Low noise level unmasks late potentials on signal-averaged electrocardiography.

INTRODUCTION: Identification of late potentials requires the reduction of random noise by signal averaging. The importance of using a very low noise level (NL) as the end point of the averaging process in patients with ventricular tachycardia, the variation of results when a lower than standard NL was used and the modification of the sensitivity of the test when a very low NL was reached were evaluated. METHODS AND
RESULTS: Signal-averaged electrocardiograms were recorded in 36 patients with ischemic heart disease and spontaneous or induced sustained or nonsustained ventricular tachycardia. Thirteen patients showed negative or indeterminate results on recordings with an NL of 0.3 μV. Eight patients (group 1) underwent a second recording with an NL of 0.1 μV. Eight normal volunteers constituted the control group (group 2). The total duration of the filtered QRS vector magnitude (QRSd), the root mean square voltage of the terminal 40 ms of the vector magnitude (RMS(40)) and the low amplitude signal duration under 40 μV in the terminal portion of the vector magnitude (LAS) modifications were evaluated. A significant difference (P<0.01) in these parameters was observed in group 1 (15.88%, 48.25% and 68.5%, respectively) when both recordings were compared. Tests were positive in all patients (100%) with NL reduction. In group 2, tests were negative in all patients (100%) at both NLs (0.3 μV and 0.1 μV). QRSd was 1.18% longer, RMS(40) was 1.38% lower and LAS was 3.55% longer with NL reduction.
CONCLUSION: Late potentials in patients with ischemic heart disease, ventricular tachycardia, and a negative or indeterminate signal-averaged electrocardiogram may be detected if the NL is reduced to 0.1 μV. Reduction of the NL increased the sensitivity of the test without modifying its specificity.