CHAPTER 14. Emergency Medicine
Patricia A. Schenck
FLUID THERAPY
I. Indications
A. Resuscitation from shock
B. Correction of dehydration, hypokalemia, and metabolic acidosis
C. Correction of other electrolyte disturbances
D. Parenteral nutrition
E. Treatment of anemia and coagulopathies
II. Distribution of body water and electrolytes
A. Water
1. Total body water is about 50% to 70% of body weight in adults. Cats have slightly less body water; young animals and neonates have a higher percentage of body water
2. Intracellular water is about 40% of body weight
3. Extracellular water is about 20% of body weight
4. Interstitial fluid is about 12% to 14% of body weight, and intravascular fluid is about 6 to 8% of body weight
B. Electrolytes
1. Sodium and chloride are high in extracellular fluid and low in intracellular fluid
2. Potassium, magnesium, and phosphorus are low in extracellular fluid and high in intracellular fluid
III. Maintenance requirements
A. Water intake should equal water loss in normal animals
B. Insensible water loss is about 20 mL/kg/day
C. Sensible losses are 20 to 40 mL/kg/day in normal animals consuming food
D. A 0.45% sodium chloride solution makes a good maintenance solution but does not contain adequate potassium. Lactated Ringer’s and 0.9% sodium chloride are not good maintenance solutions because they contain too much sodium and chloride
IV. Replacement needs (dehydration)
A. Causes of dehydration
1. Decreased water intake (decreased thirst and appetite centers in sick animals, decreased food intake)
2. Increased water loss (e.g., polyuria, vomiting, diarrhea, salivation, burns)
B. Type of dehydration
1. Isotonic dehydration
a. Normal serum sodium concentration in the presence of dehydration
b. Occurs when there is a loss of water and electrolytes in proportion to that in serum.
2. Hypertonic dehydration
a. Elevated serum sodium concentration in the presence of dehydration
b. Occurs when there is water loss in excess of electrolytes in serum
3. Hypotonic dehydration
a. Low serum sodium concentration in the presence of dehydration
b. Loss of isotonic fluid with intake and absorption of hypotonic fluids with a net dilutional effect
C. Detection of dehydration
1. Dehydration is detectable when approximately 5% of body weight in water has been lost. An acute loss of more than 12% body weight in water is life-threatening
2. History of decreased water intake or vomiting, polyuria, diarrhea, excessive panting or salivation may suggest dehydration
3. Physical examination
a. Findings associated with dehydration include depression, acute loss of body weight, sunken eyes, decreased skin turgor, dry mucous membranes, tachycardia, diminished capillary refill, and signs of shock
b. Skin turgor
(1) When skin is lifted a short distance, it should return quickly
(2) As dehydration progresses, the time required for the skin to return to its normal position increases
(3) Dehydration is as much as 5% to 10% before a loss of skin turgor can be detected
D. Assessment of dehydration
1. Packed cell volume (PCV) and total plasma protein both increase with dehydration
2. Urine specific gravity should increase in a dehydrated animal if the kidneys are healthy
3. Metabolic acidosis is common in dehydration
E. Correction of dehydration
1. Volume of fluid = % dehydration × weight (kg) = liters of fluid to be replaced
2. Type of fluid is chosen based on electrolyte status and acid-base status
V. Types of fluids
A. Crystalloid solutions redistribute quickly. Best for rehydration or replacement of fluid loss
1. Osmolality
a. Hypotonic: Osmolality less than 300 mOsm/kg (0.45% saline)
b. Hypertonic: Osmolality greater than 300 mOsm/kg (5% dextrose in 0.9% saline)
c. Isotonic: Osmolality near 300 mOsm/kg (lactated Ringer’s solution, Plasma-Lyte 148, Normosol-R, 0.9% saline)
2. Replacement fluids are formulated for specific electrolyte deficits
3. Maintenance fluids may differ greatly from serum, and becausee they usually contain glucose, their effect is hypotonic. The energy available from these fluids is insufficient to meet nutritional needs
B. Colloid solutions contain large macromolecules and attract water out of interstitial spaces. They are ideal for replacing intravascular volume but are not good for replacing extravascular fluid loss
1. Natural colloids are produced in the body and harvested for later use (blood transfusion components)
2. Synthetic colloids (such as hetastarch) are complex polysaccharide molecules
a. Used in conjunction with crystalloid fluid therapy. They are used primarily in patients with hypoproteinemia and low oncotic pressure and in the treatment of shock
b. Use with caution in those at risk for noncardiogenic pulmonary edema, in patients with congestive heart failure (CHF), and in those with renal origin oliguria or anuria
C. Fluids to keep on hand
1. The fluid of choice in the absence of laboratory data is an alkalinizing basic electrolyte solution (lactated Ringer’s solution, Plasma-Lyte 148, Normosol-R)
2. If extra sodium or chloride is needed to maintain volume expansion or correct metabolic alkalosis, 0.9% saline is chosen
3. When treating patients with hypernatremia or a water deficit, choose a low-sodium fluid (0.45% saline in 2.5% dextrose, Plasma-Lyte 56, Normosol-M, or 5% dextrose in water)
4. Keep a synthetic colloid on hand to administer in conjunction with crystalloid fluids when rapid intravascular expansion is needed or if there is lowered oncotic pressure
VI. Supplementation of parenteral fluids
A. Potassium
1. Added to fluids if serum potassium concentration is less than 3.5 mEq/L. May still be indicated if potassium is between 3.5 and 4.5 mEq/L if ventricular arrhythmias are present
2. Add approximately 20 to 30 mEq/L of potassium to maintenance fluids if needed
3. The rate of infusion is most important. Do not exceed a rate of 0.5 mEq/kg/hr
4. Use potassium supplementation with caution as death can result due to hyperkalemia
B. Alkali
1. Add sodium bicarbonate if alkali replacement is needed quickly
2. Do not add sodium bicarbonate to fluids containing calcium
3. Do not use in patients with respiratory dysfunction
C. Magnesium
1. Indicated in patients with hypokalemia, diabetic ketoacidosis, and CHF
2. Magnesium sulfate or magnesium chloride is commonly used
3. Do not use in oliguric patients
D. Dextrose: Indicated in hypoglycemia owing to sepsis, insulinoma, overdose of insulin, and liver disease
E. Calcium
1. Use either calcium gluconate or calcium chloride
2. Do not use subcutaneously (SC)
3. Indicated in hypocalcemic seizures or tetany. Monitor heart rate; bradycardia is a sign of toxicity
F. Phosphorus
1. Only significant hypophosphatemia requires phosphorus supplementation (usually in diabetic ketoacidosis)
2. Avoid oversupplementation
VII. General guidelines for fluid selection
A. Serum sodium level as a guide
1. If sodium is normal (normonatremia), use fluids that are normal in sodium and osmolality (lactated Ringer’s, Plasma-Lyte 148, Normosol-R, or 0.9% NaCl)
2. If serum sodium is elevated (hypernatremia), the fluids should contain more water than salt. Choose a hypotonic solution (5% dextrose in water, 0.45% NaCl in 2.5% dextrose)
3. If serum sodium is low (hyponatremia), fluids should contain more salt However, hypertonic fluids are usually not used unless hyponatremia is severe
B. Preservatives
1. Benzoic acid may be used as a preservative, and can be toxic
2. Signs of toxicity are neurologic
3. Do not use fluids containing preservatives in cats, puppies, or small dogs
VIII. Routes of administration for parenteral fluids
A. SC route
1. Do not use in shock, severe dehydration, hypothermia, or emergency replacement of fluids
2. Do not give 5% dextrose in water SC with severe dehydration
3. Choose a site on the trunk for administration
B. Intravenous (IV) route
1. Use for accurate delivery of large amounts of fluids
2. Use the jugular vein in cats and small dogs, or if serious disease is present
3. Cephalic, femoral, or lateral saphenous veins are also used
4. Use 20- to 22-gauge in cats and small dogs, and 14- to 20-gauge in medium to large dogs
5. Place catheter aseptically, and change every 72 hours. Monitor for fever, white blood cells (WBCs), and heart murmurs
IX. Rate of fluid infusion
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