Page 419 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 409

must be monitored for several hours to avoid hypoglyce- anesthetics because they readily cross the blood-brain
mia. b-Adrenergic agonists such as albuterol and barrier normally. The hyperosmolar state associated with
salbutamol have been used to manage hyperkalemia, hypernatremia may increase the dose of inhalant required
and their activity may be enhanced with the use of insu- for anesthesia. 180
lin. 6,113 One study in dogs documented the effect of epi-
nephrine and ritodrine in reducing hyperkalemia. 61 After HYPOOSMOLALITY
the animal is anesthetized, ventilation should be moni- This invariably is associated with an excess of free water
tored and controlled if necessary because hypercapnia and hyponatremia and should be managed as described
may decrease pH and facilitate potassium efflux from previously.
cells. Depolarizing muscle relaxants (e.g., succinylcho-
line) should be avoided because they may cause release HYPOGLYCEMIA
of potassium from cells. Nondepolarizing relaxants Hypoglycemia in an awake patient usually is manifested
should be used cautiously (50% to 70% of the normal by somnolence progressing to coma. In the anesthetized
dose) to prevent prolonged effects. The patient should animal, there may be no outward signs, and unless blood
be monitored carefully by electrocardiography and fre- glucose concentration is being monitored, it is unlikely
quent measurements of serum glucose, potassium, and that hypoglycemia would be detected. Hence, it is impor-
ionized calcium concentrations and acid-base status tant to recognize and manage hypoglycemia preopera-
(see Chapter 5). tively. Most animals regulate their blood glucose
concentration closely, but this may not be the case in very
HYPOCALCEMIA young animals, those with insulinomas, and animals with
Decreased calcium concentrations are associated with portosystemic shunts. It usually is unnecessary to remove
increased neuromuscular excitability. In the heart, this very young animals from their dam until the time of
may manifest itself as a prolonged QT–interval and other premedication if they are receiving a liquid diet only.
arrhythmias (e.g., ventricular premature contractions, If they have been orphaned or are ill and have not been
ventricular fibrillation). As with the other electrolytes, taking in fluids, it is best to check blood glucose concen-
the rate of change is an important factor in the type of tration before anesthesia and treat accordingly. If blood is
clinical signs seen. It is important to treat a patient with difficult to obtain, the animal can be given some oral glu-
hypocalcemia and clinical signs before anesthesia. This cose in the form of Karo syrup (ACH Food Companies,
can be achieved rapidly while the electrocardiogram is Inc., Memphis, Tenn.) or some other clear dextrose-
monitored for signs of overly rapid correction (bradycar- containing fluid. 58 Intraoperatively, it may be best to
dia). Hyperthermia associated with hypocalcemic seizure use a 2.5% to 5% glucose solution intravenously. Postop-
activity also should be treated before anesthesia. Hypocal- eratively, these patients should be monitored carefully or
cemic patients are at increased risk from the toxic given additional Karo syrup until they can return to their
manifestations of digoxin therapy, and this risk should previous feeding regimen. Animals with insulinomas can
be taken into consideration when preparing cardiac have resting blood glucose concentrations of 30 to
patients for anesthesia. 40 mg/dL and may tolerate these low glucose
concentrations quite well. If exogenous glucose is
HYPERCALCEMIA administered as a bolus to an animal with hyperinsulin-
Signs of muscle weakness also may be seen with hypercal- ism, massive release of insulin may trigger a hypoglycemic
cemia, but arrhythmias are relatively uncommon. When crisis. Therefore it is important to use relatively dilute
they do occur, cardiovascular manifestations include bra- solutions of glucose and administer them as an infusion
dycardia with prolonged PR–interval, wide QRS com- rather than as a bolus. We typically administer 2.5% glu-
plex, and shortened QT–interval. Hypercalcemia is cose to these patients the night before surgery at 1 to 1.5
difficult to treat acutely and usually requires treatment times the normal maintenance rate. Intraoperatively,
for at least 24 hours before anesthesia (see Chapter 6). blood glucose concentration is monitored carefully, and
glucose infusions are continued as necessary. After
HYPEROSMOLALITY the tumor is removed, blood glucose concentration
Hyperosmolality usually is associated with usually returns rapidly to the normal range. Animals
hypernatremia, hyperglycemia, ketoacidosis, uremia, or with portosystemic shunts may become hypoglycemic,
the presence of exogenous toxins (e.g., ethylene glycol). and glucose supplementation may be needed in the peri-
In some cases, it may be impossible to reverse the operative period. In one retrospective series, 2 of 13 dogs
hyperosmolar state adequately before anesthesia because with portosystemic shunts were reported to have devel-
therapy (e.g., hemodialysis) may require an invasive pro- oped hypoglycemia intraoperatively. 105 Postoperative
cedure. Hyperosmolality may be associated with disrup- administration of dexamethasone (0.1 to 0.2 mg/kg
tion of the blood-brain barrier leading to greater IV) may be helpful in managing hypoglycemia in
uptake of some drugs. 200 This is unlikely to affect most these cases. 87