Choosing the right fluid to counter hypovolemic shock........

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.

Hypertonic fluids have a greater tonicity than fluid in the extracellular compartment, so they exert more osmotic pressure. These solutions draw fluid from the intracellular to the extracellular compartment, causing cells to shrink and relieving cellular edema. But hypertonic solutions (such as 3% or 5% sodium chloride solution) raise the risk of volume overload, especially in a patient with heart failure, so assess his response to treatment frequently.

Another hypertonic solution, concentrated dextrose in water (20%, 40%, 50%, 60%, or 70%) is often added to amino acid solutions administered via central vascular access devices to correct hypoglycemia and provide calories. Monitor the patient's blood glucose levels for hyperglycemia and urine output and urine specific gravity for osmotic diuresis. Also monitor the patient's serum electrolytes.

Colloids

Colloids contain undissolved particles, such as protein, sugar, and starch molecules, which are too big to pass through capillary walls. A colloid solution draws fluid from the interstitial and intracellular spaces, increasing intravascular volume. The degree of osmotic pull that a colloid exerts depends on its particle concentration.

Colloid solutions have the same effect as hypertonic solutions and are given in smaller volumes. They also have a longer duration of action because the larger molecules stay in the intravascular compartment longer.

Albumin is the most frequently used colloid solution. A commercially prepared solution, albumin is extracted from human plasma and heated to kill pathogens. It's available in 5% or 25% concentrations (the 5% solution is isotonic) and contains no clotting components. Use albumin for volume expansion when crystalloid solutions are inadequate, as a plasma substitute when treating patients with hypovolemic shock and massive hemorrhage, and to treat patients exhibiting third-spacing of fluid into the interstitial spaces.

A patient who's lost fluid during thoracic surgery would benefit from albumin used as the primary fluid in resuscitation because it enhances blood volume, improves hemodynamics, and reduces the need for blood transfusions.

Blood and blood products

Whole blood contains red blood cells (RBCs), white blood cells, platelets, and plasma. Because storage degrades blood quality fairly quickly, units of whole blood are typically broken down into separate units of RBCs, platelets, and fresh frozen plasma. White blood cells may be removed from the plasma during processing of the blood product. Blood loss can often be managed with blood components and crystalloid and colloid solutions. Whole blood is rarely used unless it's less than 24 hours old and the patient is exsanguinating.

Packed RBCs have the same cell mass as whole blood, making them a good choice for patients who need increased RBC mass and oxygen-carrying capacity without volume overload or for patients with symptomatic anemia, hypovolemic shock, or symptomatic acute or chronic blood loss. Each unit of packed RBCs is typically infused over 1 to 2 hours, but always within 4 hours. Monitor a patient with a poor ejection fraction or a history of heart failure closely. He may require infusions of packed RBCs (smaller-volume infusions than whole blood), perhaps alternating with doses of a diuretic.

Fresh frozen plasma contains albumin, globulins, antibodies, and all other plasma proteins and clotting factors. Although it shouldn't be used for volume expansion, it's useful when clotting factors are required; for example, to counteract the effects of warfarin therapy.

Pharmaceutical plasma expanders

These colloid fluids include hetastarch (Hespan), a synthetic polymer with volume-expanding traits similar to 5% albumin, but with longer-lasting effects. Hetastarch is useful for patients with intravascular volume loss related to trauma, burns, hemorrhage, or surgery.


Dextran is available in low molecular weight (dextran 10%) or high molecular weight (dextran 6%). Composed of large glucose polymers that draw water into the intravascular space, dextran exerts its maximum effect about 1 hour after administration; however, effects may last 24 hours.

Mannitol is a sugar alcohol substance dissolved in 0.9% sodium chloride solution. Available in concentrations from 5% to 25%, mannitol contains an inactive sugar that remains in the vascular space to pull water from the interstitial and intracellular spaces, increasing plasma volume and producing an osmotic diuresis. Mannitol's primary uses are to decrease intracranial pressure from cerebral edema, reverse cerebrospinal fluid buildup, and lower intraocular pressure. Mannitol is sometimes also used for patients in hypoperfused states; for example, a postoperative patient who's had renal artery clamping during abdominal aortic aneurysm repair. During this procedure, perfusion to the kidneys is poor or absent for 15 to 20 minutes. When the clamp is released, mannitol is given to increase intravascular volume and produce osmotic diuresis, improving glomerular filtration and increasing urine flow.

Monitoring for complications

Consider any patient needing fluid resuscitation to be hemodynamically unstable and monitor him closely for complications. Besides keeping meticulous intake and output measurements, record daily weights, lab values, base deficit, serum lactate levels, and vital signs trends. Keep an eye on your patient's skin integrity, as fluid loss or displacement puts him at risk for skin breakdown.

Even a successful fluid resuscitation carries certain risks. For example, aggressive administration of crystalloid solutions can lead to volume overload, electrolyte disturbances, coagulopathy heart failure, pulmonary edema, interstitial edema, and acute respiratory distress syndrome. Colloids and blood products can trigger allergic reactions, including anaphylactic shock. During any fluid resuscitation involving blood components, implement safety measures to assess for and prevent transfusion reactions; be ready to intervene quickly if a reaction occurs.

Massive infusions of cool or room temperature solutions can cause hypothermia. Warm resuscitation fluids according to the manufacturer's guidelines and your facility's policies and procedures to prevent hypothermia.

Diving in

By watching for signs of hypovolemic shock, identifying the source of your patient's fluid loss, and choosing the right replacement, you can correct your patient's fluid imbalance and restore homeostasis.

SELECTED REFERENCES

Corrigan, A., ed: Core Curriculum for Intravenous Nursing, 2nd edition. Philadelphia, Pa., Lippincott Williams & Wilkins, 2000.

Gahart, B., and Nazareno, A.: Intravenous Medications 2003, 20th edition. St. Louis, Mo., Mosby, Inc., 2003.

Kruse, J., et al., eds: Saunders Manual of Critical Care, 1st edition. Philadelphia, Pa., W.B. Saunders Co., 2002.

McKenry, L., et al.: Mosby's Pharmacology in Nursing, 21st edition. St. Louis, Mo., Mosby, Inc., 2001.

BY LOUISE DIEHL-OPLINGER, RN, APRN,BC, CCRN, MSN, AND MARY FRAN KAMINSKI, RN, CCRN

Louise Diehl-Oplinger is an advanced practice nurse at Popkave-Mascarenhas Cardiology in Phillipsburg, N.J. Mary Fran Kaminski is clinical educator in the critical care division at Sacred Heart Hospital in Allentown, Pa.