What are Intravenous (IV) Fluids?

Different Uses of Intravenous Fluids

This lesson will discuss the appropriate uses for intravenous (IV) fluids. It may seem complicated to learn about IV fluids from a textbook, but it is easier to understand in the hospital setting. Here is a quick and easy method to understand the different IV fluids. This method will focus on understanding the pathophysiology rather than using rote memorization.

The outline for this lesson includes:

• Isotonic fluids
• Hypertonic fluids
• Hypotonic fluids

Different types of fluids have different effects on the body.

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Isotonic Fluids

Isotonic fluids are the most common IV fluids used in hospitals and other settings.

Examples are:

• 0.9% sodium chloride
• Lactated Ringer solution

0.9% sodium chloride (normal saline) is an isotonic fluid.

The prefix “iso” means equal. The concentration of solutes in isotonic fluids is equivalent to that normally found in the serum. 

The key concept is to think of 0.9% as the baseline concentration for solutes in IV fluids. Isotonic fluids have a solute percentage of 0.9%. Any concentration greater than 0.9% is considered hypertonic, and any below 0.9% is considered hypotonic. This 0.9% rule works for every IV fluid except for 5% dextrose (D5W). D5W will be discussed further in the hypotonic fluids section.

Isotonic fluid is used to replace intravascular losses because it stays within the intravascular space (space within the blood vessels). 

Isotonic fluids are most often used in emergency situations requiring the rapid replacement of fluid losses. Examples of this would be:

• Hypovolemia 
• Septic shock
• Hemorrhage

Lactated Ringer is a type of isotonic fluid used frequently in postoperative patients to replete blood that was lost during surgery. This IV fluid is formulated with electrolytes that are similar to those found in the blood.

Intravenous fluids are isotonic, hypotonic, or hypertonic.

Read: Diabetic Ketoacidosis: Fact and Fiction

Hypertonic Fluids

Hypertonic fluids have a solute percentage greater than that found in the serum, or more than 0.9%.

Examples are

• 3% sodium chloride (NaCl)
• Dextrose 5% with normal saline (D5NS)

When hypertonic fluid is administered to a patient, it makes their serum more concentrated and increases the osmolality. And where does the fluid go? The fluid will travel from places of lower concentration to places of higher concentration. This means that fluid from the cells will travel into the intravascular space. Basically, hypertonic fluid gives the serum a “pulling power.”

Hypertonic fluids are used for:

• Head injury. When patients have a head injury, the main concern is cerebral edema, which is swollen brain cells. Giving hypertonic fluids will help pull fluid out of the brain cells and reduce swelling caused by the injury.
• Hyponatremia. Hypertonic fluids are used to correct severe hyponatremia. 3% sodium chloride is commonly used.
• Hypoglycemia. Hypertonic fluids such as dextrose 50% (D50W) are given to correct severe hypoglycemia or low blood glucose.

When a patient receives a hypertonic fluid, they are at risk for intravascular fluid volume overload and may develop complications. The nurse will be measuring daily weights, recording inputs and outputs, and listening to the lungs. The nurse will also be checking the blood pressure and assessing for edema. Typically, edema will not be visible until the patient has gained an excess of 3 liters of fluid.

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Hypotonic Fluids

The solute concentration of hypotonic fluids is less than that found in the serum, or less than 0.9%. Administration of hypotonic fluids makes the intravascular space more dilute. 

Where does the fluid travel? Fluid will travel from places of low concentration to areas of high concentration. Therefore, the fluid is going to travel out of the intravascular space into the cells.

Examples of hypotonic fluids are:

• 0.45% sodium chloride (also called half-normal saline)
• 0.225% sodium chloride
• Dextrose 5% in water 

Dextrose 5% in water (D5W) is an exception to the 0.9% rule. Although 5% is greater than 0.9% and may seem hypertonic, D5W is actually hypotonic. D5W is dextrose 5% that is diluted in water. It is initially isotonic in the bag, but the dextrose is metabolized when administered, leaving water that then dilutes out into the serum and expands the extracellular space. Therefore, D5W is considered a hypotonic fluid.

Hypotonic fluids are used in cases of:

• Extreme hypernatremia. Hypotonic fluids help dilute the sodium level but must be given slowly so as not to bring the serum sodium down too rapidly and cause adverse effects.
• Diabetic ketoacidosis (DKA). Initially, a patient with DKA will receive normal saline to replace volume losses. Then, they will receive half-normal saline. After that, they will receive D5W to prevent a sudden drop in glucose levels when their numbers start coming down. 
• Heart failure. Hypertonic fluids should not be given to patients with heart failure because that can put them at risk for intravascular volume overload. 

A very important note about hypotonic fluids is that they should never be given to patients with potential or actual head injuries. This includes stroke patients because a common symptom is cerebral edema. Giving these patients hypotonic fluids could make the cerebral edema worse.

Hypotonic fluids cause cells to swell, while hypertonic fluids pull fluid out from the cells.

Summary Table

Below is a summary table of IV fluids and uses.