Uncertainty principle of signal-averaged electrocardiography

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 noi se levels of 0.3+/-0.1 and 0.5+/-0.2 mu V. For the 0.5- and 0.3-mu V noise recordings, QRS duration (QRSd) was 101.2+/-11.3 and 104.6+/-9.6 ms, respec tively (P<0.0001), and the differences in paired QRSd (Delta QRSd) were nor mally 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; Delta QRSd variance was 3.3 and 217.9 ms(2) (P<0.0001). In group 3, greate r than or equal to 10 SAECGs were acquired at noise levels of 0.2 to 0.8 mu V, in 0.1-mu V increments. QRSd increased as noise level decreased. The va riance was greater in low-noise (0.2 to 0.4 mu V) versus higher-noise (0.5 to 0.8 mu V) recordings. In group 4, SAECGs were analyzed with bidirectiona l and Bispec filters, with no difference in QRSd between the 2 filters and a normally distributed Delta QRSd. A computer simulation demonstrated that alterations in the phase relationship of noise to signal results in a norma l distribution of signal end points. Conclusions-Within the acceptable noise range for SAECG, lower noise result s in longer QRSd and larger variance, suggesting that more accurate recordi ngs may have less reproducibility. The random timing of noise relative to s ignal results in the distribution/variance of repeated measurements. Statis tical strategies may be used to reduce some of this variance and may enhanc e the diagnostic utility of SAECG.