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

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


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

In This Article


This investigation was a retrospective review of data on patients admitted to a cardiovascular care unit (i.e., an intensive care, intermediate care, or acute care unit) at University of North Carolina (UNC) Hospitals, a 750-bed academic medical center in Chapel Hill. The study design was approved by the UNC biomedical institutional review board, and the data were reviewed and analyzed by the primary authors and statistics consultants at UNC.

Inclusion and Exclusion Criteria

All cases in which patients were admitted to a cardiovascular care unit for the management of any cardiovascular condition were eligible for inclusion in this study. Eligible courses of magnesium therapy were those administered to patients who had a baseline SMC of 1.2–1.8 mg/dL and received a total dose of magnesium sulfate 2 g i.v. or magnesium oxide 800, 1200, or 1600 mg (typically split into two or three doses) orally within a 24-hour period. The baseline SMC range was chosen on the basis of a preinvestigation analysis that determined the SMCs at which oral and i.v. magnesium orders were written in approximately equal frequencies. Courses of therapy were excluded from analysis if (1) samples for follow-up measurement of SMC were collected less than 6 or more than 24 hours after the final magnesium dose of the course, (2) severe renal impairment (estimated creatinine clearance of <30 mL/min) was present,[7] or (3) magnesium was administered by both i.v. and oral routes within a 24-hour period. Only the courses of therapy not meeting these criteria were excluded from analysis; the exclusion of one course administered to a given patient did not result in the exclusion of other courses administered to that patient.

Data Collection

A list of eligible patients and courses of magnesium therapy was generated using the pharmacy information software at our institution. Demographic and laboratory data collected for each patient included age, sex, height, weight, and serum creatinine concentration. The route of administration for each magnesium dose was also recorded. The effect of magnesium replacement on the change in SMC was evaluated by comparing baseline SMCs with SMC determinations 6–24 hours after the completion of a course of magnesium replacement. SMCs were measured by our institution's clinical laboratory with a Vitros 5600 Integrated System (Ortho-Clinical Diagnostics, Inc., Rochester, NY). At a mean SMC of 1.8 mg/dL, the Vitros 5600 has a within-day and a within-laboratory standard deviation of 0.01 and 0.03 mg/dL, respectively.[8]

The influence of medications on the absorption or excretion of magnesium was evaluated by recording the concomitant use of diuretics, calcineurin inhibitors, proton pump inhibitors, histamine H2-receptor antagonists, vitamin D, and calcium supplements. Finally, the number of bowel movements within the 12-hour periods before and after each course of magnesium replacement was recorded using our nursing documentation software.


The primary outcome for the analysis was the change in SMC from the baseline value, compared between i.v. and oral routes of magnesium administration. Secondary outcomes included the influence of other variables (i.e., age, sex, dose of oral magnesium, renal function, concomitant medications, timing of SMC measurement) on the change in SMC, and the change in the number of bowel movements, compared between i.v. and oral magnesium.

Statistical Analysis

A two-sample t test was performed to evaluate the change in SMC associated with each course of i.v. or oral magnesium for a given baseline SMC. This test was also performed to detect any dose–response relationship for oral magnesium. Finally, linear regression analysis was performed to evaluate the influence of selected variables on the change in SMC. Linear regression was also used to evaluate the influence of oral magnesium oxide on the change in the number of bowel movements. The a priori level of significance was 0.05.


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.