Outcome-based justification for implementing new point-of-care tests: there is no difference between magnesium replacement based on ionized magnesium and total magnesium as a predictor of development of arrhythmias in the postoperative cardiac surgical patient.

Objective: To determine whether introducing a new laboratory test, ionized magnesium (iMg++), would affect outcome, where outcome was defined as the rate of arrhythmias in a population of postoperative cardiopulmonary bypass (CPB) patients.

Design: A prospective randomized trial.

Setting: Cardiothoracic surgical intensive care unit of a university hospital.

Patients: One hundred fifty consecutive post-CPB patients with randomized to two groups, one of which received routine reporting of iMg++ levels on all postoperative electrolyte requests while the other had access to total magnesium (tMg++) levels on demand and no access to iMg++ levels. Groups were compared for rate of arrhythmias, total amount of magnesium repleted, and demographics.

Measurements and main results: Eighty-five patients were randomized to the tMg++ group and 65 to the iMg++ group. The two groups did not differ in the rate of arrhythmias (chi-square test): 13/85 (15%) of the tMg++ patients and 12/65 (18%) of the iMg++ patients developed an arrhythmia. The groups also did not differ in the amount of magnesium sulfate (MgSO4) administered (2 tailed t-test): tMg++ patients received 1.5 +/- 0.15 (SEM) gm of MgSO4, whereas iMg++ patients received 1.3 +/- 0.15 gm.

Conclusion: The study does not support the hypothesis that magnesium repletion titrated to iMg++ reduces arrhythmia development in post-CPB patients. The lack of a difference in the amount of magnesium replacement between the two groups suggests that tMg++ level is a reasonable indicator of iMg++ level. Routine measurement of iMg++ does not, therefore, appear to have advantages over tMg++ in the postoperative management of CPB patients.