Eighty percent of the calculated fluid deficit can be replaced in the first 24 hours. More rapid administration of an animal's
estimated fluid deficit can result in diuresis and loss of the fluid administered. After successfully treating hypovolemic
shock and replacing fluid deficits estimated based on the percentage of dehydration, you can administer only maintenance fluids
until the animal can maintain hydration on its own, provided no signs of dehydration or ongoing excessive fluid losses are
present. An objective way to assess whether the fluid volume is adequate is to evaluate body weight regularly throughout the
day. Acute weight loss is commonly associated with fluid loss and can be used to determine whether the patient is at risk
of becoming dehydrated again.
TYPES OF INTRAVENOUS FLUIDS
A variety of crystalloid and colloidal fluids are available, including isotonic, hypotonic, and hypertonic solutions and natural
and synthetic colloids.
A wide variety of crystalloid fluids are available. Crystalloid fluids contain crystals or salts that are dissolved in solution.
Specific crystalloid fluids are indicated for some diseases or conditions and may be contraindicated in others. So whenever
you use a crystalloid fluid, carefully consider it to be another drug in the armamentarium, and assess the particular fluid's
use or potential misuse in each patient.7
Basic categories of crystalloid fluids include isotonic, hypotonic, and hypertonic solutions, depending on the concentration
and type of solute it contains relative to normal body plasma. A maintenance fluid contains electrolyte concentrations similar
to serum, whereas replacement solutions contain slightly higher concentrations of potassium and slightly lower concentrations
of sodium relative to serum.
Isotonic fluids have tonicity, or solute relative to water, similar to that of serum. Examples of isotonic fluids are 0.9%
(normal) saline solution, lactated Ringer's solution, Normosol-R (Abbott Laboratories), and Plasmalyte A (Baxter Healthcare).
Isotonic fluids are indicated to restore fluid deficits, correct electrolyte abnormalities, and provide maintenance fluid
requirements. Long-term use of isotonic fluids can lead to mild hypernatremia in some patients.
Hypotonic solutions have a tonicity less than that of serum. Examples of hypotonic fluid solutions are 0.45% saline solution,
0.45% sodium chloride plus 2.5% dextrose solution, and 5% dextrose solution in water. Dextrose-containing fluids are isotonic
in the bag but become hypotonic once the glucose is metabolized in the body. Essentially, free water is being administered
without the risk of causing iatrogenic hemolysis. Hypotonic fluids are indicated when treating patients with disease processes
that cause sodium and water retention, namely, congestive heart failure and hepatic disease. Hypotonic fluid therapy is also
indicated in patients with severe hypernatremia and allows you to slowly correct a free water deficit. To calculate a patient's
free water deficit, use the following formula8 :
Free water deficit (L) = 0.6 × lean body weightkg × [(patient serum sodium concentration/140) – 1]
Correct the free water deficit slowly to avoid iatrogenic cerebral edema. Ideally, the patient's serum sodium concentration
should not decrease by more than 15 mEq/L during a 24-hour period.
Hypertonic solutions draw fluid from the interstitial fluid compartment and into the intravascular space to correct hypovolemia.
Hypertonic fluid administration is absolutely contraindicated if interstitial dehydration is present. Hypertonic solutions
such as 3% or 7% saline solution have solute in excess of fluid relative to serum. Administer hypertonic saline solution in
bolus increments of 3 to 7 ml/kg as a rapid infusion. The rapid rise in capillary hydrostatic pressure can force sodium to
flow down its concentration gradient into the interstitium. Administering a low-sodium crystalloid fluid after a hypertonic
saline solution can lead to interstitial edema. Therefore, always administer hypertonic saline solution in combination with
a colloid (10 ml/kg hetastarch or dextran 70) to help retain fluid within the intravascular space, thus maintaining intravascular
fluid volume and avoiding the secondary complication of interstitial edema.