Page 222 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 8

Disorders of Magnesium: Magnesium

Deficit and Excess

Shane Bateman

Historically, magnesium has received very little atten- MAGNESIUM REGULATION
tion in veterinary medicine as an electrolyte worthy of AND BALANCE
consideration. Studies conducted in animal models in
the early twentieth century documented the devastating
effects of dietary magnesium deficiency in DISTRIBUTION OF MAGNESIUM
dogs. 25,26,117,171 Dietary magnesium became a topic The precise distribution of magnesium in the bodies of
of interest in the 1970s and 1980s as a potential risk dogs and cats under differing conditions has not been
factor for cats with struvite urolithiasis and urethral well studied. The distribution of magnesium within the
obstruction. 31,93,127 Today, following significant study bodies of human beings has been documented more
of the syndrome in cats and identification of numerous effectively. In humans, current estimates suggest that only
other risk factors, magnesium in the diet is no longer about 1% of the total body magnesium is located outside
considered a risk factor for the formation of feline uro- the cell in the extracellular fluids and that the remaining
lithiasis. 23,24,56,91 Since that time, magnesium has been 99% is located in intracellular stores. 126 Approximately
the focus of very little research when compared with two thirds (67%) of body magnesium is stored in the bone
other electrolytes. with calcium and phosphorus, 20% is found in muscle tis-
Veterinary critical care has made significant develop- sue, and 11% in other soft tissues not including mus-
mental strides over the past 20 years as it follows in the cle. 124 Bone and muscle account for the major
footsteps of its human counterpart discipline. Magne- intracellular stores of magnesium in humans. Exchange
sium has gained considerable importance within the dis- between intracellular magnesium and the extracellular
cipline of critical care because of the prevalence of fluid is difficult to study, but current estimates suggest
magnesium-related metabolic dysfunction documented that only 15% of these stores are considered to be
in human and veterinary patient populations. Study of exchangeable with the extracellular fluid. It appears that
magnesium-related disease has proved to be difficult, bone, muscle, and red blood cell stores of magnesium are
most likely, because approximately 99% of the body’s very slow to liberate magnesium to the extracellular pool,
magnesium is stored inside the cell, where it participates and that soft tissues are much more able to liberate mag-
in vital behind-the-scenes metabolic activities of the cell. nesium to the extracellular space in humans. 126 In dogs,
As technology has advanced, however, our understanding however, similar radioisotope studies suggest that bone
of the important role magnesium plays in maintaining magnesium is the most labile pool and will be scavenged
normal homeostasis of important body systems, such as during a magnesium deficit. 16 Regulatory control of
the cardiovascular and neuromuscular systems, has magnesium shifts between intracellular and extracellular
increased significantly. At the beginning of the twenty- spaces is poorly understood and is likely to be complex
first century, the field of magnesium study is rich and ripe and multifactorial. Extracellular magnesium is present
with opportunities. Our efforts have only just begun to in three forms (like calcium): an ionized or free form
scratch the surface of understanding the importance of (55%) that is believed to constitute the biologically active
magnesium. fraction, a protein bound form (20% to 30%) and a

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