Heart position and orientation in forward and inverse electrocardiography

A study has been made of the influence of the position and orientation of the heart within the thorax on computed ECG waveforms (forward model) and on computed activation sequences (inverse model) in three normal cases. Results show that differences in heart position and orientation, associated with shifts relative to the precordium of the order of 0.5 cm, may result in amplitude differences or QRS waveforms of typically tenths or millivolts, which constitute part of the observed interindividual variability of the ECG. The inverse study shows that, in spite of similar errors in estimated heart position and orientation, stable solutions of the ventricular activation sequence can still be found. However, in the case where the heart is very close to precordial leads, the stability of the inverse solution is found to be intrinsically poor.