• Eduardo A Osawa, Peter Biesenbach, Salvatore L Cutuli, Glenn M Eastwood, Johan Mårtensson, George Matalanis, Jessica Fairley, and Rinaldo Bellomo.
• Department of Intensive Care, Austin Hospital, Melbourne, Vic, Australia. rinaldo.bellomo@austin.org.au.
• Crit Care Resusc. 2018 Sep 1; 20 (3): 209-216.

BackgroundMagnesium therapy may reduce the risk of atrial fibrillation after cardiac surgery. However, studies are heterogeneous in relation to dosage and method of delivery and no studies have directly compared the biochemical effect of different delivery strategies.AimsWe conducted a before-and-after study to compare the effects of two strategies of magnesium delivery after cardiac surgery.MethodsWe conducted a prospective interventional before-and-after study. We enrolled patients admitted to the intensive care unit (ICU) after cardiac surgery and with no history of renal failure. The before period consisted of a single 20 mmol of magnesium sulfate bolus administered over one hour. The after period comprised a 10 mmol magnesium loading dose over one hour followed by a continuous infusion at 3 mmol/h for 12 hours. We measured serum and urine magnesium levels at baseline (T0), at the end of loading dose (T1), 6 (T2) and 12 hours after the intervention (T3).ResultsWe enrolled 60 patients (30 in each group) with similar baseline characteristics. In the before period, patients had a higher peak serum magnesium level at T1 (1.88 ± 0.06 v 1.59 ± 0.04 mmo/L; P < 0.001) compared with the after period. However, at 6 hours, patients in the after period had a significantly higher magnesium level (1.61 ± 0.04 v 1.29 ± 0.26 mmol/L; P < 0.001) and this level remained higher at 12 hours (1.70 ± 0.05 v 1.17 ± 0.02; P < 0.001), leading to increased time-weighted magnesaemia (P < 0.001). These changes occurred despite a significantly increased urinary magnesium concentration, fractional excretion of magnesium, and magnesium clearance, which paralleled changes in magnesaemia (P < 0.001).ConclusionsThe strategy of a 10 mmol magnesium bolus followed by a continuous infusion over 12 hours achieved a more sustained and moderately elevated magnesium concentration in comparison to a single 20 mmol bolus, despite increased urinary losses of magnesium. Further studies are required to assess a more extended continuous infusion.

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