Measuring serum electrolytes great ally in diagnosis, treatment

Q: What is a major shortcoming of some selected veterinary practices in year 2002?

A:Veterinary practices still do not do serum electrolyte determinations on a regular basis!

Each dog or cat that enters a veterinary practice ill enough to warrant receiving fluid therapy should immediately have serum (or plasma) electrolytes determinations done, especially measurements for existing concentrations of serum sodium and potassium. The primary concerns related to these serum electrolytes in dogs and cats of all ages are whether there is hypernatremia or hyponatremia, and hyperkalemia or hypokalemia present or not.

Table 1: Selected causes of altered sodium and potassium balance

See Table 1 for selected causes of altered sodium and potassium balance that may occur in dogs and cats. Measurements of these serum electrolyte concentrations and administration of appropriate fluid therapy just go together in any clinical practice.

With the assistance of Dr. Stephen P. DiBartola, DVM, dipl. ACVIM of The Ohio State University, will help us understand and select effective fluid therapy for that particular ill dog or cat.

Crystalloid solutions

A balanced fluid solution, such as lactated Ringer's solution, resembles extracellular fluid in its composition whereas an unbalanced fluid solution, such as normal saline solution, does not. Fluids may be either crystalloid solution or colloid solution.

Crystalloid solutions, such as 5% dextrose solution, 0.9% saline solution, and lactated Ringer's solution, contain electrolyte and non-electrolyte solutes capable of entering all body fluid compartments.

Colloid solutions are large molecular weight substances that are restricted to the plasma compartment and include plasma, dextrans and hydroxyethyl starch (hetastarch). The primary advantage of colloid solutions is that more of the administered solution remains in the plasma compartment and there is less risk of producing edema.

Crystalloid solutions may be equally effective in expanding the plasma compartment, but about three times the fluid volume must be given since the crystalloid solutions will be distributed to the interstitial space and intracellular space too.

Replacement or maintenance?

The crystalloid solutions may be replacement or maintenance solutions. The composition of replacement solutions resembles that of extracellular fluid. Maintenance solutions contain less sodium (40-60 mEq/L) and more potassium (15-30 mEq/L) than do replacement fluids. A simple maintenance solution can be formulated by mixing one part 0.9% saline solution with two parts 5% dextrose solution and adding 20 mEq potassium chloride per liter of final solution.

The approximate composition of such a fluid mixture would be: 51 mEq/L sodium, 20 mEq/L potassium, 71 mEq/L chloride and 16.7 g/L dextrose.

The fluid mixture would provide 57 kcal/L and have an osmolality of 235 mOsm/kg. The veterinarian can achieve a similar effect by alternating administration of 5% dextrose in water with 0.9% saline solution in a 2:1 ratio over a 24-hour period.

Another commonly used crystalloid solution is 5% dextrose solution. Administering 5% dextrose solution is equivalent to giving water. Therefore, the primary reason for giving 5% dextrose solution is to replace a pure water deficit.

Mangaging fluid therapy

The veterinarian can manage most small animals requiring fluid therapy with a limited number of crystalloid and additive solutions. The most useful crystalloid solutions for routine use are a balanced electrolyte solution such as Ringer's solution or lactated Ringer's solution, 0.9% saline solution and 5% dextrose in water solution. Supplementation of these fluid solutions with potassium chloride will be essential when losses have included large amounts of potassium.

When additives are used, the veterinarian should keep in mind that the final osmolality of the fluid may be quite high. The final osmolality may be approximated by adding the number of mEq/L of electrolyte and mMol/L of non-electrolyte solutes found in the solution.

The final osmolality of the solution also may differ and depend on how the fluid solution was formulated. For example, if 500 ml lactated Ringer's solution is mixed with 500 ml 5% dextrose solution to create a 2.5% dextrose solution, the resulting solution will have an approximate osmolality of 276 mOsm/kg - nearly the same as lactated Ringer's solution.

On the other hand, if 50 ml of 50% dextrose solution is added to one liter of lactated Ringer's solution, the resulting fluid solution will have an approximate osmolality of 393 mOsm/kg, which is substantially higher.

A reduced interstitial compartment causes decreased skin turgor and dryness of the mucous membranes. reduced plasma volume causes increased heart rate, altered peripheral pulses and collapse of peripheral veins.

Fluid deficits

The fluid deficit in a particular dog or cat is difficult to determine with any accuracy because of the subjectivity of skin turgor evaluation and the possibility of undetected ongoing fluid losses. Skin turgor in the puppy or kitten younger than 6 weeks and geriatric dog or cat cannot be used to estimate dehydration.

Thus, the crude clinical estimate of hydration status and the animal's response to fluid administration become important tools in evaluating the severity of dehydration and in formulating ongoing fluid therapy. If clinical assessment of hydration status is suggestive of hypovolemia, a replacement fluid solution should be administered rapidly.

In animals with acute, severe losses and hypotension or shock, fluids can be given safely at a rate of one blood volume per hour (about 80-90 ml/kg per hour in dogs or 50-55 ml/kg per hour in cats) provided the cardiac and renal functions are normal. If there are no signs of hypovolemia, the hydration deficit and maintenance needs may be combined and administered during the next 24 hours.

This approach allows adequate time for equilibration of fluid and electrolytes with the intracellular compartment and avoids potential complications of edema or effusion from increased hydrostatic pressure, diuresis and loss of administered electrolytes in urine.

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.

Parenteral fluids

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 or dialysis.

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.