Variability maps of body surface ECG in normal subjects

A method for quantifying the fluctuations of the QRS electrocardiogram over the flat (unrolled) thoracic surface in normal subjects is presented. Serial comparisons were performed on 32-lead ECG recordings and body surface maps from seven healthy men, using as similarity measures correlation and RMS-difference in both time-signal and 2D map domain. Recordings were made on one and the same day, and on different days. The time-signal correlation and RMS-differences were plotted as 2D distributions. Correlation was higher on the front of the torso and RMS-differences were largest in the precordial area. The average time-signal correlation calculated over the QRS in comparisons of signals taken on different days was 0.9717, whereas the corresponding RMS-difference was 57.8 microV. Additionally, 2D comparisons between reconstructed map frames at different time instants in the QRS were carried out, using the 2D versions of the correlation and RMS-difference. The averaged results of the 2D comparisons were close to the time-signal comparison values (respectively 0.9696 and 68.9 microV). Finally, 2D comparisons between isointegral maps (mapping the QRS signal integral) were performed. They yielded even higher correlations (0.98, 3.453 microV s). Although the number of subjects studied was not large, the investigation brought to light important variability ranges that may serve as a basis when detecting pathologies. In addition, the topology of normal ECG variability on the body surface was revealed. This study confirmed the pertinence of correlation and RMS-difference as measures of time-signal and map similarity.