | Crystalloids | Colloids |
|---|---|---|
| Composition | Aqueous solution of mineral salts (e.g., sodium, chloride) | Larger molecules, such as proteins (e.g., albumin, plasma) |
| Cost | Inexpensive and widely available | Expensive and often less accessible |
| Fluid Shift | Replaces volume but moves freely between intravascular and interstitial spaces | Stays in the intravascular space longer due to larger molecules, but leaks during high permeability phase |
| Use in Initial 24 Hours | Preferred for initial resuscitation due to severe capillary leak | Generally avoided in the first 12–24 hours as they can worsen edema by leaking into tissues |
| Effect on Edema | Can cause edema, especially with over-resuscitation | Theoretically reduce edema by maintaining oncotic pressure intravascularly, but effectiveness debated in early phase |
| Risk Profile | Risk of hyperchloremic acidosis with large volumes of normal saline | Higher cost, potential for allergic reactions, and questionable benefit in early resuscitation |
Fluid Calculation: A Foundation for Fluid Administration
To guide initial crystalloid administration, healthcare providers rely on established formulas, with the Parkland formula being one of the most widely used globally. This formula is a starting point for estimating fluid requirements during the first 24 hours post-burn. It is calculated based on factors such as patient weight and the percentage of total body surface area burned.
It is crucial to understand that such formulas are merely guides. Factors like inhalation injury, electrical burns, and a delay in resuscitation can influence fluid needs. Formulas provide a starting point, which must then be adjusted based on the patient's ongoing response.
Monitoring for Optimal Resuscitation
Effective fluid resuscitation hinges on continuous patient monitoring. The goal is to provide enough fluid to maintain adequate organ perfusion while avoiding the complications of both under- and over-resuscitation. The most reliable and readily available metric for monitoring resuscitation adequacy is hourly urine output, which is measured using an indwelling urinary catheter.
Monitoring parameters typically include:
- Urine Output: The target range for urine output is a critical indicator of adequate organ perfusion.
- Heart Rate and Blood Pressure: Assessment of vital signs helps gauge cardiovascular status.
- Clinical Signs: Mental status, peripheral perfusion (e.g., skin color, temperature), and capillary refill provide additional clues about tissue oxygenation.
Careful titration prevents complications. Under-resuscitation can lead to acute kidney injury and further tissue damage. Conversely, over-resuscitation can cause excessive edema, a phenomenon known as “fluid creep,” leading to compartment syndromes, respiratory failure, and multiple organ dysfunction.
Conclusion
In severe burn injuries, the systemic inflammatory response leads to widespread capillary leak and a profound shift of fluid from the bloodstream to the interstitial tissues, resulting in burn shock. Crystalloid solutions are the treatment of choice for initial resuscitation because they are readily available, inexpensive, and effective at restoring intravascular volume and electrolyte balance. Formulas provide a necessary starting point for determining fluid needs, but success depends on careful, continuous monitoring and adjustment based on patient response. This meticulous approach ensures that vital organ function is maintained, allowing for improved outcomes and survival in burn patients.
Fluid Resuscitation in Adult Burns - Current Medical Issues
Disclaimer: Information is for general knowledge, should not be taken as medical advice, and should consult with a healthcare provider.
