All nurses have a topic or two they didn’t understand well as a new nurse. (Well, maybe more than a couple!) As a med-surg and CCU nurse, I found I really didn’t understand the IV fluid choices for my patients. Why did some have normal saline while others had ½-normal saline (0.45%) or ¼ normal saline (0.2%)? Are these fluids, also called crystalloids, that different? (Crystalloids are fluids containing small minerals and molecules.) I have reviewed the terms isotonic, hypotonic, and hypertonic many times. For some reason, it wouldn’t stick. So, I decided to write about this topic to help myself and others. For some, this is just a review of the basics of IV fluids. Understanding IV fluid choices is a priority for nurses.
Fluids Are Vital
The body needs a constant supply and balance of fluids and electrolytes. Both dehydration and overhydration will challenge the body and lead to multisystem disorders. Likewise, inappropriate electrolyte values impair the function of vital organ systems. Ensuring adequate fluid and electrolytes is an important nursing role. When patients are unable to take fluids orally or through the GI tract, providers will prescribe fluids intravenously (IV). The big questions of IV therapy include:
1. What fluid would be the right choice for the patient?
2. Does the patient need additional electrolyte replacement?
Our bodies are about 60% water. But, giving only water can cause shifting in fluids throughout the body. Water moves where needed based on the principle of osmotic pressure. We have water in our cells and around our cells (in intracellular and extracellular spaces). The water mixes with electrolytes, proteins, glucose, and other substances called solutes. Solutes are dissolved particles. Through the principle of osmosis, water moves to areas that have a higher concentration or more solutes. The concentration of fluids in the body, called osmolality is measured as milliosmoles (mOsm) of solutes in water per kilogram of body weight. Another measurement is osmolarity, milliosmoles of solutes in water per liter. Normal serum osmolality is approximately 275-295 mOsm/kg.
Choices: Isotonic, Hypotonic, or Hypertonic
Words describing the concentration of fluids and blood are isotonic, hypotonic, and hypertonic. Isotonic solutions have the same osmotic pressure and concentration as plasma. Hypotonic solutions have fewer solutes than plasma. Hypertonic fluids have a higher concentration than the body’s fluids. Isotonic fluids have similar osmolality as plasma, When infused, these fluids stay in the blood vessels. They enhance circulation by increasing the blood’s volume.
Many times, isotonic fluid is chosen to allow fluid to stay in the blood. Normal saline (0.9% sodium chloride) or lactated ringer;s solutions are examples of isotonic solutions. In review, normal saline is water with sodium chloride added (154 mEq/L). Lactated ringer’s solution has added electrolytes in the approximate concentrations as the plasma. These electrolytes include sodium chloride, sodium lactate, potassium, chloride, and calcium.
Isotonic Fluids
Isotonic fluids are used to replace the loss of fluids or blood. We see isotonic fluids given for fluid resuscitation. Examples are those suffering from dehydration, hemorrhage, surgical fluid loss, burns, or sepsis. Patients with vomiting, diarrhea, or GI suction loss benefit from isotonic IV fluids. Isotonic normal saline is also used when administering blood transfusions.
Water is a hypotonic solution. Water alone is not a choice for IV fluid replacement. It will leave the blood and move into other spaces that are more concentrated. This includes the intracellular spaces as well as extracellular spaces. When patients need IV fluid, they need this fluid to stay in the blood to enhance circulation. So, giving only water would not help. The water would displace into other areas. This could be a problem when a patient is ill or injured and edema is occurring in one or many areas of the body. Infusing only water increases fluid loss from the blood and increases peripheral edema.
However, there is a type of isotonic fluid given intravenously. D5W may be considered a hypotonic solution. Although dextrose makes the fluid isotonic, the body will use the glucose, leaving only water, a hypotonic solution. Other hypotonic solutions are those with fewer electrolytes than the plasma. These include D50.45% sodium chloride in addition to those solutions with 0.33% and 0.2% sodium chloride.
Hypotonic and Hypertonic Fluids
Hypotonic solutions are used carefully. Yet, they provide fluid to patients who are hypovolemic and dehydrated. They are needed when treating patients having intracellular dehydration, such as in diabetic ketoacidosis. Hypotonic solutions are also used to help renal excretion of solutes. They help treat those with hypernatremia.
Hypertonic solutions have a higher concentration and osmotic pressure than plasma. These solutions include 3% sodium chloride or 5% sodium chloride. Having a higher concentration or tonicity as the blood, hypertonic solutions pull fluid from surrounding tissues into the blood. They also may increase blood sodium levels. Use cautiously and infused slowly as they may lead to intravascular fluid overload.
Infusing the Right Solution
Providers choose the type of I.V. fluid to administer. Many factors go into the decision. These include the amount of fluid lost by the patient and the fluid and electrolyte balance. Surgeons may have a preferred or standard I.V. fluid used postoperatively. Evidence-based fluid strategies for fluid management are being developed (Kayilioglu, et al., 2015). They may be in place in some agencies. The nurse’s role is to administer the prescribed fluids and providing ongoing monitoring. Frequent assessment of vital signs, level of consciousness, heart and lung sounds, intake and output, and lab values are key. By understanding the importance and risks of I.V. fluid administration, the nurse can provide quality care to the patient.
References and Resources:
Crawford, A. & Harris, H. (2011). I.V. fluids: What nurses need to know. Nursing, 41(5), 30-38. doi: 10.1097/01.NURSE.0000396282.43928.40
I.V. Fluids. Nursing Pocket Care, Lippincott Nursing Center.
Kaylioglu, S., Dinc, T., Sozen, I., Bostanoglu, A., Mukerrem, C., & Coskun, F. Postoperative fluid management. World Journal of Critical Care Medicine, 4(3), 192-201. doi: 10.5492/wjccm.v4.i3.192
Open Educational Resources for Nurses. Nursing Fundamentals.
RegisteredNurseRN.com. Isotonic, Hypotonic, & Hypertonic IV Fluid Solutions NCLEX Review Notes.
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