AIMS: Severe hypokalaemia can aggravate arrhythmia tendency and prognosis, but less is known about risk of mild hypokalaemia, which is a frequent finding. We examined the associations between mild hypokalaemia and ambulatory cardiac arrhythmias and their prognosis.

METHODS AND RESULTS: Subjects from the cohort of the 'Copenhagen Holter Study' (n = 671), with no history of manifest cardiovascular (CV) disease or stroke, were studied. All had laboratory tests and 48-h ambulatory electrocardiogram (ECG) recording. The median follow-up was 6.3 years. p-Potassium was inversely associated with frequency of premature ventricular complexes (PVCs) especially in combination with diuretic treatment (r = -0.22, P = 0.015). Hypokalaemia was not associated with supraventricular arrhythmias. Subjects at lowest quintile of p-potassium (mean 3.42, range 2.7-3.6 mmol/L) were defined as hypokalaemic. Cardiovascular mortality was higher in the hypokalaemic group (hazard ratio and 95% confidence intervals: 2.62 (1.11-6.18) after relevant adjustments). Hypokalaemia in combination with excessive PVC worsened the prognosis synergistically; event rates: 83 per 1000 patient-year in subjects with both abnormalities, 10 and 15 per 1000 patient-year in those with one abnormality, and 3 per 1000 patient-year in subjects with no abnormality. One variable combining hypokalaemia with excessive supraventricular arrhythmias gave similar results in univariate analysis, but not after multivariate adjustments.

CONCLUSION: In middle-aged and elderly subjects with no manifest heart disease, mild hypokalaemia is associated with increased rate of ventricular but not supraventricular arrhythmias. Hypokalaemia interacts synergistically with increased ventricular ectopy to increase the risk of adverse events.