Page 959 - Small Animal Internal Medicine, 6th Edition

CHAPTER 53 Electrolyte Imbalances 931

probably is not indicated for asymptomatic animals in which causes of clinically significant hypomagnesemia in dogs and
the serum phosphorus concentration is greater than 1.5 mg/ cats include disorders leading to small intestinal malassimi-
VetBooks.ir dL and is unlikely to decrease further. Phosphate therapy is lation; renal disorders associated with high urine output; the
osmotic diuresis of diabetic ketoacidosis; and the shift of
indicated if clinical signs or hemolysis is identified, or if the
serum phosphorus concentration is less than 1.5 mg/dL,
lular to the intracellular compartment that occurs within the
especially if a further decrease is possible. Phosphate supple- potassium, phosphorus, and magnesium from the extracel-
mentation is not indicated in dogs and cats with hypercalce- first 24 hours of therapy for DKA (Box 53.10). Magnesium
mia, oliguria, or suspected tissue necrosis. If renal function is predominantly an intracellular cation. The nature of the
is in question, phosphorus supplementation should not be translocation of magnesium between intracellular and extra-
provided until the status of renal function and the serum cellular compartments is similar to that of potassium in that
phosphorus concentration are known. factors that promote a shift of potassium into the intracel-
The goal of therapy is to maintain the serum phosphorus lular compartment (e.g., alkalosis, insulin, glucose infusion)
concentration greater than 2 mg/dL without causing hyper- promote a similar shift of magnesium.
phosphatemia. Injectable potassium phosphate solution is
available; however, unless urgent due to severity, oral phos- Clinical Features
phate supplementation is preferred (e.g., K-Phos Neutral or Hypomagnesemia is reported to be the most common elec-
Phospha 250 Neutral); bovine milk contains 0.032 mmol/ trolyte disorder in critically ill dogs and cats, and magnesium
mL of elemental phosphorous and can be used for oral deficiency may predispose animals to a variety of cardiovas-
phosphate supplementation, balanced commercial diets, cular, neuromuscular, and metabolic complications. Clinical
or a combination of these. IV phosphate supplementation signs of hypomagnesemia do not usually occur until serum
is usually required to correct severe hypophosphatemia, total and ionized magnesium concentrations are less than
especially in animals with diabetic ketoacidosis. Potassium 1.0 mg/dL and 0.4 mmol/L, respectively, and even at these
phosphate solutions are typically used. If potassium supple- low levels many animals remain grossly asymptomatic.
mentation is contraindicated, sodium phosphate solutions However, magnesium deficiency can result in several non-
can be substituted. Potassium and sodium phosphate solu- specific clinical signs, including lethargy, anorexia, muscle
tions contain 3 mmol of phosphate per milliliter and either weakness (including dysphagia and dyspnea), muscle fas-
4.4 mEq of potassium or 4 mEq of sodium per milliliter. The ciculations, seizures, ataxia, and coma. Concurrent hypoka-
initial dosage of phosphate is 0.01 to 0.03 mmol/kg/h, prefer- lemia, hyponatremia, and hypocalcemia occur in animals
ably administered by constant-rate infusion in calcium-free with hypomagnesemia, although the prevalence of these
IV fluids (i.e., 0.9% sodium chloride). In dogs and cats with electrolyte abnormalities may differ between species. These
severe hypophosphatemia, it may be necessary to increase electrolyte abnormalities may also contribute to the develop-
the dosage to 0.03 to 0.12 mmol/kg/h. Because the dose of ment of clinical signs. Magnesium is a co-factor for all
phosphate necessary to replete an animal and the animal’s enzyme reactions that involve ATP, most notably the sodium-
response to therapy cannot be predicted, it is important to potassium ATPase pump. Deficiencies in magnesium may
initially monitor the serum phosphorus concentration every cause potassium-losing nephropathy and increased urinary
4 to 8 hours and adjust the phosphate infusion accordingly. potassium losses, and the resultant hypokalemia may be
Adverse effects from overzealous phosphate administration refractory to appropriate potassium replacement therapy
include iatrogenic hypocalcemia and its associated neuro- unless hypomagnesemia is corrected. Magnesium deficiency
muscular signs, hypernatremia, hypotension, mineralization may inhibit PTH secretion from the parathyroid gland and
of soft tissues, and renal failure. Serum total or preferably may promote calcium uptake into bone, resulting in hypo-
ionized calcium concentration should be measured at the calcemia. Magnesium deficiency also causes the resting
same time as serum phosphorus concentration and the rate membrane potential of myocardial cells to be decreased and
of phosphate infusion decreased if hypocalcemia is identified. leads to increased Purkinje fiber excitability, with conse-
quent generation of arrhythmias. Electrocardiographic
changes include a prolonged PR interval, a widened QRS
HYPOMAGNESEMIA complex, a depressed ST segment, and peaked T waves.
Cardiac arrhythmias associated with magnesium deficiency
Etiology include atrial fibrillation, supraventricular tachycardia, ven-
Hypomagnesemia is present if serum total and ionized tricular tachycardia, and ventricular fibrillation. In addition,
magnesium concentrations are less than 1.5 mg/dL and hypomagnesemia predisposes animals to digitalis-induced
0.4 mmol/L, respectively, although reference ranges may arrhythmias.
vary between laboratories. Hypomagnesemia results from
decreased oral intake, increased loss from the gastrointestinal Diagnosis
tract (e.g., malabsorption syndromes), increased renal loss Many disorders and predisposing factors in dogs and cats are
(e.g., postobstructive diuresis), endocrine causes (e.g., diabe- associated with hypomagnesemia (see Box 53.10). However,
tes mellitus), or translocation of the cation from the extracel- measurement of serum total vs. ionized magnesium is some-
lular to the intracellular compartment. The most common what controversial. Serum total magnesium represents 1% of