Comparison of signal-averaged electrocardiograms with different levels of noise: time-domain, frequency-domain, and spectrotemporal analysis.

In order to test the effect of noise on the various parameters of the SAECG, 83 patients underwent three consecutive recordings at different noise levels. The high noise (HN) recordings had a noise level of 0.60-0.74 microV, the intermediate noise (IN) had 0.31-0.59 microV, and the low noise (LN) had < or = 0.30 microV. For the calculation of noise we used the standard deviation of the mean noise of the composite lead high pass filtered at 40 Hz. The recordings were compared using time-domain, frequency-domain, and spectrotemporal analysis. The time-domain parameters of the LN recordings, using 25-Hz, 40-Hz, and 60-Hz high pass cutoffs, were significantly different from those of the HN or IN recordings (P < 0.05). In the frequency-domain analysis, significant differences were found in some of the parameters of the LN compared to the HN. The spectrotemporal analysis of the X and Z leads also showed significant differences among the LN and the other recordings. In the time-domain analysis, both at 40 Hz and 25 Hz, there were more abnormal LN compared to the HN recordings (P < 0.05). In the spectrotemporal analysis, there were significantly more abnormal HN and IN recordings compared to the LN (P < 0.001 and P < 0.01, respectively). Therefore, the level of noise, even within the acceptable range, can significantly affect the SAECG. In the time domain at the lower noise levels the parameters become more abnormal, while the opposite seems to occur in the spectral and the spectrotemporal analysis.