Effects of filtering on automatic repolarisation measurements using magnetocardiography

Cardiac repolarisation can be detected by magnetocardiograms (MCGs) which non-invasively measure the variation in magnetic field strength at the body surface. The aim of this study was to assess quantitatively the influence of MCG filtering on repolarisation interval measurements. A technique for automatic analysis of ECG signals, which used modelling of the terminal T wave section to determine the end point, was extended and applied to multichannel MCG recordings of 8 normal subjects. Automatic repolarisation interval measurements were made following the addition of high and low pass filters. An experienced analyst also manually measured repolarisation intervals of the unfiltered data. The automatic technique underestimated repolarisation interval in the unfiltered data relative to manual measurement by 34.1 (8.9) ms (mean (standard deviation)). Low pass, 40Hz, filtering increased repolarisation intervals relative to unfiltered measurements by 5.1 (1.3) ms. High pass, 0.5 Hz, filtering decreased the values by 7.6 (5.0) ms.