Serum and tissue magnesium concentrations in patients with heart failure and serious ventricular arrhythmias. OBJECTIVE: To compare magnesium concentrations in serum and tissue from patients with heart failure. Two groups of patients were compared, those with or without serious ventricular arrhythmias. DESIGN: Consecutive enrollment. Blinded laboratory analyses. SETTING: Referral inpatient service of the cardiology division of a university hospital. PATIENTS: Twenty-three patients with idiopathic dilated cardiomyopathy and mild to moderately severe congestive heart failure (New York Heart Association functional class II to IV) were divided into two groups: 9 patients with sustained ventricular tachycardias and 14 patients without serious ventricular arrhythmias (control). INTERVENTIONS: Medications for heart failure were withdrawn 12 hours or more before study. Antiarrhythmic therapy was continued throughout the study. MEASUREMENTS AND MAIN RESULTS: The patients had skeletal muscle biopsies, myocardial biopsies, and blood sampling for the analysis of magnesium concentrations. No statistically significant differences in the mean magnesium concentrations in serum, circulating mononuclear cells, skeletal muscle, and myocardium were found when the 9 patients with ventricular arrhythmias were compared with the 14 control patients without serious ventricular arrhythmias. CONCLUSIONS: In a general, ambulatory sample of patients with heart failure, magnesium depletion in serum and tissue does not appear to occur more commonly in patients with serious ventricular arrhythmias than in patients without serious ventricular arrhythmias. 