Fourier transform versus bidirectional digital filters for late potentials recording in the electrocardiogram

Late potentials occur after the QRS complex and can be detected by using signal averaging and filtering techniques. In this study the filtered averaged electrocardiograms (ECGs) obtained using two types of filters were compared: the bidirectional digital recursive filter with a 40 Hz high-pass cut-off frequency, and the Fourier transform non-recursive digital filter with a 60 Hz high-pass cut-off. Strong correlation was found between the numerical values obtained when using these two filtering procedures. In addition, close diagnostic concordance was found between the late potential parameters in patients with normal and abnormal signal averaged ECGs. Visual comparison between the bidirectional and the Fourier plots revealed a close morphological similarity between the vector magnitudes. There was no significant difference in the mean root mean square voltage of the terminal 40 ms and the late potential duration, less than 40 mV, between the bidirectional and the Fourier filters in patients with a normal or an abnormal signal averaged ECG. The QRS duration was found to be significantly longer using Fourier filtering. This can be explained by the contributions of two factors: using the bidirectional procedure with a phase delay from each end of the data window, a shortening of the filtered QRS duration may occur and, alternatively, when using the Fourier filter, minimal stretching of the QRS complex may occur. It was also found when using the bidirectional filter that the energy from the P wave is shifted into the PR interval, which in ECG waveforms with a short PR interval may lead to an incorrect detection of QRS onset. It was concluded that both filters were functionally equivalent and that the results of the late potential parameters obtained were highly correlated.