Noise in the signal-averaged electrocardiogram and accuracy for identification of patients with sustained monomorphic ventricular tachycardia after myocardial infarction

Prevalence of late potentials in a sample of 487 healthy, middle-aged men from southwestern France

Late potentials (LPS) have been shown to be predictive of ventricular tachycardia in coronary artery disease subjects, but the prevalence of LP in the general population is not as well-known. The study included 487 men without a history of cardiovascular disease (aged 50-59) living in Southwestern France. Standard-averaged high resolution electrocardiogram was performed using Butterworth filtering at 40-250 Hz. LPS were defined as two or more of the following criteria: QRS > 114 ms, duration of the low amplitude signals in the terminal portion of QRS > 38 ms, root mean square (RMS) voltage of the last 40 ms < 20 microV. The mean QRS duration was 97 +/- 12 ms (mean +/- SD), duration of the low amplitude signals in the terminal portion of QRS was 32 +/- 10 ms, and RMS voltage in the last 40 ms was 39 +/- 27 microV. Eight percent of subjects (95% confidence interval [CI]: 6%-11%) had a QRS duration > 114 ms; 22% (95% CI: 18%-26%) had a duration of low amplitude signals > 38 ms, and 25% (95% CI: 22%-29%) had RMS voltage in the last 40 ms < 20 microV. Finally, the prevalence of LP was 21% (95% CI: 18%-25%). In conclusion, according to commonly used criteria, the prevalence of LP in this healthy population of middle-aged men is 21%, close to the values found in the literature for myocardial infarction patients. Those findings indicate the need for reconsidering the definition of LPS.

Recovery of beat-to-beat variations of QRS

There is a growing interest in the analysis of beat-to-beat variations of the morphology (BBM) of cardiac waves in electrocardiograms (ECG). Such analyses are confronted with the low BBM-to-noise ratio. An ECG clustering technique is introduced that brings the benefits of signal averaging to BBM analysis and recovers the beat-to-beat pattern of BBM. ECG clustering aligns waves and sorts them into clusters. The precision of the alignment was enhanced by sub-sample alignment. Kohonen's self-organising neural networks identified the clusters of the cardiac waves during training. The subsequent clustering of a wave results in a label for the closest cluster, a distance to the cluster and optimal alignment. Furthermore, ECG clustering avoids base-line variations and amplitude modulation sufficiently to be applied to the QRS wave in the raw ECG. The technique is demonstrated on 14 subjects with coronary heart disease and no myocardial infarction, myocardial infarction, or inducible ventricular tachycardia. ECG clustering is a general-purpose technique for beat-to-beat analysis, where the variations are cyclic as in the sinus rhythm. Results show that beat-to-beat variations in the QRS morphology are in general cyclic, with a main period of about four cardiac cycles. All calculations were performed with the Cardio software.

Effect of parasympathetic blockade on the signal-averaged electrocardiogram

Low parasympathetic activity is associated with late potentials detected at a noise level of 0.4 microV in a signal-averaged electrocardiogram (SAECG) following myocardial infarction. In contrast, at a noise level of 0.2 microV, lowering parasympathetic activity influences late potential parameters in the opposite direction in healthy subjects. The aim of this study was to estimate the relationship between parasympathetic activity and the SAECG obtained at noise levels of 0.4 and 0.2 microV in healthy subjects. Two SAECG recordings in 10 healthy subjects were obtained at noise levels of 0.2 and 0.4 microV before and after parasympathetic blockade using atropine (1 mg). Signal-averaged QRS duration (SA-QRS), late potential duration (LPD) defined as duration of terminal signals below 40 microV, and root mean square voltage of the terminal 40 ms of the averaged QRS (RMS40) were measured. At a noise level of 0.2 microV SA-QRS reduced from 124 +/- 14 to 114 +/- 17 ms (P = 0.008), LPD from 37 +/- 10 to 28 +/- 14 ms (P = 0.01), and RMS40 increased from 26 +/- 22 to 41 +/- 25 microV (P = 0.006) during parasympathetic blockade compared to baseline values. At a noise level of 0.4 microV the SA-QRS (115 +/- 15 ms) and LPD (29 +/- 11 ms) were lower and the RMS40 (37 +/- 23 microV) was higher compared to the noise level 0.2 microV, and no systematic alterations of the three variables were found during parasympathetic blockade. The parasympathetic nervous system may induce a very low-amplitude late potential in the SAECG. The data suggest that parasympathetic activity and a low noise level may lead to a false late potential-positive SAECG in low arrhythmia risk subjects. Therefore, we recommend the use of a noise level of 0.4 microV or identification of high arrhythmia risk patients by late potential and low parasympathetic activity.