Changes in fluid content
Examples of common disorders causing changes in fluid content
- Renal disease
- Urinary obstruction
Suspect hyperkalemia in cases of obvious urinary obstruction, uroabdomen, acute kidney injury, diabetic ketoacidosis (DKA), or changes on an electrocardiogram. If life-threatening hyperkalemia is either suspected or present (K . > 6 mmol/L), begin fluid therapy immediately along with medical therapy for hyperkalemia.35
There are several benefits associated with administering K-containing balanced electrolyte solutions pending laboratory test results. Volume expansion associated with the fluid administration results in hemodilution and lowering of serum K concentration. The relief of any urinary obstruction results in kaliuresis that offsets the effect of the administered K. The relative alkalinizing effect of the balanced solution promotes the exchange of K with hydrogen ions as the pH increases toward normal.
Most K-containing balanced electrolyte solutions contain lower K concentrations than those typically seen in cats with urethral obstruction, so the use of such solutions does not affect blood K in those cats.36 LRS contains 4 mmol/L, which is typically much lower than the serum K levels in cats with urethral obstruction.
Charts are available in many texts to aid in K supplementation of fluids and determination of administration rate.37 It is essential to mix added KCl thoroughly in the IV bag as inadvertent K overdoses can occur and are often fatal. Do not exceed an IV administration rate of 0.5 mmol/kg/hr of K.38 If hypophosphatemia exists along with hypokalemia (e.g., DKA), use potassium phosphate instead of KCl.
Hypernatremia may be common, yet mild and clinically silent. Causes of hypernatremia include loss of free water (e.g., through water deprivation), and/or iatrogenically (through the long-term use [.>24 hr] of replacement crystalloids). Another cause of hypernatremia is salt toxicity (through oral ingestion of high salt content materials).
Provide for ongoing losses and (in hypotensive patients) volume deficits with a replacement fluid having a Na concentration close to that of the patient’s serum (e.g., 0.9% saline). Once volume needs have been met, replace the free water deficit with a hypotonic solution (e.g., D5W). Additionally, for anorexic patients, provide maintenance fluid needs with an isotonic balanced electrolyte solution. The cause and duration of clinical hypernatremia will dictate the rate at which Na levels can be reduced without causing cerebral edema. Do not exceed changes in Na levels of 1 mmol/hr in acute cases or 0.5mmol/hr in chronic cases because of the risk of cerebral edema. Although the complexities of managing Na disorders often benefits from the involvement of a specialist/criticalist, this is not always feasible. The amount of free water (in the form of D5W) to infuse over the calculated timeframe (to decrease the Na concentration by the above guidelines) can be calculated as follows:
Volume (L) of free water (D5W) needed ([current Na concentration/normal Na concentration] - 1) x (0.6 x body weight [kg])33
Hyponatremia is most commonly seen in DKA and with water intoxication. Changes in serum Na levels must occur slowly, as with hypernatremia. Monitor electrolyte levels frequently, and use a fluid with Na content similar to the measured plasma Na to keep the rate of change at an appropriate level.
In patients with water intoxication, restrict water and/or use diuretics with caution. Patients with DKA may have pseudohyponatremia associated with osmotic shifts of water following glucose into the intravascular space. In pseudohyponatremia, a relationship exists between serum glucose and serum Na levels: the higher the glucose, the lower theNa. Specifically, for every 100 mg/dL increase in serum glucose over 120 mg/dL, the serum Na will decrease by 1.6 mmol/L.39
Colloid osmotic pressure is related to plasma albumin and protein levels and governs whether fluid remains in the vascular space. Fluid loss into the pulmonary, pleural, abdominal, intestinal, or interstitial spaces is uncommon until serum albumin is ,<15 g/L or total protein is ,<35 g/L.<sup>19,40</sup> Evidence of fluid loss from the vascular space is used in conjunction with either serum albumin or total solid values in determining when to initiate colloid therapy.
Guidelines for fluid therapy when treating hypoalbuminemia include the following:
- Nutritional support is critical to treatment of hypoalbuminemia.
- Plasma administration is often not effective for treatment of hypoalbuminemia due to the relatively low albumin levels for the volume infused. Human serum albumin is costly and can cause serious hypersensitivity reactions.41 Canine albumin is not readily available in most private practice settings but may be the most efficient means of supplementation when available.42
- Synthetic colloids (e.g., hydroxyethyl starch) are beneficial because they can increase oncotic pressure in patients with symptomatic hypoalbuminemia to maintain fluid in the intravascular space; however, synthetic colloids will not appreciably change total solids as measured by refractometry. Therefore, patient assessment determines response.43 Use up to 20 mL/kg/day of hetastarch for dogs and 10–20 mL/kg/day for cats.29–31
Fluid therapy in hyperglycemic patients is aimed at correcting dehydration and electrolyte abnormalities. Monitor the patient to guide the rate of correction. As with hyperkalemia, the choice of initial replacement fluid is not as important as correcting the patient’s hydration status. See the AAHA Diabetes Management Guidelines for details on managing hyperglycemia.44
Initial therapy for hypoglycemia is based on severity of clinical signs more than on laboratory findings. Treatment options include oral glucose solutions, IV dextrose-containing fluids, or food (if not contraindicated). To prepare a dilute dextrose solution of 2.5–5% dextrose, add concentrated stock dextrose solution (usually 50% or 500 mg/mL) to an isotonic balanced electrolyte solution (e.g., add 100 mL of 50% dextrose to 900 mL of fluid to make a solution containing 5% dextrose).
Anemia and Polycythemia
Blood products may be needed to treat anemia. The decision to transfuse the anemic patient is not based on either the packed cell volume or hematocrit alone, but on multiple factors as described in the “General Principles and Physical Assessment” section of this document. Use of blood products is not addressed in this document. Blood loss and hemorrhage are discussed above in volume changes.
Treatment of symptomatic polycythemia involves reducing the number of red blood cells through phlebotomy and replacing the volume removed with balanced electrolyte solutions to reduce viscosity and improve blood flow and O2 delivery.
Multiple Content Changes
Many patients present with multiple serum chemistry abnormalities, making appropriate fluid choice problematic. The vast majority of patients will benefit from early empirical fluid therapy while awaiting lab results, knowing that more specific treatment will be tailored to individual needs as diagnostic information becomes available.