Interindividual variability of multilead electrocardiographic recordings: influence of heart position.

The electrocardiogram (ECG) of normal, healthy subjects shows a large interindividual variability. Part of this variability is due to the heart position and orientation relative to the electrodes. In this report, the interindividual variability is quantified using the relative variability measure, computed as the averaged standard deviation in the ECGs, scaled by the average root mean square of the ECGs. The relative variability in the QRS complex is estimated as 0.52. The heart position and orientation relative to the lead positions is documented in 25 normal subjects. The long axis angle varies considerably among the subjects (27.1+/-8.8 degrees to the transversal plane and 38 degrees +/-5 degrees to the frontal plane). Moving the electrodes in the frontal plane to a position relative to a common reference point at the base of the heart (shift: 0.8+/-0.7 cm leftward and 2.4+/-2.3 cm downward) did not reduce the interindividual variability.