Session S44.1

The Effect of Aging and Cardiac Disease on that Portion of QT Interval Variability that Is Independent of Heart Rate Variability

V Starc*, TT Schlegel

University of Ljubljana
Ljubljana, Slovenia

It is known that beat-to-beat QT interval variability (QTV) is heavily influenced by the simultaneous RR interval variability (RRV). In this study, we isolated that part of QTV that can be readily explained by RRV and/or by other extrinsic factors ascertainable from the ECG, such as changes in voltage amplitude (Va) and QRS-T angle (RTa) occurring with respiration. Our goal was to remove all readily explainable portions of QTV from total QTV so as to derive the purest possible measure of intrinsic QTV.
Five-min supine high-fidelity 12-lead ECGs were acquired from 101 individuals: 20 healthy young men (age 21±1) and 27 age- and gender matched individuals in each of the following three groups: asymptomatic middle-aged persons (52±11 years); patients with known coronary artery disease (CAD); and patients with cardiomyopathy (CMP). Singular value decomposition was used to derive both RTa and Va, the latter being the squared sum of all QRS eigenvalues. The corresponding values of RR, RTa, and Va thereafter represented the signals from which we reconstructed the explained part of the QT interval signal using the following model: QT explained = QT(RR, Va, RTa, t), where t represents time. We assumed that changes in the QT interval have both a fast component that varies linearly with the RR, RTa and Va signals, and also a slow component that decays to a steady QT* value, QT* = QT0 x (RR/RR0)^j. By fitting our model data to the measured QT interval signal, we obtained the explained part of QTV (eQTV), and the difference between the measured and the calculated QT signal represented the intrinsic or “unexplained” part of QTV (uQTV). The ratio of unexplained to explained QTV was further quantified logarithmically as: log(uQTV/eQTV).
Mean ± SD values for log(uQTV/eQTV) in the first eigenvector were -0.03±0.25 for the young male group, 0.09±0.26 for the asymptomatic middle-aged group, 0.22±0.27 for the CAD group (P<0.01 vs. the young male group) and 0.25±0.25 for the CMP group (P<0.001 vs. the young male group and P<0.05 vs. the asymptomatic middle-aged group).
Results suggest that the ratio of unexplained to explained QTV increases with both age and cardiac disease.

(Abstract Control Number: 8)