A cause of falsely high noise level in signal averaged electrocardiogram recordings

Signal averaged electrocardiogram (SAECG) is a well-established noninvasive method of exploration in patients at risk for sudden cardiac death. The time-domain SAECG analysis has a set of well-defined standards, including the value of accepted noise level. In very rare instances, the final noise level appears to remain unacceptably high even after carefully preparing the skin of the patient and averaging a great number of QRS complexes. We encountered three such cases in patients who had a SAECG done for a 40 Hz high-pass filter, which showed a high noise level in contrast with the visual impression of a good quality of the recording.