Fluid therapy is one of the cornerstones of medical management. Fluids are used to correct dehydration, treat shock, treat
hypotension, diurese, correct electrolyte abnormalities, provide an avenue for other therapeutic agents to name a few. There
are different types of fluids available and they have different advantages and disadvantages depending on the clinical setting
for which they are used. The following discussion will provide a general overview of fluids and fluid selection and emphasize
their clinical indications.
Crystalloids are solutions that contain electrolyte components able to enter the intravascular, intracellular and interstitial
spaces. Their impact on the intravascular space is short lived and their primary effect is on the intracellular and interstitial
compartments. The major advantages of crystalloid solutions are their availability and their cost effect. The major disadvantage
of crystalloid solutions is that only approximately 25% of the volume administered remains in the vascular space 1 hour after
administration. Crystalloids can further be divided into replacement solutions and maintenance solutions.
Replacement solutions are designed for rapid volume expansion, to correct dehydration, diuresis and management of different
types of shock states. The consistent characteristic of all replacement solutions is their high sodium and chloride content
when compared to maintenance solutions. These solutions are designed to approximate the solutes found in plasma.
Lactated ringers solution
Lactated Ringers solution has been a mainstay of fluid therapy for the veterinary clinician. In many cases this crystalloid
has been the default fluid selection. The reason this fluid is commonly chosen is that it is a buffered, isotonic electrolyte
solution that contains high concentrations of sodium and chloride, small amounts of potassium and calcium and provides lactate
as a buffer. The amount of potassium and calcium is not enough for maintenance; therefore these components must be supplemented.
Physiologic saline is a commonly used crystalloid. Among the isotonic solutions, it contains the highest concentration of
sodium and chloride. It also is a fluid that is compatible with many other therapeutics including transfusion products and
can be used as a carrier for these other agents. It is also a fluid that can easily be supplemented with potassium and magnesium
if required by the patient.
Normosol R is a solution very similar to Lactated Ringers solution. It contains a similar concentration of sodium, chloride
and potassium. In addition it contains magnesium but at a lower concentration than is considered maintenance. The major
difference in these solutions is the buffer agent. Rather than lactate being used as a precursor to bicarbonate, acetate
and gluconate are found. These compounds can be converted to bicarbonate in striated muscle and do not require metabolism
via the liver.
Plasmalyte is very similar to Normosol R but has acetate and lactate as bicarbonate precursors and about double the amount
of potassium. These solutions are often used in similar clinical circumstances.