Keeping electrolytes and fluids in balance, part 1
Keeping
electrolytes & fluids
in
part 1
The “critical care shuffle” of
abnormalities can threaten patient
outcomes. Find out how to respond.
C
By Alicia L. Culleiton, DNP, RN, CNE, and
Lynn C. Simko, PhD, RN, CCRN
Critically ill patients are diverse in terms of illness,
but many experience electrolyte abnormalities or
fluid imbalances that can compromise their clinical
status and adversely affect outcomes. These shifts
in electrolytes and fluids—the “critical care shuffle”—can be attributed to an underlying chronic disease state, an acute condition that manifests during
the course of the patient’s hospitalization, or the
administration of certain medications. Monitoring
and carefully managing electrolytes and fluid balance is an integral part of assessing and caring for a
critically ill patient. This series provides a general
overview of the electrolytes tested and I.V. fluids
used in critical care areas, as well as the common
causes, signs and symptoms, and available treatments to correct electrolyte abnormalities and fluid
imbalances. This article describes sodium and fluid
imbalances. A later article will describe imbalances
in potassium, calcium, and magnesium.
The balancing act
Fluid and electrolyte balance play an important role
in homeostasis, and critical care nurses assume a
vital role in identifying and treating the physiologic
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stressors experienced by critically ill patients that
disrupt homeostasis.1
Electrolytes, found in body fluids, are electrically
charged particles (ions). Cations are positively
charged ions; anions are negatively charged ions.
Electrolytes play a crucial role in transmitting
impulses for proper heart, nerve, and muscle function. The number of positively and negatively
charged ions should be equal; when this balance
is upset, electrolyte abnormalities can occur.
Electrolytes can further be classified as extracellular
(EC, outside the cell) or intracellular (IC, inside the
cell). Sodium is the most abundant EC electrolyte;
potassium is the most abundant IC electrolyte.
Just as too little or too much of any one electrolyte can become a problem in maintaining a critically ill patient’s stability, imbalances in fluid homeostasis can also present unique challenges for both
you and your patient.
Fluids are in constant motion in the body. Total
body water (TBW) normally accounts for about
60% of an adult’s body weight. Forty percent of
TBW is in the IC space, and EC water accounts for
20% of body weight: 14% in the interstitial space,
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doesn’t support the numbers, perform a follow-up
test—an error could have occurred in the lab, blood
draw, or be the result of blood sample hemolysis.
Now let’s look more closely at sodium and fluid
balance.
Sodium: Water follows
Sodium is the key EC cation and plays a major
role in serum osmolality. When serum sodium
levels change, so does serum osmolality and water
movement. Normally, serum osmolality is 280 to
300 mOsm/kg; the value is considered critically
abnormal if it’s 240 mOsm/kg or lower, or 320
mOsm/kg or higher.5 Sodium also is a primary
contributor to nerve impulse transmission and
muscle contraction. If you suspect that your critically ill patient has a sodium imbalance, think of
water, and specifically, disorders of water balance.
Remember the saying, “where sodium goes, water
is sure to follow.” In the simplest of terms, sodium
in the body is accompanied by water. Although
serum sodium levels often reflect changes in water
balance, the body’s sodium concentration may be
increased, decreased, or normal, so the patient’s
volume status must also be evaluated before
beginning treatment.1,6 Hypotonic (dilutional)
hyponatremia is the most common type of hyponatremia, and can further be classified as hypovolemic, isovolemic (euvolemic), or hypervolemic
based on the accompanying EC fluid volumes.2
How body water is distributed2
The graphic shows the approximate size of body
compartments in a 154.3-pound (70 kg) adult.
Total body water = 60% body weight
14%
200
28 liters
100
Interstitial
10 liters
5% 1%
Transcellular 1 liter
300
Extracellular water
20% body weight
Plasma 3.5 liters
Intracellular water
40% body weight
Osmolarity - mOsm/L
and 5% in the intravascular space. Transcellular
fluid (cerebral spinal fluid and fluid contained in
other body spaces such as joint spaces, and the
pleural, peritoneal, and pericardial spaces) make
up about 1% of total body weight.2 See How body
water is distributed.
To maintain homeostasis, fluids need to be stable
in the intravascular, interstitial, and IC spaces. The
amount of IC fluid is rather stable in the body;
intravascular fluid is the least stable and fluctuates
in response to fluid intake and loss. Interstitial fluid
is the reserve fluid, replacing fluid in the intravascular and IC spaces as needed.1,3
Almost all pathologies affect the fluid balance
within the body, especially in critically ill patients.
Fluid movement within the various body spaces
depends on osmosis—movement of water through
a selectively permeable or semipermeable membrane from a solution that has a lower solute
concentration to one with a higher solute concentration—and diffusion, or the free movement
of molecules or other particles in solution across
a permeable membrane from an area of higher
concentration to an area of lower concentration,
resulting in an even distribution of the particles
in fluid. Fluid balance also is regulated by certain hormones:
• Aldosterone, the principal mineralcorticoid produced by the adrenal cortex, promotes sodium
retention by the distal tubules, while increasing
urinary losses of potassium. This helps to prevent
water and sodium losses through the kidneys.
• Antidiuretic hormone (ADH), also known as vasopressin, is synthesized in the hypothalamus and
stored in and released by the posterior pituitary
gland. ADH triggers the renal tubules to reabsorb
water and return it to the intravascular space.
• Natriuretic peptides, such as atrial natriuretic peptide, released from the heart in response to cardiac
chamber stretch and overfilling, increase sodium
and water excretion by the renal distal and collecting tubules.2
Because the kidneys are the major organs
involved in electrolyte and fluid homeostasis, determine the patient’s renal function before attempting
to correct a patient’s electrolyte or fluid imbalance.4
Also, don’t let lab numbers or the numerous equations used for fluid replacement override sound
clinical judgment. If the patient’s clinical condition
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31
Hyponatremia
only be used in severe cases of
In hyponatremia, water moves
symptomatic hyponatremia
from an area of low intravascular
because it can rapidly induce
sodium concentration to an area
fluid volume overload.)7 We’ll
of high sodium concentration.
discuss the use of crystalloid
The result is excess fluid volume
solutions later.
in the IC compartment and a
fluid volume deficit in the EC
Hypernatremia
compartment.
In hypernatremia, sodium pulls
Signs and symptoms of hypowater from the IC space into the
natremia vary depending on the
intravascular space. The dehyspeed of onset, magnitude of
drated cells shrink or shrivel, and
sodium deficit, and cause. When
the EC space becomes fluid overRemember that where
serum sodium is less than 135
loaded. Signs and symptoms of
sodium goes, water is
mEq/L (hyponatremia), signs and
hypernatremia include thirst (a
sure to follow.
symptoms generally reflect
compensatory mechanism that’s
hypoosmolality and movement
troublesome in critically ill
of water into muscle, neural, and gastrointestinal
patients, who often are fluid-restricted or physically
(GI) tissue resulting in muscle cramps, muscle
unable to drink); signs and symptoms related to
weakness, decreased deep tendon reflexes, headhyperosmolality and movement of water out of
ache, mental status changes, dizziness, anorexia,
neural tissue, including irritability, restlessness,
nausea, vomiting, abdominal cramps, and diarheadache, disorientation, and decreased deep tenrhea.2 Serum and urine osmolality will be
don reflexes; signs and symptoms related to
decreased. In severe hyponatremia (serum sodium decreased IC fluid including dry skin and mucous
of 115 mEq/L or less), you may notice muscle
membranes, decreased skin turgor, and decreased
twitching, focal weakness, papilledema, and signs
salivation and lacrimation.2 The patient’s urine speof increasing intracranial pressure, such as letharcific gravity and osmolality increase, and central
gy, confusion, hemiparesis, and seizures. Without
venous pressure decreases. In severe hypernatretreatment, increased intracranial pressure can be
mia (serum sodium of more than 160 mEq/L), the
fatal.5
patient may develop seizure activity and coma.4
Hyponatremia is often caused by or associated
Hypernatremia in critically ill patients can be
with treatments and diagnoses common to patients
caused by vomiting, diarrhea, open wounds,
in CCUs. For instance, administering hypotonic I.V. hyperventilation, fever, hypertonic enteral tube
solutions and some medications (such as thiazide
feedings without water supplementation, nasogasdiuretics, nonsteroidal anti-inflammatory drugs,
tric suctioning, GI drains, excessive administration
and selective serotonin reuptake inhibitors) may
of sodium-containing fluids such as 3% sodium
contribute to the development of hyponatremia.
chloride solution and sodium bicarbonate, and
Hyponatremia also is associated with heart, liver,
medical conditions such as diabetes insipidus and
and kidney failure, which are common among criti- Cushing syndrome.5,7-9
cally ill patients.7
As with treating hyponatremia, you’ll implement
When managing a patient with hyponatremia,
measures to restore fluid balance in addition to corthe goal is to identify and treat the underlying cause recting the underlying cause of the sodium imbalof the sodium imbalance. Treatment will also
ance. Treatment measures will depend on whether
depend on whether the patient has a fluid balance
the patient is hypovolemic, isovolemic (also called
abnormality.
euvolemic), or hypervolemic. Most critically ill
Depending on the underlying cause, you may
patients with hypernatremia are also hypovolemic,
administer diuretics, restrict water intake, or
so you’d administer isotonic or hypotonic sodium
administer hypertonic saline (for example, 3% sodichloride solution as prescribed.10 Crystalloid soluum chloride solution, although this fluid should
tions are discussed later.
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33
Keeping electrolytes and fluids in balance, part 1
Fluid balance:
Most critically ill patients will
Dry or wet?
need I.V. fluid replacement.
Fluid imbalances are common
Generally, replace fluid based on
in critically ill patients. Fluid
the type of fluid lost: blood transand electrolyte balance are
fusions for blood loss, isotonic
closely related in the sense that
solutions for patients experiencelectrolyte imbalances can be
ing diarrhea.11
avoided when you pay close
Other I.V. fluids that are availattention to the patient’s nutriable for the treatment of hypovoletional status and the use of I.V.
mia are colloids, which are highfluids.10 Causes of fluid volume
molecular-weight substances that
deficit in the critically ill patient
under normal conditions don’t
A critically ill patient
can include GI loss, infection,
pass through the semipermeable
can be dehydrated
renal loss, and third-space fluid
membrane in the vascular space.
while
appearing fluid
shifts. In contrast, causes of
Colloid solutions are hypertonic,
fluid volume excess are overadincreasing capillary oncotic presoverloaded.
ministration of fluids, heart failsure and pulling fluid from the
ure, renal failure, and certain medications.
interstitial space into the vascular compartment.
Medications may be indicated to treat the causes
Dry: Dehydration
of dehydration, such as antidiarrheals, antiemetics,
Dehydration, or fluid volume deficit, is characterand antipyretics.12
ized by a decrease in EC fluid and occurs when fluid
Third-spacing is another way a critically ill
intake is less than the body’s fluid needs, when
patient can be dehydrated while appearing fluid
patients experience excessive loss of body fluids, or
overloaded. Fluid that accumulates in these spaces
when third-spacing sequesters EC fluid where it
is physiologically useless because it’s not available
can’t contribute to cardiac output. Isotonic fluid def- for use as reserve fluid or function. Causes of thirdicit is the most common type of fluid volume deficit, spacing include:
in which there are proportionate losses in sodium
• Injury or inflammation such as burns, sepsis, crush
and other electrolytes and water. It’s almost always
injuries, massive trauma, cancer, intestinal obstruccaused by a loss of body fluids and is often accomtion, and abdominal surgery. These all increase cappanied by a decrease in fluid intake.2 Without comillary permeability and let fluid, electrolytes, and
pensatory mechanisms, perfusion to vital organs
proteins leak from the intravascular space.
may be decreased, so fluid replacement is vital.
• High vascular hydrostatic pressure from renal failure
Clinical manifestations of fluid volume deficit can or heart failure, which pushes abnormal volumes of
develop rapidly. If you suspect that your patient is
fluid from blood vessels.
at risk for developing hypovolemia, assess for acute
• Malnutrition and liver failure from cirrhosis, chronweight loss, increased thirst, decreased skin turgor,
ic alcohol abuse, or starvation. These conditions
dry mucous membranes, oliguria, high urine specifprevent the liver from producing albumin, thus
ic gravity, weak and rapid pulse, flattened neck
lowering capillary oncotic pressure.3
veins, increased temperature, decreased central
Treating third-spacing can be challenging.
venous pressure, muscle weakness, postural hypoOsmotic diuretics can be used to mobilize some of
tension, and cool, clammy pale skin related to
the fluid back into the intravascular space for elimiperipheral vasoconstriction.2,5 Being familiar with
nation by the renal system. However, this is usually
these clinical findings lets you quickly identify and
only a temporary measure because of the nature of
intervene for patients and avoid potential medical
the disease processes that caused the third-spacing.
emergencies.
Large third space fluid collections can be physically
Treatment of fluid volume deficit includes
removed (by paracentesis for ascites or thoracenteprompt identification and treatment of the underlysis for pleural effusion), and then the vascular space
ing cause and fluid replacement.
can be rehydrated with I.V. fluids.3
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Wet: Overhydration
in this series will focus on manFluid overload or fluid volume
aging the remaining electrolytes
excess is a clinical sign of fluid
that are part of the “critical care
retention or intake that exceeds
shuffle.” ❖
the body’s fluid needs.
Hypervolemia refers to increased
fluid in the intravascular and
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Staying in balance
As you’ve learned, the levels of sodium and body
water can shift up or down. Assessing the volume
status of the critically ill patient is notoriously more
challenging to detect than electrolyte abnormalities,
which can be measured directly.10 The next article
www.nursing2011criticalcare.com
Scales K, Pilsworth J. The importance of fluid balance in clinical
practice. Nurs Stand. 2008;22(47):50-57.
At Duquesne University School of Nursing in Pittsburgh, Pa., Alicia L.
Culleiton is an assistant clinical professor and Lynn C. Simko is an associate
clinical professor.
DOI-10.1097/01.CCN.0000394395.67904.4d
March l Nursing2011Critical Care l
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