Fluid therapy is one of the most important aspects of patient management in veterinary medicine. It is however, important
to realize that fluid therapy is a supportive measure and the underlying disease process that lead to aberrations in water,
electrolyte and acid-base status must be identified and treated. The fluid therapy prescription is based on the assessment
of the individual patient and must include the amount, type, route and rate of administration required. Normal homeostatic
mechanisms, particularly in patients with normal heart and renal function, provide the clinician with a considerable "buffer-zone"
when administering fluids. This is fortunate, as it is very difficult to accurately assess the patients fluid deficit and
even more difficult to determine the exact type of fluid that the patient requires.
Review of Body Fluid Compartments
In a healthy adult animal, approximately 60% of the total body weight is due to water. However, age, sex, and nutritional
status may cause this to vary. In young animals water content may be as much as 70-80%, while older animals may have water
content that is 50-55% of their body weight. Almost 66% of the total body water is intracellular fluid (ICF) and the remaining
33% is extracellular fluid (ECF). Extracellular fluids can be further divided into intravascular or plasma water (water in
the vascular space) (25% of the ECF volume) and interstitial fluid (fluid that is present in the spaces between cells) (75%
of the ECF volume). Transcellular fluid (including cerebral spinal fluid, synovial fluid, plural fluid, peritoneal fluid,
aqueous humor, and gastrointestinal secretions) is found in very small amounts and is also considered an ECF fluid. The distribution
of fluid within the body s very important as the concentration of solutes in the ECF and ICF fluid compartments are strikingly
different. ECF contains high concentrations of sodium, calcium, chloride and bicarbonate with only small amounts of potassium,
phosphate, magnesium, and protein. In ICF the distribution is quite different with potassium, phosphate, and magnesium being
the principle electrolytes.
A healthy animal with access to water will maintain a neutral fluid balance (i.e., water in = water out). The primary source
of water for the body is that which is drunk or ingested in food. However a small amount of water (0.1g/kcal energy) is also
produced via metabolism. When a patient develops dehydration, it is due to either decreased water intake, increase water loss,
or a loss of fluid to an abnormal body compartment making it functionally unavailable to the animal. (AKA third space fluid).
A decrease in the amount of water consumed may occur when the patient is anorexic due to an underlying disease condition or
has restricted access to water. Primary adipsia is quite rare in veterinary medicine, but should be considered in a chronically
dehydrated otherwise healthy, animal with free access to water.
Fluid may be lost from the body through urine, feces, sweat, saliva, respiration and draining wounds. Excessive fluid loss
form the urinary tract may occur in many disease states with a few of the more common including kidney disease, diabetes mellitus
and Cushing's syndrome. Another significant source of fluid loss from the body is the gastrointestinal tract. Typically only
a small amount of fluid is excreted in feces, but animals with diarrhea may lose a significant amount of fluid and electrolytes.
Likewise, vomiting patients may lose a significant amount of water and electrolytes. As most vomitus includes proximal duodenal
contents, patients lose water, H+ , Cl- , Na+ , K+ , and HCO3- , resulting in dehydration alog with a metabolic acidosis. However, patients with pure gastric vomiting lose water, H+ , Cl- , Na+ , and K+ and tend to have a metabolic alkalosis. Third space fluid loss refers to fluid that is functionally lost from the animal,
but is still contained within the body. i.e., ascites, pleural effusion, fluid-filled obstructed bowel, hematoma.