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Focus on
PHARMACOLOGY
ESSENTIALS FOR HEALTH PROFESSIONALS
CHAPTER
24
Drugs Used to
Treat Fluid and
Electrolyte
Balances
Figure 24-1
The urinary system.
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Kidney Structure
• Inner medulla and outer cortex
• Nephrons: functional units located in
renal cortex
– Renal corpuscle: filtering unit with
glomerulus and glomerular capsule
– Renal tubule: proximal convoluted tubule,
loop of Henle, distal convoluted tubule
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Figure 24-2
The kidney with an expanded view of the nephron.
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Renal Physiology
• Urine production promotes homeostasis
by regulating volume and composition
of blood.
• Three organic waste products:
– Urea: most abundant waste
– Creatinine: produced in skeletal muscle
tissue
– Uric acid: formed by recycling nitrogenous
base from RNA molecules
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Diuresis in Elderly People
• Older adults have:
– Decreased ability to concentrate urine
– Decreased ability to tolerate dehydration or
water loads
– Fewer nephrons
– Reduce drug elimination
– More drug accumulation in plasma, causing
possible toxic reactions
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Urine Formation
• Begins with filtration of plasma by
glomerular capillaries (glomerular
filtration)
• Glomerular filtration produces 180 L of
fluid each 24 hours.
• Kidneys return filtered fluid to internal
environment through tubular
reabsorption.
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Urine Formation
• Waste products are excreted in urine.
• Tubular secretion removes hydrogen
ions and toxins faster than through
filtration.
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Figure 24-3
Sites of resorption and secretion in a nephron.
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Homeostasis
Glomerular filtration
+Tubular secretion
−Tubular reabsorption
____________________
Urinary excretion
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Fluid Imbalances in Children
• The urine of infants and children is
more dilute than that of adults because
of:
– Higher blood flow
– Shorter loops of Henle
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Fluid Imbalances in Children
• Pediatric patients are more affected by
fluid imbalances resulting from:
– Diarrhea
– Infection
– Improper feeding
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Fluid Imbalances in Children
• Pediatric patients have limited ability to
quickly regulate change in:
– pH
– Osmotic pressure
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Control of Urine Volume
• Urine volume regulated by reabsorption
of water:
– By osmosis in proximal convoluted tubule
and descending loop of Henle
– Antidiuretic hormone (ADH) needed to
increase water permeability of tubules.
– Absence of ADH can cause diabetes
insipidus.
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Medications and Urine Retention
• Several medications cause urine
retention:
– Anticholinergics and antispasmodics
– Antidepressants and antipsychotics
– Antihistamines
– Antihypertensives
– Beta-adrenergic blockers
– Opioids
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Fluid and Electrolyte Balance
• Levels of water and electrolytes in body
are interdependent:
– Electrolytes are dissolved in water of body
fluids.
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Figure 24-4 Electrolyte balance. Electrolyte intake occurs through the ingestion of
foods and fluids through metabolic reactions. Electrolyte output occurs through the
excretion of feces, sweat, and urine.
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Dehydration in Infants
• At birth, total body water (TBW)
represents about 75 to 80% of total
body weight.
• Decreases to 67% during first year of
life
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Dehydration in Infants
• Infants are particularly susceptible to
significant changes in TBW because of
high metabolic rate and greater body
surface area.
• Dehydration may occur.
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Water Deficit
• Can indicate both sodium and water
loss
• Pure water deficits are rare.
• Comatose or paralyzed patients have
insensible water loss through the skin
and lungs.
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Water Balance
• Water intake = water output
• Water balance regulated by secretion of
ADH and perception of thirst.
• ADH is secreted when plasma
osmolality increases or circulating blood
volume decreases and blood pressure
drops.
• Increase plasma osmolality occurs with
water deficit.
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Fluctuating TBW
in Elderly Patients
• Decreased TBW in elderly patients
results in part from increased body fat,
decreased muscle, and the kidneys’
reduced ability to regulate sodium and
water balance.
• With stress or illness, this decreased
TBW can become life threatening.
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Table 24-1
Normal Water Gains and Losses*
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Electrolytes
• Electrolytes: conduct electricity
(cations) or are decomposed by
electricity (anions)
• Electrolytes of greatest importance:
sodium, potassium, calcium,
magnesium, chloride, sulfate,
phosphate, bicarbonate, hydrogen
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Table 24-2
The Main Electrolytes in the Body
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Electrolyte Imbalances
• Elderly people are at higher risk for
fluid and electrolyte imbalance because
of:
– Decreased thirst sensation
– Decreased ability of kidneys to concentrate
urine
– Decreased intracellular fluid and TBW
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Electrolyte Imbalances
• Elderly people are at higher risk for
fluid and electrolyte imbalance because
of:
– Decreased response to body hormones that
regulate fluid and electrolytes
– Increased use of diuretics
– Decreased fluid and food intake
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Diuretics
• Group of drugs that promote water loss
from body into urine
• Principal action at level of nephrons
• Remove excess extracellular fluid into
interstitial tissue which would otherwise
result in edema
• 5 classes: carbonic anhydrase inhibitors
and osmotic, thiazide, loop, and
potassium-sparing diuretics
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Table 24-3
Osmotic Diuretics
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Osmotic Diuretics: Uses
• Primarily work by decreasing solute
content resulting in less water
reabsorption by loop of Henle and
collecting duct and less sodium
reabsorption in proximal tubule
• Highly effective in cerebral edema
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Osmotics: Adverse Effects
• Common adverse effects of osmotics
include:
– Headache
– Tremor or convulsions
– Dizziness
– Hypotension or hypertension
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Osmotics: Adverse Effects
• Common adverse effects of osmotics
include:
– Thrombophlebitis
– Blurred vision
– Dry mouth
– Nausea and vomiting
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Osmotics: Contraindications
• Osmotics are contraindicated in the
following:
– Anuria
– Pulmonary congestion or edema
– Severe congestive heart failure
– Metabolic edema
– Intracranial bleeding
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Osmotics: Contraindications
• Osmotics are contraindicated in the
following:
– Shock
– Severe dehydration
– Pregnancy and lactation
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Osmotics: Patient Information
• Advise patients to report thirst, muscle
cramps or weakness, paresthesia,
dyspnea, or headache.
• Instruct female patients to avoid
breastfeeding.
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Table 24-4
Carbonic Anhydrase Inhibitors
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Carbonic Anhydrase
Inhibitors: Uses
• Work by converting carbon dioxide into
bicarbonate ions
• Used to treat absence, tonic-clonic, and
focal seizures; to reduce intraocular
pressure in glaucoma; to treat highaltitude sickness
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Carbonic Anhydrase Inhibitors:
Adverse Effects
• Common: anorexia, nausea and
vomiting, weight loss, dry mouth,
thirst, diarrhea, fatigue, dizziness,
drowsiness, hyperglycemia
• Serious: bone-marrow depression,
hepatic dysfunction
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Carbonic Anhydrase Inhibitors:
Contraindications
• Contraindicated in renal and hepatic
dysfunction, Addison’s disease and
adrenocortical insufficiency,
hyponatremia, hypokalemia,
hypochloremic acidosis
• Safety in pregnancy and lactation not
established
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Carbonic Anhydrase Inhibitors:
Contraindications
• Use cautiously in diabetes mellitus,
gout, obstructive pulmonary disease
and respiratory acidosis, and in patients
taking digitalis.
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Carbonic Anhydrase Inhibitors:
Patient Information
• Instruct patients to avoid driving or
hazardous activities if drowsiness or
dizziness occurs.
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Table 24-5
Thiazides and Thiazide-like Diuretics
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Thiazide Diuretics: Uses
• Increase urinary excretion of sodium
and water by inhibiting sodium
reabsorption; also increase excretion of
chloride, potassium, and bicarbonate
ions
• Used to treat hypertension and as
adjunctive therapy in edema associated
with heart failure and cirrhosis
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Thiazide Diuretics:
Adverse Reactions
• Common: anorexia, gastric irritation,
nausea, vomiting, cramping, diarrhea,
constipation, jaundice, hypokalemia,
pancreatitis
• Serious: anaphylactic reactions,
respiratory distress
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Thiazide Diuretics:
Contraindications
• Contraindicated in patients with
diabetes, history of gout, severe renal
disease and impaired liver function, and
in elderly patients
• Not recommended during lactation
• Interactions occur with corticosteroids,
lithium, probenecid, antidiabetic agents
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Thiazide Diuretics:
Patient Information
• Instruct patients with diabetes to
monitor blood glucose levels with care.
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Table 24-6
Loop Diuretics
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Loop Diuretics: Uses
• Act directly on loop of Henle to inhibit
sodium and chloride reabsorption
• Drugs of choice in acute pulmonary
edema of heart failure
• Also used to treat hypercalcemia and
hypertension
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Loop Diuretics:
Adverse Reactions
• Common: fluid/electrolyte imbalances,
skin rashes, photosensitivity,
hypotension
• Serious: circulatory collapse,
thromboemboli, hepatic
encephalopathy, ototoxicity, shock,
cardiac arrhythmias, bone-marrow
depression
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Loop Diuretics: Contraindications
• Do not use in infants or lactating
women.
• Avoid in patients with severe diarrhea,
dehydration, electrolyte imbalance, or
hypotension.
• Use cautiously in patients with hepatic
cirrhosis, diabetes mellitus, pulmonary
edema, pregnancy, and history of gout.
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Loop Diuretics: Contraindications
• Interact with NSAIDs, lithium,
anticoagulants, aminoglycosides, and
thiazide diuretics
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Loop Diuretics:
Patient Information
• Instruct patients to take in the morning
and not at bedtime.
• Advise women to avoid during
lactation.
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Table 24-7
Potassium-Sparing Diuretics
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Potassium-Sparing
Diuretics: Uses
• Prevent sodium reabsorption in distal
tubule
• Used to treat edema of heart failure,
hepatic cirrhosis with ascites, nephrotic
syndrome
• Used mainly in combination with other
drugs to treat hypertension
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Potassium-Sparing Diuretics:
Adverse Effects
• Common: kidney stones, gynecomastia
• Serious: life-threatening hyperkalemia,
renal failure
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Potassium-Sparing Diuretics:
Contraindications
• Contraindicated in patients with acute
renal insufficiency, impaired renal
function, or hyperkalemia
• Use cautiously in pregnancy and
lactation, or in patients with cirrhosis.
• Interact with potassium supplements,
NSAIDs, and lithium
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Potassium-Sparing Diuretics:
Patient Information
• Teach patients about symptoms of
hypokalemia, such as muscle cramps
and weakness, lethargy, anorexia,
irregular pulse, and confusion.
• Advise patients of symptoms of
hyperkalemia, including thirst, dry
mouth, and drowsiness.
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