Page 423 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 413

alkalosis; decreased serum calcium, magnesium, and pro- associated with more aggressive fluid therapy
tein concentrations; and decreased hematocrit. Similar (>500 mL/hr), which in turn was associated with a lower
changes have been documented in dogs. Serum protein risk of postoperative congestive heart failure. 34 In the
concentrations tend to decrease during pregnancy, with past, it has been recommended that fluids containing
the most marked change being a decrease in serum albu- low concentrations of sodium be administered to cardiac
min concentration. 28 Hematocrit decreases with a pro- patients (e.g., 0.45% saline in 2.5% dextrose). Most of
portionately greater decrease with increasing numbers these patients have an increase in total body sodium
of fetuses. 5,99 The pregnant dog has a decreased barore- and an increase in total body water. The latter tends to
ceptor response to hypotension and is more susceptible to exceed the former, and affected patients may be
7
hypotension with blood loss. 22,23 Thus the pregnant ani- hyponatremic. Thus it seems illogical to give a solution
mal may be more susceptible to the negative circulatory that contains additional free water. If such a patient is
effects of anesthetics and may require an increased vol- hypovolemic, it is more appropriate to use a balanced
ume of fluids during a surgical procedure. In bitches electrolyte solution. If the patient is not hypovolemic
and queens that have been in labor for some time, dehy- or it already has excessive volume, fluids may not be
dration and endotoxemia also may be present and add to needed.
circulatory instability. Affected patients may benefit from In other myocardial diseases, it also is important to
fluid therapy before anesthesia. assess the patient preoperatively for signs of dehydration
and heart failure (e.g., distended jugular veins, slow jug-
CHANGES IN FUNCTION ular emptying, jugular pulses, ascites, pulmonary edema,
pleural effusion). Invasive monitoring as described earlier
may be necessary to optimize fluid therapy during anes-
CARDIOVASCULAR DISEASE thesia and surgery. Blood may flow best at a hematocrit
If the heart is failing, it may not tolerate an increased fluid of 25% to 30%, but it may be necessary to maintain higher
load. Increased preload in this setting may not result in values to maintain optimal tissue oxygenation. If an ani-
increased cardiac output because of changes in the mal with heart failure also is anemic, consideration should
Frank-Starling curve. Conversely, even a failing heart be given to preloading the animal with packed red cells to
does not function optimally if preload is allowed to optimize oxygen delivery.
decrease too much. In a prospective study of human
patients, it was found that the frequency of postoperative COAGULATION DEFECTS
heart failure was highest in patients who had received less
than 500 mL/hr of fluids intraoperatively. 34 The most Any coagulation defect that is likely to increase
common cause of congestive heart failure in dogs is mitral intraoperative blood loss should be corrected before sur-
insufficiency. This condition is characterized by excessive gery if possible. If an animal has a known coagulation
retrograde flow with an increasing volume load on the defect (e.g., hemophilia, hepatic failure, coumarin poi-
heart. Treatment often involves use of vasodilators soning, von Willebrand’s disease), it should be given
(e.g., nitroglycerin, hydralazine, angiotensin-converting the appropriate therapy such as fresh frozen plasma,
enzyme inhibitors) to decrease afterload, and diuretics cryoprecipitate, fresh plasma, or fresh whole blood, and
and salt restriction to decrease circulating volume. Con- vitamin K in the case of coumarin poisoning. These
sequently, cardiac patients have the potential to be treatments should be given within a few hours of surgery
hypovolemic. The diagnosis of relative hypovolemia in because the half-lives of most clotting factors are relatively
these patients is based on clinical signs, such as skin tur- short. Although fresh frozen plasma and fresh plasma may
gor, mucous membrane color, capillary refill time have sufficient clotting factors to reverse the coagulation
(CRT), and jugular venous distention. Evaluation of defect, such therapy often fails in animals with severe
renal function (including urine output) may assist in defects. In dogs with von Willebrand’s disease, infusion
deciding whether the animal is adequately hydrated. Tho- of cryoprecipitate is a more effective treatment than fresh
35,175
racic radiographs can be used to help assess pulmonary frozen plasma alone. Therapy with plasma from
venous distention (i.e., lack of pulmonary venous disten- donors receiving desmopressin (DDAVP) may be more
96
tion implies lower left atrial pressure and hence a lack of effective than plasma from untreated donors. When
excessive preload). The most useful measurement in these DDAVP is given to dogs with typ. 1 von Willebrand’s dis-
patients is PCWP. PCWP is obtained by inserting a bal- ease, there is a measurable increase in the binding of von
loon-tipped catheter into the pulmonary vein from either Willebrand factor to collagen, suggesting an improve-
97
the jugular or femoral vein. Such invasive monitoring cer- ment in clotting ability during surgery. Cryoprecipitate
tainly is warranted in some cardiac patients and provides often is prepared from a number of donors and therefore
the best guide to fluid therapy. If the animal has right- has the potential to provide greater antigenic stimulation
sided heart failure, monitoring CVP provides similar or transmit disease. Cryoprecipitate contains 10 to 20
information. In one study, use of CVP or PCWP was times the normal amount of clotting factors and can be