Prophylactic Magnesium Supplementation and New-Onset Atrial Fibrillation in a General Critical Care Population: A Prescribing Preference Instrumental Variable Analysis

Aims Atrial fibrillation is frequently encountered in critical illness and causes adverse effects including haemodynamic decompensation, stroke and longer hospital stay. It is common to supplement serum magnesium for the purpose of preventing new-onset atrial fibrillation. However, no randomised studies support this practice in the non-cardiac surgery critical care population, and its effectiveness is unclear. We sought to investigate the effectiveness of magnesium supplementation in preventing new-onset atrial fibrillation in a mixed critical care population. Methods We conducted a single centre retrospective observational study of adult critical care patients. We employed a natural experiment design, using the supplementation preference of the bedside critical care nurse as an instrumental variable. Using the electronic patient record, magnesium supplementation opportunities were defined and linked to the bedside nurse. Nurse preference for administering magnesium was obtained using multilevel modelling. The results were used to define pro and anti supplementation groups, which were inputted into an instrumental variable regression to obtain an estimate of the effect of magnesium supplementation. Results 9,114 magnesium supplementation opportunities were analysed, representing 2,137 critical care admissions for 1,914 patients. There was significant variation in magnesium supplementation practices attributable to the individual nurse, after accounting for covariates. The instrumental variable analysis showed magnesium supplementation was associated with a 3% decreased chance of experiencing new-onset atrial fibrillation (95% CI -0.06 to -0.04, p = 0.03). Conclusions This study supports the strategy of routine magnesium supplementation, but further work is required to identify optimal serum magnesium targets for prophylaxis of atrial fibrillation.