The origin and characterization of the primary signal, noise, and interference sources in the high frequency electrocardiogram

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Abstract
An analysis technique was developed that enabled accurate measurement of the spectral and spatial characteristics of the predominant signal, noise, and interference components in high frequency electrocardiograms. A selective sampling technique, utilizing the isoelectric interval during the T-P segments, enabled the charactetistics of the internally generated noise and external interference to be obtained independently of the heart signal. Using this sampling technique, it was found that the electrical activity of skeletal muscles was the major source of noise in the high frequency electrocardiogram. The amounts of skdetal muscle noise present in the high frequency electrocardiograms of resting patients were measured quantitatively through the use of anesthetics and muscle relaxant drugs. In the normal resting state, the electrical activity of skeletal muscles contributed up to 15 dB of random noise to the spectra of waveforms generated by the electrical activity of the heart. Moderate, voluntary contraction of small groups of skeletal muscles increased their electrical output by 15 to 20 dB. Spectral analysis of high frequency electrocardiograms indicated that a dynamic range of approximately 66 dB is requized to adequately measure the signal and noise components present. Power spectra of high frequency electrocardiograms indicated that the 60-Hz power lines were the major source of interference Harmonics of the 60-Hz power line added significant interference components at frequencies as high as 1 kHz. The magnitude-squared coherence function between pairs of electrocardiographic leads, in conjunction with segment sampling and analysis techniques, was used to measure the spatial coherence of waveforms from electrical source associated with heart and skeletal muscle activity. The high values of spatial coherence of heart signal waveforms indicated that the body spatial transfer functions for electrical sources associated with heart activity acted as linear, noiseless transmission channels. The low spatial coherence of the waveforms generated by the electrical activity of skeletal muscles indicated that the body spatial transfer functions for electrical sources associated with skeletal muscle activity acted as noisy (nondeterministic) transmission channels. The spatial coherence of isoelectric interval waveforms was inversely proportional to both the distance between measurement sites and the amount of skeletal muscle activity.