Tonicity and IV Fluids
By Kelli Rosenthal, MS, RN, BC, CRNI, ANP, APRN, BC
In adults, water accounts for approximately 60% of body weight (~80% in neonates, decreasing to ~45 Ė 50% as we age). There are variations, of course, based on age, sex, and amount of body fat. The majority of fluid (~40% of body weight) is found inside cells, in the intracellular compartment. The balance is located in the extracellular compartment (outside cells) including intravascular (within the blood vessels), interstitial (between the blood vessels and cells) and transcellular (cerebrospinal, pericardial and synovial) fluids. The body regulates movement of water and electrolytes between compartments so that their distribution and composition remains stable.
One of the bodyís major means of regulating fluid and electrolyte balance is through the process of osmosis. Osmosis is the movement of water through a semipermeable membrane from an area of lower concentration of solute to higher concentration of solute. This tends to equalize the amount of solute ions on either side of the membrane, since solute molecules canít pass through (thatís the definition of "semipermeable"). In the vascular system, the most important semipermeable membranes are the tunica intima (or just "intima", the innermost lining of the veins), the capillary walls, and the cell membranes of red blood cells in the bloodstream.
The rate of osmosis is based on the osmotic pressure within the patientís tissues. Osmotic pressure is the pull that draws the water through the membrane to the more concentrated side Ė either into or out of the cell. When infusing a solution, the amount of osmotic pressure exerted is directly related to how concentrated the infusate is.
The movement of fluids out of (or into) the blood vessels will also be influenced by hydrostatic pressure (pressure of the intravascular fluid against the wall of the vein) and oncotic pressure (sometimes called colloidal osmotic pressure). Higher hydrostatic pressure tends to push fluid out of the vasculature. Oncotic pressure, which is created by the presence of large protein molecules such as albumin and transferrin in the blood, tends to retain fluid in the capillaries. When plasma proteins are low, such as in the case of severe proteinuria or protein-calorie malnutrition (Kwashiorkor), fluid moves into and stays in the interstitial spaces, where it is not accessible to the body to meet its hydration needs. This is an example of third spacing, which can also occur in trauma, burns, lymphatic blockage, and other situations.
Broadly, IV fluids are ordered for the following purposes:
To maintain fluid balance (replace insensible water losses + sweat + urine output when patients are NPO or otherwise unable to drink as much as they need to for replacement)
To replace volume losses (i.e., surgical blood volume loss, losses from the GI tract from vomiting or diarrhea)
To repair imbalances (electrolyte imbalances, acidosis/alkalosis).
The patientís health status and any disease processes play a major role in how effectively the body manages its fluid and electrolyte balance. Since many alterations can be anticipated due to diagnosis, scheduled procedures, and environmental factors, infusion therapy orders can be written accordingly. For example, fever, restlessness/delirium, and high ambient temperatures increase fluid intake needs; situations like hypothermia, high humidity, increased intracranial pressure, reduced urine output, and a decreased level of consciousness cause a drop in the bodyís need for fluid intake. Fluid orders may be adjusted later in response to laboratory values and clinical observations.
An IV solutionís effect on body fluid movement depends in part on its tonicity, or concentration. This term is sometimes used interchangeably with osmolarity, although they are subtly different. Osmolarity is the number of osmols or moles of solute per liter of solvent plus solute. Tonicity is the relative osmolality of a solution.
A solution is isotonic if its tonicity falls within (or near) the normal range for blood serum Ė from 275 to 295 mOsm/kg. A hypotonic solution has lower osmolarity (<250), and a hypertonic solution has higher osmolarity (>350). IV solutions with very high (>500mEq/L) tonicities should only be given via central lines to prevent tissue death within the peripheral veins.
Letís follow how isotonic, hypotonic, and hypertonic solutions behave when theyíre infused.
Remember, in the usual situation the intravascular and extravascular fluids have more or less settled into a stable balance (whether or not itís a healthy one). When an isotonic solution is given, there is little or no change in the concentration of solute and water in the bloodstream, so osmosis neither moves water into the circulation nor pulls it out. Thatís why isotonic solutions like normal saline (0.9% sodium chloride), Ringerís lactate, Ringerís acetate, and D5W (5% dextrose in sterile water) are given to replace fluid losses. Since these solutions replenish and may expand the intravascular compartment, closely monitor the patient for signs of fluid overload Ė especially if there is a history of hypertension or CHF. Although isotonic in the bag, D5W acts like a hypotonic solution (see below) once it enters the bloodstream because its low concentration of dextrose is quickly metabolized by the cells of the lining of the vein and the circulating cells in the bloodstream. The liver converts lactate to bicarbonate, so donít give lactated Ringerís if the patient has a diagnosis of severe liver disease, because he or she wonít be able to metabolize the lactate and may become acidemic. Also avoid LR if the patientís blood pH is already alkaline (above 7.50).
Commonly infused hypotonic fluids include 0.45% saline or 0.25% saline (with or without dextrose). Potassium chloride is sometimes added in low concentrations. When hypotonic solutions are administered, more water (relative to solute) is being infused than is already present in the vessel and inside the cells. Therefore, water moves into the cells, including the cells of the tunica intima of the vein at the catheter insertion site. This extra water causes the cells to swell and burst, exposing the basement membrane of the vein and starting the process of inflammation that can potentially become phlebitis and lead to infiltration (due to swelling of the venous pathway).
When the vein swells, it narrows the lumen, and infused fluids may infiltrate, tracking back up the path of the IV cannula into the surrounding tissue. Watch any hypotonic IV site carefully for the signs of infiltration: coolness, swelling, and discomfort.
Hypotonic fluids have the potential to cause sudden fluid shifts out of blood vessels and into cells, which can cause cardiovascular collapse from intravascular fluid depletion and increased intracranial pressure from fluid shift into brain cells. Thus, hypotonic fluids should not be given to patients already at risk for increased ICP, like those being treated for cerebrovascular accident, head trauma, or neurosurgery. Also, donít give hypotonic solutions to patients at risk for third-space fluid shifts (abnormal fluid shifts into the interstitial compartment or a body cavity) Ė for example, patients suffering from burns, trauma, or low serum protein levels from malnutrition or liver disease. In general, they should not be administered indefinitely, or when the patient is able to meet fluid needs PO.
Hypertonic solutions are those with tonicities exceeding 350 mEq/L. Most admixed medications infused intravenously fall into this category, since they are generally mixed into a solution that was isotonic to begin with and the drug formulations tend to be quite hypertonic. When hypertonic fluids are infused, osmosis pulls water out of the cells. This causes the cells to shrink. When they shrink at the site of IV infusion, the basement membrane of the lining of the vein is exposed, subjecting it to the same complications seen with hypotonic infusions. Hypertonic solutions are used in repairing electrolyte and acid/base imbalances, and also include total and partial parenteral nutrition solutions.
Remember, hypertonic solutions will cause greater damage to the vein as their tonicity increases. Thatís why the Standards of Practice for Infusion Nursing from the Infusion Nurses Society mandates that all fluids with a tonicity exceeding 500 mEq/L be infused through a central venous access device, where the more rapid blood flow of the superior vena cava quickly whisks the solution into the circulation and away from venous tissue. This includes solutions containing more than 10% Dextrose, 5% protein hydrosylate, or high electrolyte concentrations. If youíre not sure about the tonicity of a solution, check with your pharmacy. Other nursing considerations for hypertonic fluids:
Closely monitor the patient for circulatory overload (since they pull fluid from the cells into the intravascular compartment)
Donít give hypertonic solutions to patients with a condition causing cellular dehydration Ė for example, diabetic ketoacidosis
Donít give hypertonic solutions to patients with impaired heart or kidney function, since they may not be able to handle the extra fluid.
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