Plasma magnesium levels and postoperative atrial fibrillation after cardiac surgery: a cohort study

OBJECTIVES: Postoperative atrial fibrillation (POAF) is common after cardiac surgery, and hypomagnesemia has been proposed as a modifiable risk factor. In centers with active magnesium repletion protocols, however, overt hypomagnesemia has become uncommon. We aimed to determine whether postoperative plasma magnesium concentration is independently associated with POAF. METHODS: We conducted a retrospective cohort study of 435 adults undergoing cardiac surgery at a hospital specializing in cardiovascular disease. Postoperative plasma magnesium concentration, assessed at intensive care unit admission, as the mean value, and as the lowest value, was evaluated as the primary exposure in multivariable logistic regression models adjusted for relevant preoperative, intraoperative, and postoperative covariates. RESULTS: Isolated coronary artery bypass grafting was the most frequent procedure (46.6%). Overall mortality was 3.2%, and POAF incidence was 16.8%. Under active repletion, hypomagnesemia occurred in only 3.4% of patients. None of the three magnesium measures was independently associated with POAF. In contrast, age, type of surgical procedure, severe left ventricular dysfunction, and postoperative acute kidney injury were independently associated with POAF. CONCLUSIONS: Under a protocol of active postoperative magnesium repletion, we did not detect an independent association between plasma magnesium concentration-within the observed normal-to-high range-and POAF. This complication was primarily related to age, type of surgery, and severe ventricular dysfunction, factors that should guide postoperative risk stratification; acute kidney injury showed a concurrent association with POAF and may act as a marker of shared postoperative risk factors rather than a causal factor.