Frequency of hypomagnesemia in hospitalized patients receiving digitalis

We examined the frequency of hypokalemia and hypomagnesemia in patients receiving digitalis. Serum sodium, magnesium, and potassium levels were determined in 136 serum samples sent to the laboratory for digoxin assay. Hyponatremia (less than or equal to 130 mEq/L) occurred most frequently (21%), followed by hypomagnesemia (less than or equal to 1.25 mEq/L) in 19%, hypokalemia (less than or equal to 3.5 mEq/L) in 9%, and hypermagnesemia (greater than or equal to 2.25 mEq/L) in 7%. The twofold frequency of hypomagnesemia (19%) contrasted with hypokalemia (9%) indicates that clinicians are more attuned to avoiding hypokalemia than hypomagnesemia in patients receiving digitalis. Because hypokalemia and/or hypomagnesemia may contribute to the toxic effects of digitalis, our observation suggests that hypomagnesemia may be a more frequent contributor than hypokalemia to induction of toxic reactions to digitalis. Routine serum magnesium determination in patients receiving digitalis, who often are also receiving potent diuretics, may assist in identifying additional patients at risk for the toxic effects of digitalis.