"QT clock" to improve detection of QT prolongation in long QT syndrome patients

BACKGROUND The QT interval is a risk marker for cardiac events such as torsades de pointes. However, QT measurements obtained from a 12-lead ECG during clinic hours may not capture the full extent of a patient's daily QT range. OBJECTIVE The purpose of this study was to evaluate the utility of 24-hour Holter ECG recording in patients with long QT syndrome (LQTS) to identify dynamic changes in the heart rate-corrected QT interval and to investigate methods of visualizing the resulting datasets. METHODS Beat-to-beat QTc (Bazett) intervals were automatically measured across 24-hour Holter recordings from 202 LQTS type 1, 89 type 2, and 14 type 3 genotyped patients and a reference group of 200 healthy individuals. We measured the percentage of beats with QTc greater than the gender-specific threshold (QTc >= 470 ms in women and QTc >= 450 ms in men). The percentage of beats with QTc prolongation was determined across the 24-hour recordings. RESULTS Based on the median percentage of heartbeats per patient with QTc prolongation, LOTS type 1 patients showed more frequent QTc prolongation during the day (similar to 3 PM) than they did at night (similar to 3 AM): 97% vs 48%, P similar to 10(-4) for men, and 68% vs 30%, P similar to 10(-5) for women. LQTS type 2 patients showed less frequent QTc prolongation during the day compared to nighttime: 87% vs 100%, P similar to 10(-4) for men, and 62% vs 1000/o, P similar to 10(-3) for women. CONCLUSION In patients with genotype-positive LOTS, significant differences exist in the degree of daytime and nocturnal QTc prolongation. Holter monitoring using the "QT clock" concept may provide an easy, fast, and accurate method for assessing the true personalized burden of QTc prolongation.