Today's Veterinary Practice

SEP-OCT 2018

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Page 46 of 79 SEPTEMBER/OCTOBER 2018 45 FEATURES serum sodium concentration of 150 mEq/L has a corrected serum sodium of 164.4 mEq/L, assuming a normal serum glucose of 100 mg/dL. This situation raises concerns for hyperglycemic hyperosmolar syndrome (HHS), an uncommon form of diabetic crisis characterized by severe hyperglycemia (glucose level ≥ 600 mg/dL), minimal to absent urine ketones, and serum osmolality in excess of 350 mOsm/kg. 2 HHS must be rapidly recognized because treatment and prognosis differ from those for dogs presenting with diabetic ketoacidosis. 2,6-9 Treatment of HHS should be tailored to slowly decrease blood glucose and sodium in order to prevent cerebral edema and neurologic deterioration. Recommended rates of sodium decrease are 10 to 12 mEq/L per 24 hours or 0.5 to 1 mEq/L per hour; 10 for glucose, decreases of no more than 50 to 75 mg/dL per hour are recommended. 3 POTASSIUM Potassium is the primary intracellular cation in mammalian cells, with 95% to 99% of total- body potassium located within cells. 10,11 The most important role of potassium is in maintaining resting membrane potential, which determines a cell's degree of excitability. 10 Potassium disorders are among the most frequent electrolyte disturbances in human and veterinary patients and can be life-threatening when severe. 10,12 Hyperkalemia can arise from increased potassium loads, decreased urinary excretion, transcellular maldistribution, or pseudohyperkalemia (factitious). Hyperkalemia may cause muscular weakness, arrhythmias, and cardiac conduction abnormalities, most commonly leading to bradycardia and atrial standstill if severe. 1,10,11,13 Hypokalemia is caused by disorders of internal balance (i.e., metabolic alkalosis, insulin administration, and increased levels of catecholamines) and disorders of external balance (e.g., renal potassium wasting, inadequate intake, diuretics, liver disease, and hyperaldosteronism). Hypokalemia may also result in decreased cellular excitability, causing neuromuscular weakness and cardiac conduction abnormalities, such as ventricular tachyarrhythmias, atrioventricular dissociation, and ventricular fibrillation. 1,10,11 In dogs and humans, serum potassium concentrations are strongly correlated with total-body potassium stores, but serum potassium concentrations may not always reflect potassium balance because so much resides intracellularly. 10,11 Pseudohyperkalemia is an artifactual increase in potassium caused by several factors. In veterinary patients, thrombocytosis or leukocytosis may result in pseudohyperkalemia. 10,11 Hemolysis may also result in pseudohyperkalemia, particularly in neonates and Japanese breeds of dog, in which potassium concentrations in red blood cells are higher than those of normal adult canines. Additionally, dogs with phosphofructokinase deficiencies may exhibit pseudohyperkalemia in the presence of hemolysis. 11 Finally, incorrect order of blood draw can result in sample contamination with potassium ethylenediaminetetraacetic (EDTA), leading to pseudohyperkalemia. 14 For this reason, we recommend that when blood samples are collected, blood tubes be filled in the following order: first, red top (for serum collection); next, green top (containing citrate); and finally, purple top (containing EDTA). Artifactual or pseudohyperkalemia may mask true hypokalemia in the aforementioned situations. For patients exhibiting clinical signs consistent with hypokalemia and existing pathology that may lead to such electrolyte derangement, reported "normal" potassium values should be reassessed by collecting blood in a heparinized tube to determine plasma potassium concentration because this should not be affected by platelet or white blood cell numbers. 11 Care should also be taken to avoid contamination of the sample with EDTA in these cases. In cases of upper or lower urinary tract obstruction, normal or borderline-high potassium concentrations may give the false impression of a normal potassium balance, but veterinarians should know that the obstruction causes decreased excretion of potassium. Unilateral ureteral obstruction may pose an even greater challenge in determining normal electrolyte Potassium disorders are among the most frequent electrolyte disturbances in human and veterinary patients and can be life-threatening when severe. 10,12

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