Page 461 - Small Animal Internal Medicine, 6th Edition

CHAPTER 28 General Therapeutic Principles 433

TABLE 28.1 to choose the correct fluid to prevent electrolyte imbalances,
especially hypokalemia. In general, potassium should be
VetBooks.ir General Guidelines for Initial Potassium supplemented if the animal is hyporexic or vomiting, has
diarrhea, or is receiving prolonged or intense fluid therapy
Supplementation of Intravenous Fluids
PLASMA AMOUNT OF POTASSIUM (see guidelines for administration in Table 28.1). The animal
should be monitored for iatrogenic hyperkalemia. Generally,
POTASSIUM CHLORIDE (KCl) TO ADD no more than 0.5 mEq K/kg/h should be administered IV. If
CONCENTRATION TO FLUIDS GIVEN AT the animal is not vomiting, oral (PO) potassium supplemen-
(mEq/L) MAINTENANCE RATES* (mEq/L)
tation is often more effective than parenteral supplementa-
3.7-5.0 10-20 tion. Cats receiving IV fluids often show an initial decrease
3.0-3.7 20-30 in their serum potassium concentrations, even if the fluids
contain 40 mEq or more of potassium chloride per liter;
2.5-3.0 30-40 therefore, severely hypokalemic cats should initially receive
2.0-2.5 40-60 oral potassium gluconate, if possible.
≤2.0 60-70 Dehydrated animals not in shock are treated by replacing
the estimated fluid deficit. First, the degree of dehydration
*Do not exceed 0.5 mEq/kg/h potassium except in animals in must be estimated. Prolonged skin tenting is usually first
hypokalemic emergencies, and then only with constant close noted at 5% to 6% dehydration, but any patient with substan-
electrocardiographic monitoring. Be sure to routinely monitor
plasma potassium concentrations whenever administering fluids with tial weight loss may show skin tenting. Obese animals and
more than 30 to 40 mEq of potassium per liter. those with peracute dehydration often do not show skin
tenting even with severe dehydration. Dry, tacky oral mucous
membranes usually indicate 6% to 7% dehydration. However,
volumes of crystalloids to large animals. Four to five mL/ well-hydrated, panting, or dyspneic animals can have dry
kg of hypertonic saline solution (i.e., 7%) given IV over 10 mouths. Multiplying the estimated percentage of dehydra-
to 20 minutes (do not exceed 1 mL/kg/min) is effective for tion by the animal’s weight (in kilograms) determines the
approximately 30 minutes. Hypertonic solutions generally liters required to replace deficit. This amount is typically
should not be used in animals with hypernatremic dehy- replaced over 2 to 8 hours, depending on the animal’s condi-
dration, cardiogenic shock, or renal failure and probably in tion. Fluid delivery rate should generally not exceed 88 mL/
those with uncontrolled hemorrhage. If necessary, hyper- kg/h. In general, it is better to slightly (not grossly) overesti-
tonic saline may be readministered in 2 mL/kg aliquots until mate rather than underestimate the fluid deficit, unless the
a total of 10 mL/kg has been given or until the serum sodium animal has congestive heart failure, anuric/oliguric renal
concentration is 160 mEq/L. However, because the volume failure, severe hypoproteinemia, or pulmonary edema. In
expansive effects of hypertonic saline last only for approxi- general, cats are more easily harmed by excessive fluid
mately 30 minutes, the clinician needs to administer other administration than are dogs.
fluids (usually at a reduced rate until shock is controlled). Ongoing losses are typically estimated from observations
A mixture of 23.4% saline solution plus 6% hetastarch or of vomiting, diarrhea, and urination, but it is common to
pentastarch in a 1:2 ratio administered at 3 to 5 mL/kg gives underestimate losses. Weighing the animal or the cage pads
a longer duration of action than hypertonic saline solution that soak up the urine/feces is another way to estimate
alone. ongoing losses because acute weight loss is due to fluid loss.
Colloids (e.g., hetastarch or pentastarch) also draw water Development of inspiratory pulmonary crackles, a gallop
from the interstitial compartment into the vascular compart- rhythm, or edema (especially cervical) probably indicates
ment, but their effects last longer than hypertonic saline and overhydration. A new heart murmur is not always a sign of
do not increase the total body sodium load. Relatively small overhydration; severely dehydrated dogs with valvular insuf-
volumes can be administered quickly (i.e., in dogs, 5-10 mL/ ficiency may not have an audible murmur until they are
kg slow IV bolus with a maximum of 20 mL/kg/day; in cats, volume replete. Central venous pressure (CVP) is excellent
5-10 mL/kg/day), and the clinician must reduce the subse- for detecting excessive fluid administration, but it is rarely
quent rate of IV fluid administration to prevent hyperten- necessary except in animals with severe cardiac or renal
sion. Colloids should be used with caution in animals with failure and those receiving very aggressive fluid therapy.
bleeding tendencies. In rare cases, the colloid may go into CVP is normally less than 4 cm H 2 O and generally should
extravascular compartments, drawing fluid out of the vascu- not exceed 10 to 12 cm H 2 O even during aggressive fluid
lar compartment complicating the shock; therefore, close therapy. Poor technique will often produce falsely high CVP
monitoring is required. readings.
Maintenance fluid requirements for dogs weighing Oral rehydration therapy makes use of the facilitated
between 10 and 50 kg are 44 to 66 mL/kg/day with larger intestinal absorption of sodium. Co-administration of a
dogs needing less per kilogram than smaller dogs. Dogs monosaccharide (e.g., dextrose) or amino acid with sodium
weighing less than 5 kg may need 80 mL/kg/day. Cats weigh- speeds up sodium absorption and subsequent water uptake.
ing more than 3 kg need 53 to 61 mL/kg/day. It is important This approach works if the animal can ingest oral fluids (i.e.,