Abstract 2699: Improved Prediction of Cardiac Events with Assessment of Absolute QT and RR Intervals in Adolescent Patients with the Congenital Long QT Syndrome
Background: The QT interval is most commonly corrected for heart rate (QTc) by using Bazett’s formula. However, heart rate correction formulae have several limitations, particularly at fast or slow heart rates. We hypothesized that combined separate measures of both absolute QT and the RR interval provide better prognostic information for life-threatening cardiac events in long QT syndrome (LQTS) patients than standard QTc formulae.
Methods: The independent contribution of QT and RR (model 1) to the risk of aborted cardiac arrest (ACA) or LQTS-related death during adolescence was assessed in 2,772 LQTS patients enrolled in the International LQTS Registry, and was compared with the risk associated with heart rate corrected QT using the Bazett (model 2) and the Fridericia (model 3) formulae. The Akaike Information Criterion (AIC) was used to determine and compare the fit of each model, with the model with the best fit defined by having the lowest AIC.
Results: Mean (±SD) QT, RR, QTc-Bazett, and QTc-Fridericia were 453 ± 74 msec, 850 ± 200 msec, 494 ± 49 msec, and 480 ± 53, respectively. After adjustment for gender, time-dependent β-blocker therapy, and a history of prior syncope, both the absolute QT duration and the RR interval were shown to contribute independently to outcome: 10 msec increments in QT were significantly associated with a 7% increase in the risk of ACA or death (p<0.001), and 100 msec decrements in RR were associated with a further 43% increase in the risk associated with the QT interval (p<0.001). Thus, model 1, in which both QT and RR were included, was shown to have the best fit, as compared with models 2 and 3 in which QT correction formulae were incorporated (AIC=1792, 1802 and 1810, respectively
Conclusions: The prognostic information obtained from analysis of both absolute QT and RR is superior to that obtained through QT correction for heart rate. We suggest that risk assessment in LQTS patients is improved by using absolute QT and RR intervals rather than standard heart-rate correction formulae.