P-wave signal-averaged electrocardiography
P-SAECG with P-wave triggering was performed before CA using the FCP-8800 (Fukuda Denshi, Ltd, Tokyo, Japan) with 12 standard ECG leads. First, the skin at the electrode placement site was cleaned with alcohol and scrubbed with a rubbing pad because the signals were of relatively low amplitude. Subsequently, the signals from each lead were amplified and filtered between 40 Hz and 300 Hz. A modified X, Y, and Z lead system was used to trigger signal averaging from the P-wave. The signal was passed through a P-wave recognition program to eliminate atrial extrasystole, and a signal of 200 beats with an error vector magnitude of <0.4 µV was obtained. The following two ALP parameters were evaluated: root mean square voltage during the terminal 20 ms (RMS20) and filtered P-wave duration (PWD). RMS20 and PWD data were automatically calculated by the system. Positive ALP was defined as an RMS20<2.2 µV and/or a PWD >115 ms. The standard value was based on an analysis of 120 healthy individuals.10