Page 960 - Small Animal Internal Medicine, 6th Edition

932 PART VII Metabolic and Electrolyte Disorders

accurately assesses total body magnesium content than mea-
BOX 53.10 surement of serum total magnesium. However, some lines of
VetBooks.ir Causes of Hypomagnesemia and Magnesium Depletion evidence in dogs, cats, and humans indicate that evaluation
of total magnesium or the ratio or ionized to total magne-
in Dogs and Cats
ionized magnesium may be preserved at the expense of total
Gastrointestinal Causes sium is indicated with suspected chronic deficiency, because
Inadequate intake magnesium concentrations. Assessing an animal’s magne-
Chronic diarrhea and vomiting* sium status is problematic because no simple, rapid, and
Malabsorption syndromes accurate laboratory test is available to gauge total body mag-
Acute pancreatitis nesium status. A parenteral magnesium tolerance test involv-
Cholestatic liver disease ing IV infusion and measurement of magnesium in urine to
Nasogastric suction estimate retention has been described in dogs to identify
Renal Causes those with poor total body status, but this has not been
Chronic kidney disease standardized in clinical practice. In dogs and cats with low
Renal tubular acidosis serum magnesium concentration, a review of the history,
Postobstructive diuresis physical examination, CBC, serum biochemistry panel, and
Drug-induced tubular injury (e.g., aminoglycosides, urinalysis usually provides clues to the underlying cause (see
cisplatin) Box 53.10).
Post–renal transplant
Prolonged intravenous fluid therapy* Treatment
Diuretics* Treatment of hypomagnesemia usually involves ill dogs and
Digitalis administration cats that are hospitalized and have inappetence and/or exces-
Concurrent electrolyte disorders sive fluid loss from the gastrointestinal tract or kidneys.
Hypercalcemia Treatment of hypomagnesemia may also be indicated during
Hypokalemia treatment of DKA in dogs and cats with refractory hypoka-
Hypophosphatemia
lemia, hypocalcemia, or both, in dogs and cats with refeed-
Endocrine Causes ing syndrome, and in dogs or cats in heart failure with
Diabetes mellitus and diabetic ketoacidosis* concurrent ventricular arrhythmias that are being treated
Hyperthyroidism with loop diuretics, digitalis, or both.
Primary hyperparathyroidism Parenteral solutions of magnesium sulfate (8.12 mEq of
Primary hyperaldosteronism magnesium per gram of salt) and magnesium chloride
(9.25 mEq of magnesium per gram of salt) are available com-
Miscellaneous Causes mercially. The IV dose for rapid and slow magnesium
Acute administration of insulin, glucose, or amino acids replacement is 0.5 to 1 mEq/kg/day and 0.3 to 0.5 mEq/kg/
Sepsis day, respectively, administered by constant-rate infusion in
Hypothermia 5% dextrose in water or 0.9% sodium chloride. Magnesium
Massive blood transfusion
Peritoneal dialysis, hemodialysis is incompatible with solutions containing bicarbonate or
Parenteral nutrition calcium. Kidney function should be assessed before magne-
sium is administered, and the magnesium dose should be
*Common causes. reduced by 50% to 75% in azotemic animals. The use of
Modified from Bateman S: Disorders of magnesium: magnesium magnesium with digitalis cardioglycosides may cause serious
deficit and excess. In DiBartola SP, editor: Fluid, electrolyte, and conduction disturbances. Serum magnesium, calcium, and
acid-base disorders in small animal practice, ed 4, St Louis, 2012, potassium concentrations should be monitored every 8 to 12
Saunders/Elsevier.
hours. The goal of magnesium therapy is the resolution of
clinical signs or refractory hypokalemia and hypocalcemia.
the body’s magnesium stores, and serum ionized magnesium Parenteral administration of magnesium sulfate may cause
represents 0.2% to 0.3% of total body magnesium stores. As significant hypocalcemia due to the chelation of calcium
a result, serum total and ionized magnesium concentrations with sulfate; therefore, magnesium chloride should be given
do not always reflect total body magnesium status. A normal if hypocalcemia is also present or a calcium infusion may be
serum magnesium concentration may exist despite an intra- necessary. Other adverse effects of magnesium therapy
cellular magnesium deficiency. However, a low serum mag- include hypotension; atrioventricular and bundle branch
nesium concentration would support the presence of a total blocks; and, in the event of overdose, respiratory depression
body magnesium deficiency, especially when clinical signs and cardiac arrest. Overdoses are treated with IV calcium
or concurrent electrolyte abnormalities are consistent with gluconate (see Box 53.7). Oral supplementation for more
hypomagnesemia. A serum ionized magnesium concentra- chronic or less severe cases is indicated and often well toler-
tion determined with the use of an ion-selective electrode ated although a laxative effect may be seen; magnesium
has been recommended based on the assertion that it more oxide, citrate, gluconate, chelate, or a mix of forms in an oral