The maintenance fluid requirement for dogs and cats is estimated as 40-60 ml/kg daily. Large dogs are given the lower limit
(40 ml/kg daily) and cats and small dogs the upper limit (60 ml/kg daily) of this range. Approximately two-thirds (27-40 ml/kg
daily) of the maintenance requirement represents sensible losses of fluid (urine output) and one-third (13-20 ml/kg daily)
represents insensible losses (primarily fecal and respiratory water loss).
The fluid choice to administer is dependent on the type of the disease process and composition of the fluid lost. The veterinarian
should attempt to replace losses with a fluid that is similar in volume and electrolyte composition to that which has been
lost. Persistent vomiting of stomach contents results in losses of hydrochloric acid, potassium, sodium, and water and potentially
produces hypokalemia, hypochloremia and metabolic alkalosis.
Then, the initial fluid choice would be 0.9% saline solution with 20-30 mEq potassium chloride added to a liter of solution.
Except in the case of vomiting of stomach contents, lactated Ringer's solution is a good first choice for fluid therapy while
waiting for the serum chemistry profile and electrolytes results. A 0.9% saline solution would be less ideal because this
solution is not a balanced solution. It contains chloride in greater concentration than body fluids (154 mEq/L versus 110
mEq/L in dogs and 120 mEq/L in cats) and by displacing bicarbonate with chloride in extracellular fluid and initiating natriuresis
it has a mild acidifying effect.
Anions such as lactate, acetate and gluconate are added to crystalloid solutions as a source of base because their oxidative
metabolism in the body yields bicarbonate. Lactate has been introduced for the treatment of acidosis because of technical
difficulties in preparation of bicarbonate solutions suitable for intravenous use.
These technical difficulties have been overcome, but crystalloid solutions containing lactate as a source of base still are
widely used for fluid therapy in clinical practice. Most small animals treated with lactate-containing replacement solutions
respond well, probably as a result of improved tissue perfusion from the extracellular fluid volume expansion.
Harmful to some?
There has been some concern that lactate in lactated Ringer's solution may be harmful to animals with poor tissue perfusion
and severe metabolic acidosis (pH <7.2). Administration of lactate as a salt cannot contribute directly to metabolic acidosis.
Rather, the ability of the liver to metabolize lactate and the potentially detrimental effect of lactate on myocardial contractility
has been debated.
During severe hypoxia, increased lactate production in intestinal tract and muscle and decreased hepatic extraction of lactate
lead to progressive lactic acidosis. In moderate metabolic acidosis, administration of lactated Ringer's solution probably
will be beneficial, because any tendency toward lactate accumulation is likely to be offset by improved hepatic perfusion
and oxygen delivery as a result of extracellular fluid volume expansion.
Potassium chloride is the additive of choice for parenteral fluid therapy because chloride repletion also is important if
vomiting or diuretic administration is the underlying cause of hypokalemia. When administered intravenously, potassium should
not be infused at a rate greater than 0.5 mEq/kg per hour. Infusion of potassium-containing fluids initially may be associated
with a decrease in serum potassium concentration as a result of dilution, increased distal tubular flow and cellular uptake
of potassium, especially if the infused fluid also contains glucose.
This effect may be minimized by using a fluid that does not contain glucose and by administering potassium at an appropriate
rate. Potassium gluconate is recommended for oral supplementation. In cats with hypokalemic nephropathy, the initial oral
dosage of potassium gluconate is 5-8 mEq daily divided into two or three doses whereas the maintenance dosage can usually
be reduced to 2-4 mEq daily.
Careful potassium supplementation is important when using insulin to treat diabetic ketoacidosis. Chronic potassium depletion
usually is present in affected animals as a result of loss of muscle mass, anorexia, vomiting and polyuria. Serum potassium
concentrations, however, often are normal or even increased due to the effects of insulin deficiency and hyperosmolality on
serum potassium concentration.
As blood glucose concentration falls with insulin treatment, marked hypokalemia often develops if potassium supplementation
is not diligent.
Hyperkalemia may be treated by antagonizing the effects of potassium on cell membranes with calcium gluconate, driving extracellular
potassium into cells with sodium bicarbonate or glucose, or by removing potassium from the body with a cation exchange resin
Remember, any source of intake, such as potassium-containing fluids and potassium penicillin, must be discontinued during
hyperkalemic situations. Administered calcium gluconate begins to work within minutes, but its effect lasts less than an hour.
The dosage of calcium gluconate is 2-10 ml of a 10% solution to be administered slowly with electrocardiographic monitoring.
Administered glucose begins within an hour and lasts a few hours. Glucose-containing fluids (5% dextrose solution or 10% dextrose
solution) or 50% dextrose solution (1-2 ml/kg) can be used for this purpose. Unless the animal is diabetic, administration
of insulin with glucose usually is unnecessary and may cause hypoglycemia.
Sodium bicarbonate also works by moving potassium into cells as hydrogen ions come out to titrate the administered bicarbonate.
Administered sodium bicarbonate begins to work within an hour and its effects last a few hours. The usual dosage is 1-2 mEq/kg
intravenously and it can be repeated if necessary. The loop or thiazide diuretics increase distal tubular flow rate and potassium
secretion and may have adjunctive value in the treatment of hyperkalemia.
The cation exchange resin polystyrene sulfonate can be used to bind potassium and release sodium in the gastrointestinal tract.
Each gram will bind one mEq of potassium and release 1-2 mEq of sodium. If these measures fail, the veterinarian should consider
using peritoneal dialysis.