Principles of fluid therapy (Proceedings)
Fluid therapy is supportive. The underlying disease process that caused the fluid, electrolyte, and acidbase disturbances must be diagnosed and treated appropriately. In formulating a fluid therapy plan, 6 questions should be considered:
Is fluid therapy indicated?
The hydration status of the animal is estimated by careful evaluation of the history, physical examination findings, and the results of a few simple laboratory tests. Normally, fluid input consists of water consumed in food, water that is drunk, and water produced metabolically in the body. Maintenance water and electrolyte needs parallel caloric expenditure, and normal daily losses of water and electrolytes include respiratory, fecal, and urinary losses.In disease states, decreased fluid intake results from anorexia, and increased fluid loss may occur by urinary (e.g., polyuria) and gastrointestinal (e.g., vomiting, diarrhea) routes. Thirdspace loss of fluid occurs when effective circulating volume is decreased, but the fluid lost remains in the body.
The time period over which fluid losses have occurred and an estimate of their magnitude should be determined. Physical findings associated with fluid losses of 5% to 15% of body weight vary from no clinically detectable changes (5%) to signs of hypovolemic shock and impending death (15%). The hydration deficit is estimated by evaluating skin turgor, moistness of the mucous membranes, position of the eyes in their orbits, heart rate, character of peripheral pulses, capillary refill time, and extent of peripheral venous distention (i.e., inspection of jugular veins). A decrease in the interstitial compartment volume leads to decreased skin turgor and dryness of the mucous membranes. A decrease in plasma volume leads to tachycardia, alterations in peripheral pulses, and collapse of peripheral veins. When these cardiovascular signs are present, the patient is in shock and should be resuscitated promptly before correction of the hydration deficit. Thus, a crude clinical estimate of hydration status and the patient's response to fluid administration become important tools in evaluating the extent of dehydration and planning fluid therapy. The urinary bladder should be small in a dehydrated animal with normal renal function. In the absence of urinary obstruction, a large bladder in a severely dehydrated patient indicates failure of the normal renal concentrating mechanism. Body weight recorded on a serial basis traditionally has been considered a good indicator of hydration status, especially when fluid loss has been acute and previous body weight has been recorded (i.e., 1 kg loss of body weight equals a 1 L fluid deficit). In one study, however, clinician estimates of hydration in dogs and cats admitted to a veterinary teaching hospital intensive care unit did not reliably predict changes in weight after 24 to 48 hours of fluid therapy. In chronically ill animals, loss of weight also includes loss of muscle mass. Anorexic animals have been estimated to lose 0.1 to 0.3 kg of body weight per day per 1000 kcal energy requirement. Another factor that must be considered in evaluating body weight is the possibility of thirdspace loss. Fluid lost into a third space does not decrease body weight. Packed cell volume (PCV), total plasma protein concentration (TPP), and urine specific gravity (USG) are simple laboratory tests that aid in the evaluation of hydration. These results should be obtained before initiating fluid therapy.