Uncertainty Principle of Signal-Averaged Electrocardiography

Abstract

Background —Signal-averaged ECG (SAECG) reproducibility is reported to have a component that is independent of residual noise. Methods and Results —In group 1, multiple paired SAECGs were obtained to noise levels of 0.3±0.1 and 0.5±0.2 μV. For the 0.5- and 0.3-μV noise recordings, QRS duration (QRSd) was 101.2±11.3 and 104.6±9.6 ms, respectively ( P <0.0001), and the differences in paired QRSd (ΔQRSd) were normally distributed, with variances of 11.4 and 26.2 ms 2 ( P <0.0001). Paired SAECGs were obtained in group 2 patients without and with late potentials; ΔQRSd variance was 3.3 and 217.9 ms 2 ( P <0.0001). In group 3, ≥10 SAECGs were acquired at noise levels of 0.2 to 0.8 μV, in 0.1-μV increments. QRSd increased as noise level decreased. The variance was greater in low-noise (0.2 to 0.4 μV) versus higher-noise (0.5 to 0.8 μV) recordings. In group 4, SAECGs were analyzed with bidirectional and Bispec filters, with no difference in QRSd between the 2 filters and a normally distributed ΔQRSd. A computer simulation demonstrated that alterations in the phase relationship of noise to signal results in a normal distribution of signal end points. Conclusions —Within the acceptable noise range for SAECG, lower noise results in longer QRSd and larger variance, suggesting that more accurate recordings may have less reproducibility. The random timing of noise relative to signal results in the distribution/variance of repeated measurements. Statistical strategies may be used to reduce some of this variance and may enhance the diagnostic utility of SAECG.