Magnesium supplementation: Pharmacokinetics in cardiac surgery patients with normal renal function

Background: Intravenous magnesium is routinely administered in intensive care units (ICU) to treat arrhythmias after cardiothoracic surgery. There are no data on the pharmacokinetics of continuous magnesium infusion therapy. Objective: To investigate the pharmacokinetics of continuous magnesium infusion, focusing on serum and urinary magnesium concentration, volume of distribution and half-life. Methods: We administered a 10 mmol bolus of magnesium-sulfate followed by a continuous infusion of 3 mmol/h for 12 h in twenty cardiac surgery patients. We obtained blood and urine samples prior to magnesium administration and after one, six, and 12 h. Results: Median magnesium levels increased from 1.09 (IQR 1.00–1.23) mmol/L to 1.59 (1.45–1.76) mmol/L after 60 min (p <.001), followed by 1.53 (1.48–1.71) and 1.59 (1.48–1.76) mmol/L after 6 and 12 h. Urinary magnesium concentration increased from 9.2 (5.0–13.9) mmol/L to 17 (13.6–21.6) mmol/L after 60 min (p <.001). Cumulative urinary magnesium excretion was 28 mmol (60.9% of the dose given). The volume of distribution was 0.25 (0.22–0.30) L/kg. There were no episodes of severe hypermagnesemia (≥3 mmol/L). Conclusion: Combined bolus and continuous magnesium infusion therapy leads to a significant and stable increase in magnesium serum concentration despite increased renal excretion and redistribution.