Christopher J. Yarnell, Federico Angriman, Eliot R. Beaubien, J. Howard Brown, Selasi Bruce-Kemevor, Lisa Burry, Joshua Craig, Simon Donato-Woodger, Shannon Melania Fernando, Rob Fowler, Thecla Kattakkayam, Francois Lamontagne, Varuna Prakash, Tabo Sikaneta, Stephanie Sibley, Karim Soliman, G. Tomlinson, Thomas Bodley
OBJECTIVES: Most ICUs use protocolized magnesium supplementation, yet the clinical effect of this practice is unknown. DESIGN: Pseudo-randomized retrospective study comparing patients who were and were not assigned to receive magnesium supplementation, using a protocol where supplementation occurs when serum levels are less than or equal to 0.95 mmol/L (2.31 mg/dL). Primary outcome was atrial fibrillation or flutter within 24 hours. Secondary outcomes were tachyarrhythmia (supraventricular tachycardia or ventricular arrhythmia) and death within 24 hours. SETTING: ICUs with a shared magnesium supplementation protocol, in five hospitals in Ontario, Canada, from January 1, 2022, to December 31, 2024. PATIENTS: Adults (18 yr old or older) admitted to ICU with a magnesium protocol order, at their first magnesium level of 0.92-0.99 mmol/L (2.24-2.41 mg/dL). To minimize confounding, we included only patients with a level near the supplementation threshold. INTERVENTIONS: None. EXPOSURE: Magnesium level 0.92-0.95 mmol/L (2.24-2.31 mg/dL, supplementation group) vs. 0.96-0.99 mmol/L (2.32-2.41 mg/dL, no supplementation group). MEASUREMENTS AND MAIN RESULTS: We identified 4198 patients; median age 70 years, 41% female, 39% invasively ventilated; 2144 (51%) in the supplementation group, of whom 77% received magnesium, and 2054 (49%) in the no supplementation group, of whom 9% received magnesium. Atrial fibrillation or flutter occurred within 24 hours in 355 (16.6%) in the supplementation group and 375 (18.3%) in the no supplementation group. Bayesian logistic regression, adjusted for hospital, showed a 1.6% absolute risk reduction associated with supplementation (95% credible interval, 3.8% reduction to 0.8% increase; probability of reduction, 0.91). For the composite outcome of atrial fibrillation and flutter, tachyarrhythmia, and death, the absolute risk reduction associated with supplementation was 2.2% (CrI, 4.3% reduction to 0.1% increase; probability of risk reduction, 0.97). CONCLUSIONS: Protocolized magnesium supplementation at a threshold of 0.95 mmol/L (2.31 mg/dL) may be associated with reduced 24-hour incidence of atrial fibrillation and flutter in critically ill patients.