I.V.

ROUNDS

Intravenous fluids: The whys and wherefores

BY KELLI ROSENTHAL, RN,BC, ANP, APRN,BC, CRNI, MS

MORE THAN 90% of hospitalized moves from an area of

patients receive I.V. therapy during QUICK GUIDE TO I.V. SOLUTIONS low ion concentration (a
their hospital stay, typically as a hypotonic solution) to
Solutions used for I.V. therapy may be isotonic,
continuous infusion. Among the rea- an area of higher ion
hypotonic, or hypertonic. The type you give
sons a patient may receive I.V. ther- depends on whether you want to change or main-
concentration (a hyper-
apy are to: tain body fluid status. tonic solution).
• replace blood or other fluids lost When the number of
Isotonic solution
through surgery, trauma, diarrhea, protein molecules in
(280-300 mOsm/liter)
or vomiting plasma is low, such as
An isotonic solution has an
• maintain fluid balance, as when pa- osmolarity about equal to that
in proteinuria seen with
tients are N.P.O. or can’t drink of serum. Because it stays in uncontrolled diabetes or
enough fluid for other reasons the intravascular space, it protein-calorie malnu-
• correct electrolyte imbalances expands the intravascular trition known as kwa-
• provide a medium for administering compartment. shiorkor, fluid moves
medications and nutritional support. Hypotonic solution
into and stays in the
In this article, I’ll discuss when (less than 280 mOsm/liter) interstitial spaces,
various I.V. fluids are appropriate, A hypotonic solution has an where it’s unavailable to
and why. Let’s start by reviewing the osmolarity lower than that of meet the body’s hydra-
principles behind fluid and elec- serum. It shifts fluid out of the tion needs. This is a
trolyte balance. intravascular compartment, type of third-space fluid
hydrating cells and the intersti- shift, also called third-
Back to basics tial compartments. spacing. This condition
Water makes up about 60% of an Hypertonic solution (greater sequesters fluid in the
adult’s body weight and about 80% than 300 mOsm/liter) interstitial and intracel-
of a neonate’s body weight. The A hypertonic solution has an lular spaces and in a
amount of water normally varies osmolarity higher than that of third-body space (such
somewhat based on such factors as serum. It draws fluid into the as the intestinal lumen)
age, sex, and percentage of body fat. intravascular compartment where it doesn’t support
from the cells and the intersti-
Most fluid—about 40% of body circulation.
tial compartments.
weight—is in the intracellular com-
partment, or inside the cells. The bal- Taking stock of tonicity
ance is in the extracellular compart- An I.V. solution’s effect
ment, which consists of: remain stable. The rate of osmosis on body fluid movement depends in
• intravascular fluid (in the blood depends on the osmotic pressure part on its tonicity, or the concen-
vessels) within the patient’s tissues. This tration of solutes in solution. Par-
• interstitial fluid (between the pressure draws water through semi- enteral solutions are classified ac-
blood vessels and cells) permeable membranes, such as a cording to their tonicity relative to
• transcellular fluid (cerebrospinal, cell membrane. normal blood plasma. The Infusion
pleural, peritoneal, and synovial flu- Responding to osmotic pressure, Nurses Society (INS) classifies a so-
ids). fluid can move into or out of the lution as isotonic if its tonicity falls
Fluid moves between the fluid cell. The amount of osmotic pres- within (or near) the normal range
compartments by osmosis, a process sure depends on the ratio between for blood serum—between 280 and
that regulates water and electrolytes the concentration of ions in the 300 mOsm/liter. A hypotonic solu-
so that their distribution and com- infused solution and the concentra- tion has an osmolarity less than 280
position in the compartments tion of ions in cell fluid. Water mOsm/liter, and a hypertonic solu-

26 Nursing2006, Volume 36, Number 7 www.nursing2006.com

tion has an osmolarity greater than (with or without D5W). Potassium matic injuries, or low-serum protein
300 mOsm/liter. Here’s how the chloride may be added in low con- levels from malnutrition or liver dis-
three fluid types act in the body. centrations to replace losses from ease.
• When an isotonic solution is in- the gastrointestinal system. When a • When hypertonic fluids are in-
fused, water neither moves into nor hypotonic solution is administered, fused, water moves out of the cells
is pulled out of cells because it puts more water in the serum in an attempt to dilute the infusate,
roughly the same concentration of than is found inside cells. As a re- shrinking the cells. When they
solute is on both sides of the mem- sult, water moves into the cells, shrink at the I.V. infusion site, the
brane (the tonicity is equivalent). causing them to swell. basement membrane of the vein’s
That’s why isotonic solutions such Although hypotonic solutions lining is exposed, creating the risk of
as 0.9% sodium chloride, Ringer’s help replace intracellular fluid, the phlebitis and infiltration as described
lactate, Ringer’s acetate, and dex- extra water also moves into the cells above for hypotonic infusions.
trose 5% in water (D5W), are given of the tunica intima of the vein at Hypertonic solutions, used to help
to expand circulating volume and the catheter insertion site. This may reestablish equilibrium in electrolyte
replace actual fluid losses. Because cause the cells to swell and burst, and acid-base imbalances, include
these solutions expand the in- exposing the vein’s basement mem- electrolyte replacement solutions and
travascular compartment, closely brane and potentially leading to parenteral nutrition solutions. But
monitor the patient for signs and phlebitis and infiltration. Watch all because hypertonic solutions can
symptoms of fluid overload, espe- I.V. sites carefully for signs of cause severe damage to the vein, the
INS’s standards of practice mandate
that all fluids with an osmolarity
Although D5W is isotonic in the bag, it acts like a hypotonic greater than 600 mOsm/liter be
infused through a central venous
solution once it enters the bloodstream because simple sugars such access device for greater hemodilu-
as dextrose are the preferred energy source for cells. tion. This includes solutions contain-
ing more than 10% dextrose, 5% pro-
tein hydrosylate, and high electrolyte
cially if he has a history of hyper- phlebitis (erythema at the site with concentrations. If you’re unsure of a
tension or heart failure. or without pain and edema, palpable solution’s final concentration, check
Although D5W is isotonic in the venous cord, streak formation, and with your pharmacy.
bag, it acts like a hypotonic solution purulent drainage) and infiltration Closely monitor any patient receiv-
once it enters the bloodstream (coolness, swelling, and discomfort). ing a hypertonic solution for circula-
because simple sugars such as dex- Because hypotonic solutions have tory overload. Don’t give hypertonic
trose are the preferred energy source the potential to cause sudden fluid solutions to a patient with any condi-
for cells. The low concentration of shifts from blood vessels into cells, tion that causes cellular dehydration,
dextrose in D5W is quickly con- don’t administer them indefinitely. such as diabetic ketoacidosis. Nor
sumed by the cells lining the vein Stop infusing a hypotonic solution should any patient with impaired
and circulating in the bloodstream. once the patient can drink enough to heart or kidney function receive an
Use this solution with caution in meet his fluid needs. Failing to do so infusion of hypertonic solution—his
patients at risk for increased could cause cardiovascular collapse system just can’t handle the extra
intracranial pressure (ICP). from intravascular fluid depletion fluid. ‹›
The liver converts lactate to bicar- and increased ICP from fluid shift SELECTED REFERENCES
bonate, so don’t give lactated into brain cells. Infusion Nurses Society. Infusion Nursing Standards
of Practice. Norwood Mass., Infusion Nurses Soci-
Ringer’s solution if the patient has a Don’t give hypotonic fluids to ety, 2006.
serum blood pH above 7.5 or liver patients already at risk for increased Kee JL. Handbook of Fluid, Electrolyte, and Acid-
disease—he won’t be able to metab- ICP, such as those being treated for Base Imbalances. Stamford, Conn., Thompson
Learning, 2003.
olize the lactate, worsening his alka- stroke or head trauma and those Weinstein S. Plumer’s Principals and Practices of IV
losis. who’ve had neurosurgery. Also avoid Therapy, 7th edition. Philadelphia, Pa., Lippincott
• Commonly infused hypotonic so- giving hypotonic solutions to patients Williams & Wilkins, 2001.

lutions include 0.45% sodium chlo- at risk for third-space fluid shifts, Kelli Rosenthal is president and chief executive offiicer of
ResourceNurse Continuing Education, Inc., of Malverne,
ride or 0.25% sodium chloride such as those with severe burns, trau- N.Y., and president of the Association for Vascular Access.

www.nursing2006.com Nursing2006, July 27