Measurement of QT interval and duration of the QRS complex at different ECG sampling rates

The precise measurement of QT interval and QRS complex duration is important to characterize the electrical cardiac activity of the surface ECG. Variations of these intervals were evaluated when ECG sampling rate changes. The ECG recordings used in this study (n=78) were obtained from a combination of pharmacological blockage and postural changes. The ECG recordings were originally sampled at 500 Hz, and then were down-sampled by intervals of 25 Hz until the minimum sampling rate of 75 Hz. In all cases the down-sampling was implemented using cubic spline interpolation. The QRS complex duration, RR and QT intervals for each subject, condition and sampling rate were measured. The QT interval was corrected using two different techniques: Bazzett and individualized QT corrections. The effect of the sampling rate was modeled with an exponential decay function, which was used later to measure where the exponential reaches an asymptotic value (at 5tau). The 5tau value for uncorrected QT was at 290 Hz, corrected QT with Bazzett was at 303 Hz, and corrected QT with Individual method was at 253 Hz, finally QRS complex duration was at 297 Hz. An overestimation of the QT interval and QRS complex duration was observed when decreasing the sampling rate below 300 Hz