Development of a New Analytical Method for the Electrocardiogram Using Short-Time First Fourier Transforms

Abstract

continuation point was corrected using Hanning window. It was necessary to force the signal to zero at the beginning and end of the time series to reduce the effect of leakage. Hanning window is a smoothing window, which has excellent frequency and amplitude resolution. The data processed in this manner underwent frequency analysis with the FFT computation routine. Overlap processing can be used to provide high resolution in both the frequency and time axes. We processed the data of 2 ms of the preceding segment simultaneously. These segments were calculated continuously with the FFT calculation for 5 ms at 512 points (Fig 2). The processing result was displayed in 3 dimensions: the time axis (ms), frequency axis (Hz) and strength axis ( V). Subjects and MeasurementTen healthy male volunteers (mean age, 27±1.2 years), having no abnormality in their ECG, were chosen. We used f the techniques for measuring the minute electric potential of an electrocardiogram (ECG), the time domain method is usually used because it can get a high signal to noise ratio (S/N ratio) by signal averaging. [1][2][3] However, it is difficult to evaluate the highfrequency electrical potential within the QRS complex by this method, although it is excellent for detecting the late electric potential that occurs at the end of the QRS complex. Moreover, a disadvantage of this method is that the high-frequency component decreases remarkably when the number of averagings increases due to the unstable trigger position. Although the frequency domain method was considered useful for the analysis of the structure inside the QRS complex it could not detect the change in minute electric potential because it analyzes the whole QRS complex at one time. 4-9 Therefore, short-time First Fourier Transforms (SFFT), which improves the frequency resolution by high-frequency sampling, was used to analyze the ECG of healthy individuals and we then evaluated its usefulness in detecting the change in minute electric potential. Methods PrincipleThe low-frequency-component-containing ECG was removed with a high-pass filter (Fig 1). Of the data in a segment of 160 ms, including the QRS complex, the dis- To detect the minute electric potential inside the QRS complex, a new frequency domain method was designed using short-time First Fourier Transforms (SFFT) and high-frequency sampling (oversampling). SFFT improved the frequency resolution by oversampling that was applied to this analysis. The electric potential data of 15,000 points received weighted, running average processing and was subtracted from the original waveform to reduce the low-frequency component. The data in a segment of 160 ms, including QRS, was processed by frequency analysis with the SFFT computation routine. The ECG of healthy individuals was analyzed by this method and its usefulness evaluated. The processing waves of the X-axis, Y-axis, and Z-axis of a representative normal subject were formed into 3 groups of peak electric potential. SFFT enabled the detection of the structure inside the QRS com...

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