Technical aspects of the signal-averaged electrocardiogram
- Sanjiv M Narayan, MD, PhD
Sanjiv M Narayan, MD, PhD
- Professor of Medicine
- Stanford University School of Medicine
- Michael E Cain, MD
Michael E Cain, MD
- Vice President for Health Sciences, Dean of the Jacobs School of Medicine and Biomedical Sciences
- University at Buffalo, Buffalo
The signal-averaged electrocardiogram (SAECG) is a signal processing approach to detect subtle abnormalities in the surface electrocardiogram (ECG) that are not visible to the naked eye. The SAECG is derived by computing the arithmetic mean of multiple ECG complexes. This process emphasizes consistent signals between complexes while diminishing the more variable noise, thus increasing the signal-to-noise ratio of cardiac potentials to enable detection of small (ie, microvolt-level) signals than would otherwise be discernible on visual inspection of the surface ECG . Signals from the His bundle as well as subtle abnormalities of atrial or ventricular complexes, anomalies not visualized on a surface ECG, are detectable using the SAECG [2,3].
The SAECG has most often been used to identify low-amplitude signals at the end of the QRS complex, referred to as "ventricular late potentials". These late potentials represent delayed ventricular activation, which may reflect the presence of myocardial scar tissue and identify patients at increased risk for reentrant ventricular tachyarrhythmias . The SAECG is particularly useful in understanding the arrhythmic substrate and stratifying risk for ventricular tachyarrhythmias in patients with cardiomyopathies of various etiologies. (See "Reentry and the development of cardiac arrhythmias".)
However, the SAECG can also be useful in evaluating the risk for atrial arrhythmias. Studies suggest that a prolonged SAECG P-wave, equivalent to "atrial late potentials", may identify patients at risk for atrial fibrillation [5,6]. Finally, the SAECG can also improve the resolution of atrial from ventricular activation for improved ECG diagnosis of supraventricular tachyarrhythmias.
The technical aspects of the SAECG and the characteristics of late potentials will be reviewed here. The clinical use of the SAECG is discussed separately. (See "Use of the signal-averaged electrocardiogram in arrhythmia evaluation and management" and "Use of the signal-averaged electrocardiogram in ischemic heart disease" and "Use of the signal-averaged electrocardiogram in nonischemic heart disease and cardiac transplantation".)
The electrical signals of interest that reflect delayed conduction through ventricular or atrial scar are minute compared with the size of the QRS complex or P-wave seen on a surface ECG. Although simple amplification of the ECG may reveal these signals, it also amplifies ambient noise, which typically masks the useful low-amplitude signals. Conversely, averaging multiple QRS complexes (or P-waves) "reinforces" the consistent ventricular or atrial components while diminishing the inconsistent noise components, thus improving the signal-to-noise ratio. The ECG can be averaged over time ("temporal averaging") or spatially, and this process has been further enhanced by digital filtering and spectral analysis.
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