Validity of the uniform double layer in the solution of the ECG forward problem

This study attempted to solve the electrocardiographic forward problem (ie, to predict body surface potential fields from pathways of depolarization). A perfused heart was suspended in a cylinder and surrounded with fluid of the conductivity of the lung. Computation of potential fields included conductivity and boundary effects and assumed a uniform double-layer source. Three instants (early, late, and mid QRS) during normal depolarization and one instant during a stimulated beat were studied. There was good qualitative agreement between recorded and predicted fields early and late in the QRS and during the stimulated beat, but there were quantitative differences. The cardiac generator was far stronger late in the QRS. In mid QRS, the agreement between recorded and predicted fields was very poor. To resolve questions raised in this study, potentials around a small volume of stimulated tissue were recorded and predicted. Recorded potentials were compared with potentials calculated (1) on the assumption that the cardiac generator is a uniform double layer and (2) on the assumption that all current flows along the long axes of the cardiac fibers. The recorded potentials compared most favorably with the second of these. These studies led to the opinion that the uniform double-layer assumption is inadequate for prediction of body surface potential fields from depolarization pathways. Other studies of the forward and inverse problem that assume a uniform double-layer source are considered to be successful and therefore disagree with this study's conclusion. It appears that the question needs further experimental study.