Effect of channel exclusion for the automatic measurement of QT dispersion in multichannel magnetocardiograms

Recent studies have indicated that multichannel magnetocardiograms (MCGs), which non-invasively measure cardiac magnetic field strength from many sites above the body surface, may provide independent information to ECGs about QT dispersion. In this study QT dispersion measurement was investigated in 61-channel MCGs and 12-lead ECGs, recorded simultaneously using the Bochum multichannel SQUID system from 20 healthy volunteers. Two automatic methods for QT interval measurement were used to determine T wave end. QT dispersion, expressed as the QT interval range, was calculated across the 12 ECG leads and 61 MCG channels for each subject. MCG channels were then systematically excluded in random groups of 10 and QT dispersion was calculated for 51, 41, 31, 21 and 11 channels respectively. Dispersion in the 61 channel MCG was significantly greater than 12 lead ECG by 39.1 (20.4) ms (mean (SD)) (p<0.00001), across techniques and all subjects. Significant differences of 34.7 (18.7) ms (p<0.00001), 29.6 (18.8) ms (p<0.00001), 25.5 (21.3) ms (p<0.0002) and 14.9 (21.0) ms (p<0.001) were also obtained for the 51 to 21 channel data respectively. No significant differences were obtained for 11-channel data. Automatic MCG dispersion measurements were significantly greater than dispersion from ECGs even when the number of MCG channels were reduced to 21 channels. However, small numbers of MCG channels need to be selected appropriately. The results suggest that a limited number of MCG channels is of value for repolarisation dispersion