Use of the standard 12-lead ECG to simulate electrode displacements

Placement of the precordial electrodes for recording a 12-lead electrocardiogram (ECG) is subject to variation. Previous research has shown that displacement, especially in the longitudinal direction, can lead to changes in diagnosis. In practice, both the displacement and the effects of displacement on an individual ECG are unknown. To assess this effect for a given ECG, the authors developed a method to simulate ECGs at different displacements using only the recorded ECG. The material consisted of 746 body surface potential maps (BSPMs) containing 232 cases without abnormalities, 277 with myocardial infarction (MI), and 237 with left ventricular hypertrophy. By interpolating BSPMs, ECGs from closely spaced electrode positions could be derived. Taking electrode positioning errors that may be encountered in practice, 40 ECGs at different electrode displacements (displaced ECGs) for each BSPM were derived. Using half of the BSPMs, for each displacement, a transformation matrix that transforms the ECG at the standard 12-lead electrode positions (standard ECG) to the displaced ECG was determined. Using the other half of the BSPMs, each displaced ECG was compared with the ECG yielded by the corresponding transformation matrix (transformed ECG). For each comparison, the differences were assessed between the two sets of ECG signals and between the diagnostic computer classifications of the two sets. Signal differences were expressed as mean absolute amplitude differences over the QRS. Computer interpretation of MI and left ventricular hypertrophy was graded in five levels of certainty (no, consider, possible, probable, definite). For instance, for the largest longitudinal displacement studied of about one intercostal space, the 96th percentile mean absolute amplitude difference over the test set was 204 microV. The percentage of cases showing a change in MI classification of more than two certainty levels was 2.7% for this displacement. When comparing the standard ECG with the displaced ECG, these figures were 434 microV and 8.3%, respectively. It is concluded that ECGs from displaced electrodes can be well simulated by transforming the standard ECG, both for the ECG signal and diagnostic classifications.