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

OBJECTIVES

The 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.

DESIGN

A prospective, double-blind, randomized, placebo-controlled trial.

SETTING

A surgical intensive care unit (ICU).

PATIENTS

A total of 32 adult surgical ICU patients were admitted to the study on the basis of documented hypokalemia (potassium of < 3.5 mmol/L) within the 24-hr period before entering the study. Patients were randomized to receive either placebo (n = 15) or magnesium sulfate (n = 17). One patient from each group was excluded from the study due to failure to complete the full series of doses.

INTERVENTIONS

Patients 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 schedule test dose was held if hypermagnesemia (magnesium of > 2.8 mg/dL [> 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 < 1.8 mg/dL (< 0.74 mmol/L).

MEASUREMENTS AND MAIN RESULTS

Age, 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 with itself at time 0 (p < .0001), a difference maintained throughout the study. Compared with the placebo group, the total amount of elemental magnesium administered was significantly greater in the treatment group (1603 +/- 124 vs. 752 +/- 215 mg [65.7 +/- 5.8 vs. 30.8 +/- 8.8 mmol], p < .0001), as was urine magnesium excretion (1000 +/- 156 vs. 541 +/- 68 mg [41.0 +/- 6.4 vs. 22.2 +/- 2.8 mmol] p < .0001). However, the net magnesium balance (total magnesium in - total urine magnesium) was significantly more positive in the treatment group (612 +/- 180 vs. 216 +/- 217 mg [25.1 +/- 7.4 vs. 8.9 +/- 8.9 mmol], p < .005). The treatment and control groups had the same serum potassium concentrations and did not receive different amounts of potassium (245 +/- 39 vs. 344 +/- 45 mmol, respectively, p = .06), although the treatment group required less potassium replacement/6 hrs by 30 hrs compared with itself at time 0 (p < .05). Despite the same serum potassium values, the net potassium balance for 48 hrs was positive in the treatment group (+ 72 +/- 32 mmol) and negative in the control group (-74 +/- 95 mmol, p < .05). There were no complications associated with the magnesium sulfate administration.

CONCLUSIONS

Magnesium sulfate administered according to the above regimen safety and significantly increases the circulating magnesium concentration. Despite greater urine magnesium losses in the treatment group, this group exhibited significantly better magnesium retention.