Magnesium repletion and its effect on potassium homeostasis in critically ill adults: Results of a double-blind, randomized, controlled trial

OBJECTIVESThe aims of this study were to evaluate the safety and efficacy of magnesium replacement therapy and to determine its effect on potassium retention in hypokalemic, critically ill patients. DESIGNA prospective, double-blind, randomized, placebo-controlled trial. SETTINGA surgical intensive care unit (ICU). PATIENTSA total of 32 adult surgical ICU patients were admitted to the study on the basis of documented hypokalemia (potassium of less than 3.5 mmol/L) within the 24-hr period before entering the study. Patients were randomized to receive either placebo (n equals 15) or magnesium sulfate (n equals 17). One patient from each group was excluded from the study due to failure to complete the full series of doses. INTERVENTIONSPatients received a "test dose" of either magnesium sulfate (2 g, 8 mmol) or placebo (5% dextrose in water) infused over 30 mins every 6 hrs for eight doses. The next scheduled test dose was held if hypermagnesemia (magnesium of more than 2.8 mg/dL [more than 1.15 mmol/L]) was documented at any time during the study. Routine replacements of potassium and magnesium continued during the duration of the study, when clinically indicated, for serum potassium concentrations of 3.5 mmol/L or serum magnesium concentrations of less than 1.8 mg/dL (less than 0.74 mmol/L). MEASUREMENTS AND MAIN RESULTSAge, weight, and Acute Physiology and Chronic Health Evaluation II scores were recorded on entry into the study. Just before administration of each test dose, blood was drawn for magnesium and potassium, bicarbonate, pH, and glucose determinations, and an aliquot of the preceding 6 hrs urine collection was sent for magnesium and potassium determinations. Serum calcium, phosphate, urea nitrogen, and creatinine concentrations were measured daily. The amounts of magnesium and potassium administered via parenteral nutrition, tube feeding, and replacement infusions were calculated for each 6-hr interval. The amounts of magnesium and potassium excreted in the urine were similarly assessed.The groups showed no differences with regard to age, weight, Acute Physiology and Chronic Health Evaluation II scores, or initial serum magnesium concentration.Initial potassium, bicarbonate, pH, calcium, phosphate, glucose, blood urea nitrogen, and creatinine values were not different between groups. Patients receiving magnesium sulfate showed a statistically significant increase in serum magnesium concentration at 6 hrs when compared with placebo, as well as wi