A closer look at I.V. fluids

Nursing, Oct 1998 by Young, Judy

KATHY HIGGINS, 40, COMES TO THE EMERGENCY DEPARTment complaining of abdominal pain, dizziness, and vomiting that's lasted 2 days. Her BP is 102/60 and her pulse rate is 96. Her skin is pale and her mucous membranes are dry. You know she's dehydrated and needs intravenous (I.V.) therapy, but which type of fluid should she receive? The answer isn't necessarily obvious-and making the wrong choice could be harmful.

In this article, I'll review how I.V. fluids are categorized and when each type is indicated-and why. Let's start with some fluid fundamentals.

What's in the bag?

More than 200 types of commercially prepared I.V. fluids are available to treat fluid and electrolyte imbalances. One way to classify them is by osmolality (tonicity)-the concentration of ions in a solution.

The osmolality of blood plasma-about 290 mOsm/liter-sets the standard. Fluids in the range of 240 to 340 mOsm/liter are considered isotonic. Fluids with tonicities above 340 mOsm/liter are hypertonic; those with tonicities below 240 mOsm/ liter are hypotonic.

Injected I.V., a fluid exerts predictable effects according to its tonicity: Isotonic fluids stay in the vessels (extracellular space), hypertonic fluids pull fluid from the cells (intracellular space), and hypotonic fluids flow into cells. So choosing the right I.V. fluid requires knowing the fluid status of each space or compartment and whether your patient has a fluid deficit or overload. (For more on fluid deficit and fluid overload, see Too Little or Too Much ?)

Another way to classify I.V. fluids is by key ingredients-sodium, dextrose, and multiple electrolytes-which are available in various combinations. Let's look at sodium-containing fluids next.

Salt of the earth

Sodium-containing fluids range from hypotonic (0.45% sodium chloride solution) to isotonic (0.9% sodium chloride solution) to hypertonic (5% dextrose in 0.9% sodium chloride solution). Along with sodium, these fluids provide chloride and free water; some are available in combination with dextrose.

A solution of 0.9% sodium chloride is known as "normal" saline solution because its tonicity (308 mOsm/liter) closely correlates with the osmolality of plasma. This solution contains 154 mEq/liter of both sodium and chloride, close to normal plasma levels of 140 mEq/liter of sodium and 103 mEq/ liter of chloride. (For more on the makeup of common I.V. solutions, see What's in There? Contents of Some Common I.V. Fluids.)

Indications. Hypotonic saline fluids such as 0.45% sodium chloride solution, which expand the intracellular compartment, are indicated for hypertonic dehydration, gastric fluid loss, and cellular dehydration from excessive diuresis.

Isotonic saline fluids such as 0.9% sodium chloride solution can temporarily expand the extracellular compartment during times of circulatory insufficiency, replenish sodium and chloride losses, treat diabetic ketoacidosis, and replenish fluids in the early treatment of burns and adrenal insufficiency. Because their osmolality is similar to that of blood, they're also the standard flush solutions used with blood transfusions.

Hypertonic saline fluids such as 5% dextrose in 0.9% sodium chloride solution are used cautiously to treat severe hyponatremia.

Precautions. Closely monitor your patient for complications, such as electrolyte imbalances, calorie depletion, and increased intracranial pressure (ICP). Because hypertonic fluids pull water from the intracellular space into the extracellular space, fluid volume and ICP can increase. Watch for fluid overload in patients with a history of heart failure or hypertension.

Mrs. Higgins, the patient we met at the beginning of this article, is showing signs of dehydration from vomiting and needs fluid replacement in the extracellular compartment. After the physician's initial evaluation, you insert an I.V. catheter and begin an infusion of 0.9% sodium chloride solution at 150 ml/hour.

Later, the physician diagnoses a bowel obstruction, and Mrs. Higgins has a small-bowel resection. She receives 4 liters of 0.9% sodium chloride solution during surgery.

Now, on her second postoperative day, she's receiving 5% dextrose in 0.45% sodium chloride solution at 100 ml/hour. You find that her vital signs are stable but her abdomen is slightly distended with no bowel sounds. The nursing student assigned to Mrs. Higgins asks why the physician ordered 5% dextrose in 0.45% sodium chloride solution. For answers, let's look at dextrose-containing fluids.

Sugar water

Dextrose fluids, which contain dextrose and free water, are available in concentrations of 2.5%, 5%, 10%, 20%, and 50%. Each percentage represents 1 gram of dextrose per 100 ml of fluid. For example, D5W provides 5 grams of dextrose per 100 ml of water, or 50 grams/ liter. The tonicity of D5W is 253 mOsm/liter.

Dextrose fluids also are available in combination Indications. Dextrose fluids provide calories for energy, sparing body protein and preventing ketosis, which occurs when the body burns fat. They also make it easier for potassium to move from the extracellular to the intracellular compartment. Dextrose fluids flush the kidneys with water, helping them excrete solutes, and improve liver function (glucose is stored in the liver as glycogen).