Clinical Drug Experience Knowledgebase

Hypomagnesemia is a common electrolyte disturbance that affects up to 65% of intensive care unit (ICU) patients with normal renal function. Causes of hypomagnesemia are attributed to either gastrointestinal (secretory loss, impaired absorption or reabsorption, acute pancreatitis) or renal losses (alcohol, hypercalcemia, volume expansion, loop or thiazide diuretics, nephrotoxic medications, renal tubular dysfunction, inborn disorders). Consequences of magnesium deficiency are not benign and may include neuromuscular and neurologic dysfunction, cardiac arrhythmias and concomitant electrolyte abnormalities including hypokalemia and hypocalcemia. Hypomagnesemia has been associated with a significantly greater mortality rate in critically ill medical patients compared to normomagnesemic patients. In a study conducted by Rubeiz et al, 46% (17/37) of hypomagnesemic patients in the medical ICU died compared to 25% (37/147) of normomagnesemic patients (p < 0.05).

It can be difficult to assess patients for hypomagnesemia because of the unreliable relationship between serum and tissue magnesium levels. Approximately 1% of total body magnesium is found in the extracellular fluid while the remaining 99% is distributed among the bones, muscles, and soft tissues. Approximately 60% of serum magnesium is free ions; 33% is bound to proteins and 7% is complexed with anions. The most simple and commonly used test to diagnose hypomagnesemia is the total serum magnesium level which reflects free magnesium along with complexed and protein bound magnesium. The serum magnesium level, however, is not always accurate at detecting magnesium deficiency. Patients may appear to be normomagnesemic based on their serum magnesium level, yet have an underlying magnesium deficiency. Normal serum magnesium levels vary by laboratory. The normal range of values at Charleston Area Medical Center (CAMC) is 1.6-2.6 mg/dL.

Magnesium replacement depends on the clinical situation and manifestations. In critical conditions such as pre-eclampsia, arrhythmias, and tetany, large doses of IV magnesium are rapidly bolused and often followed by a continuous IV infusion. In asymptomatic patients, magnesium may be replaced by the oral or IV route depending on the clinical situation. The dose required to return patients to the normal magnesium range is variable and replacement may take several doses. Serum magnesium levels are primarily controlled by glomerular filtration and tubular reabsorption at the sites of the Loop of Henle and distal tubule. When faced with an increased filtered load of magnesium, the kidney is capable of increasing its excretion rate. Following intravenous (IV) administration, cellular magnesium uptake is slow and approximately 50% or more of the infused dose is lost due to increased excretion by the kidneys and decreased tubular reabsorption.

The investigators current practice in the Medical and Neuroscience ICUs at CAMC General Hospital is to order 8g of magnesium sulfate for replacement in patients with hypomagnesemia. When IV magnesium sulfate is ordered the pharmacy automatically sets the rate to run at 2g per hour unless otherwise specified. Often times the physician will specify for 8g to be infused over eight hours. The basis of using an extended infusion is that a slower magnesium infusion rate may increase magnesium retention by allowing a longer period of time for magnesium uptake by cells and by decreasing the magnesium load delivered to the kidneys at any given time. As far as the investigators are aware, there have been no studies completed to date that assess the rate of IV magnesium infusion on the magnesium retention rate.