Comparison of Intravenous and Oral Magnesium Replacement in Hospitalized Patients With Cardiovascular Disease

Brent N. Reed; Su Zhang, J. S. Marron; Deborah Montague

Disclosures

Am J Health Syst Pharm. 2012;69(14):1212-1217. 

In This Article

Results

Data collection was performed from October 18, 2009, through January 28, 2010. The pharmacy information system identified 525 patients who received any dose of i.v. magnesium sulfate or oral magnesium oxide. Of these patients, 291 met the inclusion criteria; they had received a total of 499 courses of magnesium therapy. A total of 147 courses were excluded—79 because samples for SMC measurement were collected less than 6 or more than 24 hours after a magnesium course, 41 because the patient received both oral and i.v. magnesium within a 24-hour period, and 27 because the patient had severe renal impairment. The exclusion of these courses resulted in 80 patients being excluded from the analysis altogether. Therefore, a total of 352 courses (188 courses of i.v. magnesium sulfate and 164 courses of oral magnesium oxide) administered to 211 patients were included in the analysis.

The baseline characteristics of patients who received i.v. and oral magnesium were similar, with the exception of the mean ± S.D. total magnesium dose, the mean ± S.D. estimated creatinine clearance, the mean ± S.D. baseline SMC, and the median time of SMC measurement after the completion of magnesium courses (Table 1).

Primary Outcome

In patients with baseline SMC values of 1.4–1.8 mg/dL, the use of i.v. magnesium sulfate resulted in a significantly greater mean change in SMC than the use of oral magnesium oxide (Table 2). In patients with a baseline SMC of 1.2 or 1.3 mg/dL, i.v. magnesium produced, respectively, a greater change in SMC and an equal change in SMC relative to oral therapy, but the sample sizes for those baseline SMC values were too small to detect any significant differences. The lower the baseline SMC, the greater the mean change in SMC associated with i.v. magnesium. Among the patients with baseline SMCs of 1.4–1.8 mg/dL, oral magnesium oxide consistently produced a median change in SMC of 0.1 mg/dL.

The mean change in SMC did not appear to differ with the size of the total oral magnesium dose (Table 3), but the numbers of courses involving total magnesium oxide doses of 1200 and 1600 mg were relatively small (18 and 19, respectively). The use of i.v. magnesium sulfate was associated with a greater mean change in SMC than any of the studied doses of oral magnesium oxide.

Secondary Outcomes

In the linear regression analysis of factors possibly affecting the absorption or excretion of magnesium, five variables were found to be significantly associated with the degree of change in SMC after either oral or i.v. magnesium replacement: (1) the route of magnesium administration, (2) the timing of SMC measurement after completion of the course, (3) renal function, (4) the concomitant use of oral loop diuretics, and (5) the concomitant use of i.v. loop diuretics. No other examined factors, including the total dose of oral magnesium oxide, were significantly associated with the change in SMC; an association approaching statistical significance (p = 0.054) was documented for one factor (the concomitant use of calcineurin inhibitors).

The change in SMC was negatively correlated with the timing of the laboratory measurement of SMC after the completion of magnesium courses: The shorter the time to SMC measurement, the greater the change in SMC (r = −0.0053, p = 0.0087). The impact of time on predicted SMC was greater with i.v. than with oral magnesium by a factor of 0.1714 (p < 0.001).

Finally, the change in the number of bowel movements within 12 hours of the completion of a magnesium course did not differ significantly between patients receiving i.v. versus oral magnesium (mean changes, 0.04 and −0.04 bowel movements, respectively; p = 0.247).

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