Choosing the right fluid to counter hypovolemic shock

Nursing, Mar 2004 by Diehl-Oplinger, Louise, Kaminski, Mary Fran

Find out how to recognize hypovolemia early and provide appropriate fluid resuscitation for your patient.

A PATIENT THREATENED by hypovolemic shock needs immediate intravenous (I.V.) fluid resuscitation to survive. Do you know which fluids are appropriate-and which to avoid? In this article, we'll examine the fluids used for resuscitation and discuss which one is right for your patient, depending on his condition. Let's start by looking at how fluid loss or shifts within the body lead to hypovolemia.

What causes hypovolemia?

The body has two main fluid compartments: Fluid in the cells is called intracellular fluid; fluid in plasma (intravascular) and interstitial space is called extracellular fluid.

In a healthy person, the amount of fluid in intracellular and extracellular spaces is relatively constant, but water and solutes, such as electrolytes, move among the compartments to maintain homeostasis. Fluid intake and output provide a rough measure of homeostasis: They must be approximately equal to maintain balance. Illness or injury upsets the balance, requiring your intervention.

Hypovolemia results from internal fluid shifts or external fluid losses:

* Internal fluid shifts leading to hypovolemia occur as fluid moves out of the intravascular compartment into another area of the body, such as the interstitial space; for example, during internal hemorrhage associated with a hemothorax, long-bone fracture, or ruptured spleen. Third-spacing occurs when fluid accumulates in the extracellular and intracellular spaces and in a third body space (such as the intestinal lumen) that doesn't support circulation.

* External fluid loss can result from bleeding, vomiting, diarrhea, nasogastric suction, diuretic therapy, diabetes insipidus, hyperglycemic osmotic diuresis, severe burns, trauma, and surgery.

The goal of fluid resuscitation is to maintain perfusion to the patient's vital organs, especially the brain and heart, by restoring circulating volume.

Warning signs of shock

Untreated hypovolemia can quickly evolve into hypovolemic shock, which produces characteristic signs and symptoms depending on severity:

* mild hypovolemic shock-diaphoresis, anxiety, increased capillary refill time, and cool extremities

* moderate hypovolemic shock-the same as for mild shock, plus increased heart and respiratory rates and decreased urine output

* severe hypovolemic shock-the same as for moderate shock, plus hemodynamic instability, hypotension, and altered mental status, including coma.

Regular assessments can help you identify and treat hypovolemia at an early stage, before the patient's condition deteriorates. Remember that very young and elderly patients are especially vulnerable to fluid imbalances.

Choosing the right fluid

Parenteral fluids can be classified in several ways; for example, crystalloid or colloid, blood and blood products, and pharmaceutical plasma expanders. Two main factors affect the choice of fluid for your patient: how the volume loss occurred and which solutes need to be replaced.

First, address the underlying problem; for example, stop the bleeding or treat the vomiting or diarrhea. Next, provide I.V. fluids to restore circulating blood volume. Let's look at how I.V. fluids are categorized and when each type is indicated.

Crystalloids

Crystalloid solutions closely mimic the body's extracellular fluid. Common examples are 0.9% sodium chloride solution and Ringer's solution. Given I.V., crystalloid solutions diffuse through the capillary walls that separate plasma from interstitial fluid. They can be used to expand both intravascular and extravascular fluid volume.

Crystalloids are further classified by tonicity, or the number of particles (or solutes) in the solution. A fluid's tonicity controls fluid movement between fluid compartments. To maintain homeostasis, fluids move from areas of lower solute concentration to areas of higher solute concentration, a process called osmosis.

Isotonic fluids have the same tonicity as plasma. They're useful in raising intravascular volume without altering fluid shifts in or out of cells or changing plasma electrolyte concentration. Common isotonic fluids include 0.9% sodium chloride solution, D^sub 5^W, Ringer's solution, and lactated Ringer's solution.

Use isotonic fluids for patients whose fluid losses stem from vomiting and diarrhea, those awaiting an infusion of blood and blood products, and patients who lost fluid during surgery. Because isotonic fluids expand circulating volume, monitor for fluid excess or overload.

Hypotonic fluids, such as 0.45% sodium chloride solution, help the body restore homeostasis by moving fluid into the intracellular compartment. Because hypotonic fluids have a lower concentration of particles than plasma, they exert less osmotic pressure than the fluid in the extracellular compartment. Hypotonic fluids often are given to patients whose sodium intake must be restricted, such as those with hypernatremia.

Monitor the patient closely; too much of a hypotonic fluid can cause intravascular fluid depletion, hypotension, and cellular edema and tissue damage.