Hypomagnesemia with diuretics

Hypomagnesemia is treated initially with oral, intramuscular, or intravenous administration of magnesium salts. Immediate control of the symptoms of acute hypermagnesemia is obtained with doses of intravenous calcium repeated hourly but extreme toxicity may require cardiac support or mechanical ventilation. Calcium gluconate and calcium chloride can also be administered as antidotes. Serum levels are lowered by reducing intake and by normal methods of excretion, with diuretics given to patients with normal renal function. Other accompanying electrolyte imbalances should be treated concurrently, followed by treatment of the condi-tion(s) that lead to the imbalances. 

Many drugs can cause hypokalemia (Table 78-5) and it is most commonly seen with use of loop and thiazide diuretics. Other causes of hypokalemia include diarrhea, vomiting, and hypomagnesemia. 

Hypomagnesemia is usually associated with disorders of the intestinal tract or kidneys. Drugs (e.g., aminoglycosides, amphotericin B, cyclosporine, diuretics, digitalis, cisplatin) or conditions that interfere with intestinal absorption or increase renal excretion of magnesium can cause hypomagnesemia. 

Therapy with hydrochlorothiazide, up to 50 mg twice daily, or chlorthalidone, 50-100 mg daily, is recommended. Loop diuretics such as furosemide and ethacrynic acid should not be used because they increase urinary calcium excretion. The major toxicity of thiazide diuretics, besides hypokalemia, hypomagnesemia, and hyperglycemia, is hypercalcemia. This is seldom more than a biochemical observation unless the patient has a disease such as hyperparathyroidism in which bone turnover is accelerated. Accordingly, one should screen patients for such disorders before starting thiazide therapy and monitor serum and urine calcium when therapy has begun. 

Because amisulpride may dose-dependently prolong QTc interval, use with caution in patients who have hypokalemia and/or hypomagnesemia or who are taking drugs that can induce hypokalemia and/or magnesemia (e.g., diuretics, stimulant laxatives. Intravenous amphotericin B, glucocorticoids, tetracosactide)... 

Hypomagnesemia is common with both loop and thiazide diuretics. In 242 patients the frequency was 19% (10). It was corrected with potassium magnesium citrate, whereas potassium citrate or potassium chloride alone had little corrective effect. 

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