Abstract 18326: Heart Rate-independent Qt Variability in Congenital Long Qt Syndrome
Introduction: Heart rate is a major factor affecting QT interval duration. In normal ambulatory subjects, modeling QT interval as a linear function of preceding RR intervals weighted with an exponential function with 60 s time-constant reproduces gradual QT accommodation and provides a better fit than any function of instantaneous RR interval. QT accommodation in LQTS is not well understood.
Methods: Digitized Holter recordings (24 hrs) from 63 genotyped LQTS patients (47 LQT1, 13 LQT2, 2 JLN, 1 unknown) were analyzed. LQTS-related symptoms (syncope, ventricular tachycardia, cardiac arrest, or appropriate device shock) were present in 21 patients. RR and QT intervals were automatically determined with custom software; all data were manually edited. QT was modeled as a linear function of preceding RR interval, or of the RR intervals in preceding 180 s weighted with a 30 s or 60 s time-constant exponential function. QTc was determined for each patient from the 60 s time-constant model.
Results: The 60 s time-constant model provided a better fit of data than preceding RR interval or 30 s time-constant model (p<0.001 for both; see Figure for an example). Residual variability (the QT variability not explained by the 60 s time-constant model) correlated positively with QTc (r=0.53; p<0.001) and was higher in LQT2 than in LQT1 subjects (p<0.005; Figure). There was a trend to higher residual QT variability in symptomatic subjects (p=0.058). QTc did not differ between LQT1 and LQT2 or between symptomatic and asymptomatic patients.
Conclusions: In ambulatory LQTS patients, QT interval depends on heart rate in the preceding 3 minutes and is well described by an exponential weight function with 60 s time-constant. Residual QT variability is higher in LQT2 than LQT1 and may be higher in symptomatic subjects. Additional research is required to determine if residual QT variability improves risk-stratification in LQTS.
Author Disclosures: S. Seethala: None. P. Singh: None. V. Shusterman: None. K.H. Haugaa: None. J. Nemec: None.
- © 2014 by American Heart Association, Inc.