Measuring serum electrolytes great ally in diagnosis, treatment

Measuring serum electrolytes great ally in diagnosis, treatment

Jul 01, 2002

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!

Table 1: Selected causes of altered sodium and potassium balance
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.

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.