Nurses hang IV fluids daily. But many nurses really don't understand the why behind the fluid type and the anticipated effect fluids have on patients. However, understanding the type of fluid and how the body will respond can be a matter of life or death!
To master fluid administration, you first have to understand osmosis. In general, fluid moves from an area with low osmolality (thin) to an area with high osmolality (thick) to maintain balance (normal osmolality). Normal osmolality in the blood is 275-295 mOS/kg.
Second, you have to understand where fluids “sits” in the body. Your body keeps fluid in basically 3 spaces: extracellular (blood vessels), intracellular (inside our cells), and interstitial (in the body’s tissues). The body responds to changes in fluid status by shifting fluids to protect the extracellular fluid space first. Why? Because it’s the fluid in your blood vessels (extracellular) that perfuse your vital organs (brain, heart, etc.).
KEY POINT: THE BODY SPENDS ENERGY EVERY SECOND OF EVERY DAY TO MAINTAIN NORMAL OSMOLALITY (FLUID BALANCE) IN THE BLOOD VESSELS.
Types of Fluid
Isotonic: The osmolality of an isotonic solution is the same as normal extracellular fluid (blood). Isotonic fluids are given to patients just to replace fluid volume. An isotonic solution does not cause any shifting. So, if your patient loses blood (GI bleed, surgery) you will give an isotonic solution. 0.9% Normal Saline, Lactated Ringers are the most common.
ISOTONIC = NO SHIFTING
Hypertonic: The osmolality of a hypertonic solution is thicker than extracellular fluid (blood). Hypertonic fluids are given to patients if they are fluid overloaded and you need to pull the extra fluid out of their interstitial space and into the blood vessel. Patients in heart failure or post-op typically receive a hypertonic solution until they unload. For example, your post-op patient may receive extra fluids interoperatively. He or she may then need a few bags of a hypertonic solution for a period of time. D5NS or D5LR are hypertonic solutions.
KEY POINT HERE: if your patient is receiving hypertonic fluids post-op, you should also expect to have a stop order!!!
HYPERTONIC = SHIFTS FLUID FROM THE INTERSTITIAL SPACE INTO THE EXTRACELLULAR SPACE
Hypotonic: The osmolality of a hypotonic solution is thinner than normal extracellular fluid (blood). This is given to a patient who is dry and needs to have fluid pulled from their blood vessel into their interstitial space. A patient that is admitted with severe dehydration will receive a hypotonic solution. 1⁄2 normal saline or 1⁄4 normal saline are examples of hypotonic solutions.
HYPOTONIC = SHIFTS FLUID FROM BLOOD VESSELS INTO INTERSTITIAL SPACE
You would never want to give a dehydrated patient a hypertonic solution because if you did, whatever fluid was left in the interstitial and intracellular space would be forced into the blood vessel. In contrast, you would never want to give a hypotonic solution to somebody who is fluid overloaded. If you did, that could potentially go into pulmonary edema.
The key is to understand WHY you are giving fluids and what EFFECT will have on your patient's fluid status.
Remember, ignorance isn't a defense! You must understand how IV fluids impact your patients. If you're not sure...ask!
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