Today's Veterinary Practice

MAR-APR 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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PEER REVIEWED 52 FLUID THERAPY: PART 2 plasma. If the patient's sodium concentration is >160 mEq/L, custom fluids can be made by adding hypertonic saline to 0.9% saline to achieve the desired sodium concentration ( BOX 2 ). Free water can be provided enterally (voluntarily or via a feeding tube) or by administering 5% dextrose in water. If 0.45% NaCl is administered, half of the volume is free water; the other half is equal to giving 0.9% NaCl. During correction of FWD, the patient should also receive a maintenance rate of a fluid that is close to isotonic relative to its plasma. This may be 0.9% NaCl if the patient is mildly to moderately hypernatremic. If the patient is severely hypernatremic, a custom solution may be required. Frequent (approximately every 4 hours) monitoring of the sodium concentration is necessary to ensure that it is not decreased too quickly and to allow adjustments to the fluid therapy plan. Ongoing hypotonic or free water losses may require higher rates of free water be administered. In patients that are hypernatremic because of sodium gain, dilution of the extracellular fluid with free water will cause further expansion of the intravascular volume and may result in volume overload. Caution should be used in patients that are oliguric or have underlying cardiac disease. Frequent monitoring for signs of volume overload ( BOX 3 ) is indicated. In patients that have mild hypernatremia with hypotonic fluid loss or decreased renal function and a decreased ability to excrete a salt load, 0.45% NaCl can be used as a maintenance fluid after volume depletion has been corrected. Hyponatremia Pathophysiology and Clinical Signs Decreased serum sodium can occur in hypo- osmolar, normo-osmolar, and hyperosmolar states. Falsely decreased sodium can be seen in patients with normal osmolality because another solute (eg, glucose, mannitol) in the plasma is causing the decrease in sodium or interfering with measurement of sodium. TABLE 1 lists the effect of various serum abnormalities on measured sodium. In the case of hyperglycemia, the effect on measured sodium is not linear, and the correction factor will be higher with blood glucose >440 mg/dL. 1 True hyponatremia (with decreased osmolality) can occur in conjunction with hypovolemia, hypervolemia, or normovolemia. Hypovolemic hyponatremia can be seen with gastrointestinal, third-space, or renal losses and is frequently seen in hypoadrenocorticism. 1 BOX 2 Creating Custom Fluids for Hypernatremic Patients If a patient's sodium concentration is >160 mEq/L, a fluid that is isotonic relative to the patient's plasma can be made by adding hypertonic saline (23.4% NaCl) to 0.9% saline. The amount of NaCl in 23.4% saline is 4 mEq/mL. The concentration of sodium in a liter bag of 0.9% NaCl varies slightly; the average is 154 mEq/L. It is helpful to measure the sodium concentration of any customized solutions on a point-of-care analyzer before administration. For a patient with a serum sodium concentration of 180 mEq/L: Desired Na concentration = 180 mEq/L 0.9% NaCl = 154 mEq/L Na 180 mEq/L − 154 mEq/L = 26 mEq/L 26 mEq/L must be added to a 1-L bag of 0.9 NaCl to match the patient's sodium concentration. To obtain the amount of 23.4% NaCl that contains 26 mEq/L NaCl: 26 mEq/L/4 mEq/L = 6.5 mL 6.5 mL of 23.4% NaCl must be added to a 1-L bag of 0.9% NaCl for this patient. BOX 3 Clinical Signs of Fluid Overload Increasing body weight Increased skin turgor Serous nasal discharge Increased respiratory rate and effort Peripheral edema Body cavity effusions Chemosis Jugular venous distention

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