Risk factors for prolonged QTc among US adults: Third National Health and Nutrition Examination Survey.

Background: QT interval prolongation can lead to torsades de pointes, a potentially fatal arrhythmia. Although research exists on the relationship between QT prolongation and clinical outcome, few studies have described risk factors for prolonged QT interval in the general population.
Methods: The Third National Health and Nutrition Examination Survey (NHANES III) collected electrocardiogram interval data on 8561 subjects over 40 years of age and projected results to the US population. QT was corrected for heart rate using Fridericia's formula. Logistic regression analyses were performed to identify factors independently associated with prolonged QTc interval, defined as being in the upper 5% of the population QTc interval distribution. Analyses were conducted separately for women and men as a result of differences in the QT distribution between the sexes and also because of potential effect modification. Analytical variables included age, race/ethnicity, electrolyte measurements, body mass index, the recent use of QT-prolonging drugs and past medical histories of stroke, thyroid disease, hypertension, diabetes and myocardial infarction.
Results: Age, female sex, hypocalcemia (men), hypokalemia (women), and a history of thyroid disease and myocardial infarction (men) were associated with a prolonged QTc interval. In addition, taking QT-prolonging medications in the past month was associated with more than a twofold increase in the odds of prolonged QTc interval in both men and women.
Conclusions: Healthcare practitioners should be aware that a prolonged QTc interval is a potential indicator of cardiovascular risk, and should exercise caution in prescribing potentially QT-prolonging medications to certain patients.