How much magnesium sulfate is needed to "keep total serum magnesium above 2.0 mg/dL"?

For patients at increased risk of life-threating ventricular arrythmias, hospitalists often administer intravenous magnesium sulfate to maintain total serum magnesium concentration (TsMg) above 2 mg/dL. How long each dose keeps TsMg above this threshold is not well known, however. We collected TsMg values from 12,618 veterans who were given 24,363 doses of intravenous magnesium sulfate during 14,901 hospitalizations for acute heart failure. Across dose amounts, the average TsMg dropped below 2.0 mg/dL within 24 h of administration. When we limited our analysis to 2 g doses (the most common dose) and adjusted for baseline TsMg, estimated glomerular filtration rate, oral magnesium supplementation, and loop diuretic dosing, we found that less than half of the adjusted TsMg values remained above 2.0 mg/dL just 12 h after dose administration. Hospitalists should expect, on average, to administer 2 g intravenous magnesium sulfate at least twice daily to maintain total serum magnesium above 2 mg/dL.