Quantification of differences in frequency content of signal-averaged electrocardiograms in patients with compared to those without sustained ventricular tachycardia.

To quantify differences in the frequency content of signal-averaged electrocardiograms between patients with and without sustained ventricular tachycardia (VT), the energy spectra of the terminal QRS and ST segments of signal-averaged orthogonal ECGs were computed in 3 groups of patients by squaring the magnitude of the fast-Fourier transformed data. The terminal 40 ms of the QRS complex and ST segment were analyzed as a single unit to enhance frequency resolution. Group I comprised 23 patients with documented, remote myocardial infarction who had manifested subsequent episodes of sustained VT; group II comprised 53 patients with previous, remote infarction without subsequent sustained VT; and group III comprised 11 normal subjects. The terminal QRS and ST segments from patients with sustained VT contained a 10- to 100-fold greater proportion of components in the 20- to 50-Hz range compared with corresponding electrocardiographic segments in patients without VT. There were no significant differences in the peak frequencies among patient groups. However, the relative contribution of the magnitudes of these peak frequencies to the overall maximum magnitude of the spectral plot differed significantly (p less than 0.0001). No frequencies above 50 Hz contributed substantially to the energy spectra of the terminal QRS and ST segments in any group. Thus, differences in the energy spectra do not result from differences in the frequencies of components, but are attributable instead to differences in the amplitudes of components within a relatively narrow range of frequencies. The quantitative approach developed should provide objective indexes for assessing effects of antiarrhythmic interventions on abnormalities recognizable by frequency-domain analysis and improve noninvasive definition of risk for development of sustained VT.