Page 415 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 17

Perioperative Management

of Fluid Therapy

Peter J. Pascoe

The normal mechanisms of fluid homeostasis are dis- PREOPERATIVE
turbed when an animal undergoes anesthesia and surgery. PREPARATION OF THE
Consequently, animals should receive fluids during the
perioperative period to maintain proper fluid balance. PATIENT
Anesthetized animals should receive fluids:
The animal’s fluid balance should be as close to normal as
1. To establish and maintain venous access. A minimal
possible before anesthesia. Almost all anesthetics have
rate of fluid administration is necessary (e.g., 3 mL/
some effects on circulatory and renal function, and it is
hr) and will ensure rapid access to the circulation
in the event of an emergency in the perioperative important that the patient’s circulating volume be opti-
mal so that these effects are not exacerbated.
period.
2. To counter the physiologic changes associated with Disturbances that require attention may be classified by
their urgency. Some can be corrected acutely (e.g.,
anesthetics. Most of the drugs and techniques used hypovolemia); some require more time to correct (e.g.,
to anesthetize animals have some effect on the
hypernatremia); and a few require completion of the pro-
circulation.
cedure before correction of the problem can occur (e.g.,
3. To replace fluids lost during anesthesia and surgery.
hypervolemia associated with acute renal failure in a
During the procedure, the animal cannot drink
patient being prepared for hemodialysis).
and its metabolic rate is reduced (decreased produc-
tion of metabolic water). At the same time, the animal
continues to produce urine, salivate, secrete fluid CHANGES IN VASCULAR
into the gastrointestinal tract, and lose water by VOLUME
evaporation from the respiratory tract. The aim should
be at least to replace the expected insensible fluid
losses. HYPOVOLEMIA
4. To correct fluid losses caused by disease and replace Hypovolemia may be caused by fluid loss directly from
ongoing losses attributed to the procedure. The vol- the vascular space (e.g., hemorrhage), a more general loss
ume of fluid lost or gained depends on the type of sur- (e.g., dehydration), or changes in vascular tone. In all
gical procedure, the skill of the surgeon, the cases, fluid should be given to replace the loss. For a sim-
preoperative state of the animal, and the equipment ple loss in which the composition of the vascular space is
used by the anesthetist. Trauma and surgery are relatively normal, the loss can be replaced effectively using
associated with increased secretion of vasopressin, an isotonic crystalloid, a hypertonic crystalloid, an artifi-
and additional secretion may occur as a result of hypo- cial colloid, or a blood product. The fluid used depends
tension or hypovolemia. Other stress hormones (e.g., on the severity of the loss and the financial resources of
cortisol, catecholamines, renin) released during the the client. Acute blood loss of up to 30% of blood volume
procedure also may play a role in upsetting normal can be replaced adequately using a crystalloid solution
fluid homeostasis and warrant perioperative fluid (assuming normal hematocrit and total protein concen-
therapy. tration before therapy), whereas a loss of 50% of blood

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