Spontaneous Variability of QTc Interval in Patients with Congenital Long QT Syndrome

Background: QTc interval assessment is an essential tool to diagnose and stratify the risk of adverse cardiac events in patients with congenital long QT syndromes (LQTS). QTc intervals can markedly vary in shape and be intermittently prolonged only before, during and after an arrhythmic event. Objectives: The aim of this study was to assess whether QTc intervals measured on different occasions vary significantly in patients with congenital LQTS. Methods: QTc intervals were measured from three ECG recorded every 7 days, in 25 patients and 15 healthy controls. Results: Both groups evidenced significant QTc interval variability. In patients with LQTS, minimum and maximum QTc were 492 ms (25-75 interquartile range (IQR) 463.5-501.5) and 522 ms (IQR 503.5-543), respectively (p<0.0001). In healthy individuals, these values were 409 ms (IQR 395-418) and 423 ms (IQR 413-431) (p<0.001). In 56% of patients, QTc intervals varied above and below 500 ms, in 24%, they were always below 500 ms and in 20% they were always above this value. QTc interval was always above 500 ms in 50% of symptomatic patients and in 10.5% of asymptomatic ones. Conclusions: QTc intervals vary significantly in patients with LQTS. A single assessment is not enough to stratify risk.