To determine the effect of the ST segment length on the fast Fourier transform (FFT), changes in the energy distribution between 0 to 50 Hz were computed at 10-Hz intervals from the energy spectrum and FFT magnitude during progressive shortening of the ST segment. Results demonstrated that the proportion of low-frequency and high-frequency components in the energy spectrum changed markedly precluding the reliable assessment of patients having short ST segments. The shift in the frequency distribution of the FFT magnitude was much less pronounced. Based on the observed changes in the frequency distribution of the FFT magnitude, a ratio that relates the relative contribution of frequencies between 20 to 50 Hz to those between 10 to 50 Hz was calculated, tested and found to be an excellent discriminator of patients with and without ventricular tachycardia independent of ST segment length.