Page 422 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

412 FLUID THERAPY

bacterial contamination of the blood and no risk of the volume of blood or cerebrospinal fluid in the cranial vault
blood containing cancer cells that could metastasize to promotes an increase in ICP. In situations in which the
other areas of the body. The blood should be passed cause is medically reversible (e.g., hyponatremia), therapy
through a filter to remove clots before it is should be carried out before anesthesia. In cases in which
autotransfused. Cats with pleuritis appear to be in great the diagnosis or treatment requires anesthesia, the preop-
pain and often are very fractious. It may be beneficial to erative assessment of the patient must include a detailed
provide sedation and analgesia (e.g., oxymorphone) examination of fluid balance. Animals with an acute
and oxygen before attempting to drain the chest. increase in ICP caused by trauma also may be
hypovolemic because of other injuries. Judicious use of
PERITONEAL FLUID hypertonic resuscitation fluids is appropriate for these
A large volume of fluid in the abdomen can increase patients because such fluids promote a reduction in
intraabdominal pressure (so-called abdominal compart- ICP while restoring circulating volume. 103,199 Patients
ment syndrome) and should be drained before anesthesia with chronically increased ICP often have had decreased
if feasible. Abdominal compartment syndrome is food and water intake for some time and may have been
associated with a number of physiologic changes, includ- treated with diuretics to reduce ICP. Consequently, such
ing hypoventilation with reduced pulmonary compliance; patients often are dehydrated and may have electrolyte
tachycardia; low cardiac output; and increased central disturbances. Whenever possible, preoperative assess-
venous pressure (CVP), mean pulmonary artery pressure, ment should include examination of the animal for signs
and PCWP. In the abdomen, the increased pressure of dehydration, an assessment of the cause of increased
reduces urine output and decreases blood flow to the ICP, an evaluation of renal function, and measurement
abdominal wall and the splanchnic vascular beds. of serum electrolytes, hematocrit, total proteins, osmolal-
Intraabdominal hypertension also may increase intracra- ity, and colloid osmotic pressure. If the animal clearly is
nial pressure (ICP) with a decrease in cerebral perfusion dehydrated, it should be given fluids before anesthesia
pressure. 94 to increase its circulating volume. If plasma osmolality
Drainage of the abdomen usually is achieved by plac- is less than 320 mOsm/kg, it may be beneficial to treat
ing a catheter in the abdominal cavity and drawing off the animal with mannitol (0.25 to 1 g/kg).
the fluid with a syringe. It is helpful if the catheter has
additional side holes cut in it before insertion so that INCREASED INTRAOCULAR
there is less likelihood of the catheter being obstructed PRESSURE
by the omentum. Most affected animals have greater Patients with glaucoma often are treated similarly to
respiratory distress lying on their backs, and the catheter patients with an increased ICP (i.e., diuretics), but they
usually is inserted with the animal on its side. I usually also are given carbonic anhydrase inhibitors (e.g.,
place the catheter about halfway between the last rib methazolamide, Teva, Sellersville, Pa.), which can cause
and the ischium, 1 to 4 inches off the ventral midline. metabolic acidosis over the course of 12 to 24 hours.
Draining fluid in this manner can take a long time, but Although correction of the acidosis may not be essential
this is actually advantageous because rapid removal can in many of these animals, treatment with sodium bicar-
result in mesenteric vasodilatation and cardiovascular col- bonate may decrease the risk associated with anesthesia.
lapse. 94 In the case of hemoabdomen, the blood may have The combination of dehydration and acidosis may sub-
been defibrinated, but it is best to collect it in an antico- stantially reduce the dose of thiopental required for
agulant (e.g., heparin, citrate). Collected blood should be induction, and care should be taken to titrate this drug
used only if there is no gross contamination of the abdo- to effect in these patients.
men and no risk of neoplasia. The blood should be passed
through a filter to remove clots before it is AGE
autotransfused. In cases of massive trauma, it may be bet- As animals get older, body water and cardiovascular
ter to leave the blood in the abdomen until the surgeon is reserve decrease. These changes make older animals more
ready to stop the bleeding. Although the accumulated susceptible to fluid overload in the perioperative period.
blood may compromise ventilation during this time, Geriatric patients admitted to the hospital several days
the increased intraabdominal pressure may reduce the before anesthesia and surgery may not have been drinking
rate of hemorrhage. well (i.e., low tolerance for a new environment) and may
be dehydrated.
INCREASED INTRACRANIAL
PRESSURE PREGNANCY
Increased ICP requires careful management in terms of Pregnancy is associated with many changes in fluid bal-
fluid balance. The cranial vault is a relatively fixed cavity, ance. In women, the typical changes associated with preg-
and any accumulated fluid tends to increase the pressure. nancy include hyponatremia; decreased blood urea
An increase in the fluid content of the brain or in the nitrogen and creatinine concentrations; respiratory