The effect of aging and cardiac disease on that portion of QT interval variability that is independent of Heart Rate Variability

Our goal was to remove all readily explainable (extrinsic) portions of beat-to-beat QT interval variability (QTV) from total QTV so as to derive the purest possible measure of intrinsic QTV. For this purpose we developed a model that describes extrinsic QTV based on other factors ascertainable from the ECG, such as changes in RR intervals, voltage amplitudes and QRS-T angles. Five-min supine high-fidelity 12-lead ECGs were acquired from 101 individuals, including from 20 healthy young men (age 21plusmn1) and from three groups of middle-aged individuals (52plusmn11 years) with increasing levels of cardiac disease. By fitting our model data to the measured QT interval signal, we calculated the explained (extrinsic) part of QTV, and the difference between the measured and the calculated QT signal represented the ldquounexplainedrdquo (or intrinsic) part of QTV. Results suggest that the ratio of unexplained to explained QTV increases with both age and cardiac disease.