Magnesium is the second most abundant intracelular cation, exceeded only by potassium. The majority of magnesium is found in bone and muscle. This cation is required for many metabolic functions, most notably as a coenzyme for the sodium-potassium ATPase pump. Magnesium functions to maintain the electrolyte gradient across all membranes. Interference with this gradient may result in changes in the resting membrane potential and disturbances in repolarization, resulting in cardiovascular and neuromuscular abnormalities.
Hypomagnesemia may be the most underdiagnosed electrolyte disorder. Incidence rates greater than 50 percent have been reported in critically ill human patients. Currently there is little or no information available regarding the incidence and significance of hypomagnesemia in hospitalized animals. Causes of magnesium deficiency can be divided into four general categories: gastrointestinal, renal, endocrine and miscellaneous. The diagnosis of magnesium depletion can be difficult since less than one percent of total body magnesium is located in serum. Alternative methods of evaluating magnesium status include determining ultrafilterable magnesium levels, mononuclear blood cell magnesium levels or by quantifying magnesium retention of an administered loading dose.