Plasma potassium, serum magnesium and ventricular fibrillation: a prospective study.

Low plasma potassium and magnesium concentrations have been advanced as risk factors for ventricular fibrillation (VF). For potassium, this assertion is based almost exclusively on retrospective data; for magnesium the evidence shows an association with ventricular arrhythmias but no direct association with VF. We studied the relationship between these electrolytes and VF prospectively. Plasma potassium and serum magnesium concentrations were measured on admission to our coronary care unit. Drug therapy, time from onset of symptoms, ECG, enzyme changes and clinical status were all recorded. VF was confirmed by analysis of 24 h monitoring tapes. Mean plasma potassium in the 21 patients with VF who had measurements prior to their arrhythmia was 3.49 +/- 0.54 mmol/l, lower than that of the 1165 patients without VF (mean K 3.88 +/- 0.57 mmol/l, p < 0.05). Plasma potassium concentrations in the 17 patients with myocardial infarction and VF were lower (mean 3.58 +/- 0.41 mmol/l) than in those without VF (n = 417, mean 3.89 +/- 0.61 mmol/l) (p < 0.05). Mean serum magnesium in the 12 patients with VF, measured prior to their arrhythmia (all with myocardial infarction) was 0.80 +/- 0.07 mmol/l, which was not different from the mean for patients without VF (n = 781, 0.82 +/- 0.09 mmol/l) or from the mean of 0.81 +/- 0.08 mmol/l for those with infarction but not VF (n = 331). Low plasma potassium concentrations are associated with increased risk of ventricular fibrillation, but low serum magnesium concentrations are not.