This study is being conducted to see if magnesium can reduce the number of shocks patients with ICDs experience and to see if magnesium supplementation improves patients quality of life.

Not all ICD shocks are for ventricular arrhythmias. Some patients receive shocks when they have arrhythmias in the atria (top chambers of the heart). These are called inapproriate shocks, but the pain is similar to the pain patients feel with an appropriate shock (a shock for a ventricular arrhythmia). This study is being conducted to determine if taking magnesium can reduce the number of shocks patients with ICDs experience and to see if magnesium supplementation improves patients quality of life. Magnesium's impact of the electrocardiogram (ECG) and intracellular magnesium concentrations will also be studied.

Comparison: Magnesium compared to placebo in patients with ICDs to evaluate the number of ICD shocks and patient perceived quality of life.

Inclusion Criteria:newly implanted ICD or recent ICD shock (within 6 months) -

Exclusion Criteria:inability to swallow, a non-cardiac disease with a survival prognosis of less than 12 months, hypermagnesemia, a creatinine clearance less than 30mL/min, lactic acidosis or systemic acidosis syndrome, or previous intolerance to magnesium L-lactate.

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