Comparison of frequency and time domain analysis of the signal-averaged electrocardiogram in patients with ventricular tachycardia and coronary artery disease: Methodologic validation and clinical relevance

Abstract

Frequency analysis with fast Fourier transform and time domain analysis after signal averaging of the electrocardiogram (ECG) have given contradictory results in patients with sustained ventricular tachycardia after myocardial infarction. Therefore, the same orthogonal ECGs were analyzed in the frequency domain (Blackman-Harris window) and the time domain after signal averaging and high gain, low noise amplification (0 to 300 Hz) in 30 patients with sustained ventricular tachycardia after myocardial infarction, 15 patients without ventricular tachycardia after infarction and 15 healthy subjects. Patients with bundle branch block were not excluded.Twenty-one of the 30 patients with ventricular tachycardia had late potentials in the time domain and abnormal Fourier transform of the ST segment (defined as increased spectral area of 60 to 120 Hz and spectral peaks >10 dB). Among the remaining nine patients with ventricular tachy cardia all had no late potentials in the time domain and one manifested abnormal frequency spectra. In contrast, of the 15 patients without ventricular tachycardia after infarction, 2 had late potentials in the time domain and only 1 demonstrated abnormal frequency spectra; none of the healthy subjects manifested either phenomenon. Patients with bundle branch block were correctly classified by Fourier analysis, but were frequently missed by time domain analysis.Normalization of the spectra and area ratios proved potential pitfalls, and the choice of an appropriate ST segment was crucial: if the segment was long with respect to the duration of the late potentials and if it extended too far into the QRS complex, fast Fourier transform yielded random results. The optimal segment proved to be one that began 20 ms before the end of the QRS complex and had a duration of 120 ms.Thus, fast Fourier transform of the surface ECG—if applied appropriately—is a sensitive and specific method for detecting delayed ventricular activation in patients with ventricular tachycardia and coronary artery disease when this delay extends beyond the end of the QRS complex.