Page 232 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

222 ELECTROLYTE DISORDERS

which some defect of renal magnesium handling may be category.* Therapeutic use of magnesium has been more
present. Urinary magnesium excretion (24 hour), urinary completely studied in several other diseases, but its ther-
magnesium clearance, and urinary fractional excretion of apeutic efficacy is still controversial. Diseases such as
magnesium have all been used as methods of evaluating myocardial infarction, acute severe asthma, hypertension,
renal magnesium handling. Patients with a magnesium and diabetes mellitus are examples that fit this category. {
deficit or those with inadequate dietary intake of Finally, several conditions have been well studied and the
magnesium would be expected to be retain magnesium efficacy of magnesium in conditions such as eclamsia/
to a greater extent than normal patients. Although these preeclamsia and several types of cardiac arrhythmias such
assays have not been widely tested in clinical patients, as digitalis toxicity, torsades de pointes, and ventricular
they have been used as a helpful tool in assessing ectopy has been shown. {
patients with inadequate dietary magnesium intake and In small animal veterinary medicine, there are several
thus increased renal reabsorptive compensatory conditions that warrant consideration of magnesium as
mechanisms. 109,159 The use of a magnesium retention a therapeutic agent. For most dogs and cats being fed a
test is favored by many human physicians in assessing commercial food, a dietary magnesium deficiency is not
the magnesium status of their patients. Several studies a concern. Most commercial dog and cat foods have
have evaluated the use of this test in human patients abundant magnesium supplementation. One recent
and found it to correlate well with clinical suspicion of study documented the likelihood that the dietary intake
magnesium deficit. 74,138 One study suggests, however, cats with heart failure was above recommended levels. 164
that the magnesium retention test assesses loss of magne- The at-risk population for magnesium deficit therefore is
sium from the exchangeable bone stores, which may not predominantly hospitalized dogs and cats, particularly
34
be reflective of total body magnesium deficit. The mag- those who have been anorexic for several days and in
nesium loading test has not, however, been standardized whom excessive gastrointestinal or renal loss of magne-
in human medicine and several variations of the test are sium could be occurring (see Box 8-1). Patients meeting
reported. 128 Although a study of a magnesium loading such criteria should be evaluated for a magnesium deficit.
test has been completed in dogs, the results have not been Documented hypomagnesemia or a magnesium reten-
published. 101 tion test suspected of being abnormal (normal values have
The diagnosis of magnesium deficit is challenging. not been established in small animal patients) should
Currently, there is no consensus regarding the best assay prompt the clinician to consider magnesium therapy to
for diagnosis. Several new technologies seem promising, correct the deficit. Although increased mortality has been
but they have not reached widespread clinical use and reported to occur in humans with a magnesium deficit,
raise more questions than answers. From the available therapy with magnesium salts has not been studied to
veterinary literature, it would appear that both ionized determine if therapeutic intervention with magnesium
serum magnesium and total serum magnesium in such patients changes the clinical outcome. Patients
concentrations may be useful when they are low and con- with refractory hypokalemia or hypocalcemia despite
sistent with the clinical suspicion of a magnesium deficit. seemingly appropriate supplementation should also be
It must be emphasized that a normal result does not rule evaluated for a magnesium deficit and treated accordingly
out a magnesium deficit. Clinical suspicion must still play if a deficit is detected. In addition, there are two kinds of
an important role. The magnesium retention test holds patient populations that are routinely encountered in
promise, but needs to be evaluated more completely in small animal emergency and critical care medicine that
veterinary species. are frequently identified with a magnesium deficit.
Patients in heart failure with concurrent ventricular
PHARMACOLOGIC USES OF arrhythmias and who are being medicated with loop
MAGNESIUM diuretics and/or digitalis constitute one high-risk group.
Significantly better control of arrhythmias (e.g., torsades
In addition to their use as therapy in patients with mag- de pointes, ventricular ectopy, digitalis toxicity) fre-
nesium deficit, magnesium salts have been used to treat a quently is gained in humans from supplementation with
number of disparate disease processes in humans. magnesium and potassium. Correction of the underlying
Although their use as a therapeutic agent in many of these electrolyte disturbances aids in normalization of the
conditions remains unproved, rigorous clinical trials have electrophysiologic state of the cardiac myocyte 76,129
not been performed to validate the use of magnesium. Ventricular arrhythmias resulting from an overload of
The use of magnesium as prophylaxis for migrainous
headache in children, as protection from endotoxin
*References 13–15, 29, 44, 78, 79, 85, 103, 121, 141, 162, 167, 172,
challenge, as management for cardiovascular signs of 185–187.
pheochromocytoma, as an adjunctive analgesic agent { References 7, 10, 18, 19, 42, 65, 67, 97, 116, 130–132, 135, 143, 147,
and as an adjunctive means of controlling muscular 151, 156, 165, 179, 180, 183, 192.
spasms of tetanus are examples of magnesium use in this { References 22, 33, 44, 58, 61, 77, 87, 104, 113, 120, 125, 129, 155.