Page 433 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Perioperative Management of Fluid Therapy 423

HYPERTONIC SOLUTIONS prolong bleeding times by interfering with fibrin clot for-
mation, reducing factor VIII and von Willebrand’s factor,
These solutions may provide rapid resuscitation in the
diluting clotting factors, and interfering with platelet
preoperative period but are seldom used intraoperatively.
function. In dogs, rapid infusion of dextran 70 caused a
They may be needed in special circumstances such as for
decrease in von Willebrand’s factor antigen and factor
an animal with rapid hemorrhage when blood products
VIII activity and increases in APTT and buccal mucosal
are unavailable, an animal with a high ICP, or a patient 39,69
bleeding time. Dextrans and HES also alter the
with hyponatremia. Most of these solutions have very
structure of the fibrin clot, giving it a weaker, more cha-
high sodium concentrations, and it is important to moni- 72
otic appearance. These effects suggest that dextrans
tor serum sodium concentrations before and after their
administration. Maintenance with an isotonic crystalloid may not be the best choice for fluid therapy when major
usually is required after administration of these solutions. surgery is planned. Clinically, it seems that infusions of
dextrans have been associated with increased bleeding,
COLLOIDS but no studies have documented increased blood loss
when dextrans have been used. Allergic reactions have
Dextrans, HES, polygelatins, and plasma are the main been reported in human patients, but the frequency
colloid solutions available. They are used to correct appears to be less than 0.1%, and such reactions have
hypovolemia, provide colloid osmotic pressure, and in not been reported in dogs or cats.
the case of fresh frozen plasma they provide clotting In humans, dextran 40 has been used to reduce the
factors. The synthetic colloids are polydisperse colloids occurrence of deep vein thrombosis. It is thought that
that, by definition, contain particles of several different this effect is caused by decreased viscosity of blood after
molecular weights. In the past, the average molecular dextran administration. There also is some evidence that
weight (M w ) of such solutions was described, but this low molecular weight dextrans alter red cell aggregation
approach favors the high molecular weight particles. and decrease clumping of red cells in the microcircula-
It now is common to describe the solution according tion. Use of dextran 70 also reduced the frequency of fatal
to the number molecular weight (M n ), which is the total postoperative pulmonary embolism from 2.0% to 0.35%.
weight of all the molecules divided by the number of In these studies, dextrans were given on the day of sur-
molecules. In the case of dextran 70, the M w is 70,000, gery. The only common conditions in dogs and cats com-
but the M n is 41,000 (Table 17-2). Use of M n allows rec- plicated by pulmonary thromboembolism are
ognition of the smaller molecular weight particles in the hyperadrenocorticism and the nephrotic syndrome, and
solution. The terms have clinical significance because the the use of dextrans has not been investigated in these
oncotic pressure exerted by the solution depends on the settings in veterinary medicine.
number of particles present, whereas the duration of A number of reports have linked dextrans to renal fail-
effect depends on the size of the particles present. The ure. This complication has been attributed to increased
duration of effect of a colloid is short if the particles rap- viscosity of the glomerular filtrate associated with early
107,201
idly leak through the endothelium. excretion of low molecular weight particles. Exper-
imental studies in dogs identified changes in proximal
DEXTRANS tubular cells but no effect on renal function. Affected
The dextran molecule is a linear polysaccharide produced human patients have received large doses of dextrans
by certain strains of Leuconostoc bacteria growing in and have had an associated increase in oncotic pressure.
sucrose-containing media. Dextrans are supplied in low Treatment by exchange transfusion to lower oncotic pres-
and high molecular weight forms (dextran 40 and 70, sure has been successful, suggesting that the renal
respectively) with plasma half-lives estimated at 1 to 3 changes are not structural but functional. 189,201 There
and 2 to 6 hours. 182 In dogs with normal renal function, are no reports of renal failure after dextran administration
70% of a dose of dextran 40 and 40% of a dose of dextran in dogs or cats.
70 are excreted unchanged in urine within 24 hours. The
remaining molecules are metabolized slowly to glucose HETASTARCH
by dextranase in the liver. Some of these molecules may HES is a synthetic polymer of glucose (amylopectin) that
be present in the body weeks after their administration. closely resembles glycogen and contains predominantly
The plasma volume expansion achieved per gram of dex- a-1,4 linkages. Starch normally is metabolized by
tran is roughly the same, regardless of molecular weight amylases, and by adding hydroxyethyl groups to positions
(approximately 20 to 25 mL water/g dextran). 86 Clini- 2, 3, or 6 on the glucose molecules, the rate of metabo-
cally, however, dextran 40 has a greater concentration lism can be reduced. Metabolic breakdown is slower with
per milliliter and provides greater plasma volume expan- increased substitution and a higher ratio of C2:C6 substi-
sion initially. tution. This understanding has led to development of dif-
Concerns about the use of dextrans include effects on ferent molecules that are described by their molecular
hemostasis and allergic reactions. Dextrans tend to weight, the proportion of substitution, and their C2: