Page 358 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

348 FLUID THERAPY

BOX 14-1 Calculation of Replacement Requirement (Hydration Deficit)

1. Hydration deficit (replacement requirement) 2. Maintenance requirement (40-60 mL/kg/day)
a. Body weight (lb) % dehydration as a decimal a. Sensible losses (urine output): 27-40 mL/kg/day
500* ¼ deficit in milliliters b. Insensible losses (fecal, cutaneous, respiratory):
b. Body weight (kg) % dehydration as a decimal ¼ 13-20 mL/kg/day
deficit in liters 3. Contemporary (ongoing) losses (e.g., vomiting, diarrhea,
polyuria)

From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor. Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 35.

*500 mL ¼ 1 lb.

initial infusion fails to restore hydration. Finally, the pos- URINE OUTPUT
sibility must be considered that the animal was not The clinician should observe the animal’s urine output
dehydrated at presentation (e.g., abnormal skin turgor carefully after fluid therapy has begun. Oliguria should
related to old age or emaciation). This should be consid- be strongly suspected in patients with acute renal failure,
ered if the animal does not gain weight despite several
especially those with possible ethylene glycol ingestion.
days of fluid therapy.
Urine output should be monitored when fluids are
administered intravenously at a rapid rate and renal func-
MONITORING FLUID tion is in question. Normal urine output is 1 to 2 mL/
kg/hr. As the patient becomes rehydrated, physiologic
THERAPY oliguria should resolve, and urine output should increase
while USG decreases. If oliguria that was present at
It is important to remember that the hydration deficit as admission persists after the hydration deficit has been
estimated by history and physical examination is only an replaced, it is prudent to divide daily fluid therapy into
estimate, and fluid therapy must be tailored to physical six 4-hour intervals if the status of renal function is uncer-
(e.g., body weight) and laboratory (e.g., PCV, TPP)
tain. The calculated insensible volume plus a volume
findings during the first few days of fluid therapy.
equal to the urine output of the previous 4 hours is
administered during each 4-hour period (known as mea-
PHYSICAL AND LABORATORY suring “ins and outs”). The risk of overhydration is
FINDINGS minimized, and fluid therapy keeps pace with urine out-
put even if oliguria is present when this technique is used.
A complete physical examination, including evaluation of
If oliguria persists, an increase in the daily fluid volume by
skin turgor and careful thoracic auscultation, should be an amount equal to 5% of body weight is justified on the
performed once or twice daily for animals receiving fluid assumption that the initial clinical estimate of dehydra-
therapy. Hematocrit, TPP, and body weight should be tion was inaccurate. If oliguria does not respond to mild
monitored. Serial body weight has been considered one volume expansion, administration of increased volumes
of the most important variables to follow, and animals of fluid may result in pulmonary edema.
receiving continuous fluid therapy should be weighed
once or twice daily using the same scale. A gain or loss CENTRAL VENOUS PRESSURE
of 1 kg can be considered an excess or deficit of 1 L of
fluid because lean body mass is not quickly gained or lost. Measurement of CVP with a jugular catheter positioned
A dehydrated patient should gain weight as rehydration is at the level of the right atrium allows the cardiovascular
achieved, and afterward weight should remain relatively response to fluid administration to be monitored. Nor-
constant. However, weight may increase without restora- mal CVP is 0 to 3 cm H 2 O. CVP increases from below
tion of effective circulating volume in patients with severe normal into the normal range when fluids are
third-space losses. Despite these traditional principles, administered to a dehydrated animal. A progressive
one study of dogs and cats hospitalized in an intensive increase in CVP above normal during fluid therapy is
care unit showed that clinical estimates of dehydration an indication to decrease the rate of fluid administration
did not reliably predict changes in body weight after 24 or to stop fluid therapy temporarily. A sudden and
to 48 hours of fluid therapy. 18 sustained increase in CVP may indicate failure of the