Measurements of Transmembrane Potential and Magnetic Field at the Apex of the Heart

We studied the transmembrane potential and magnetic fields from electrical activity at the apex of the isolated rabbit heart experimentally using optical mapping and superconducting quantum interference device microscopy, and theoretically using monodomain and bidomain models. The cardiac apex has a complex spiral fiber architecture that plays an important role in the development and propagation of action currents during stimulation at the apex. This spiral fiber orientation contains both radial electric currents that contribute to the electrocardiogram and electrically silent circular currents that cannot be detected by the electrocardiogram but are detectable by their magnetic field, Bz. In our experiments, the transmembrane potential, Vm, was first measured optically and then Bz was measured with a superconducting quantum interference device microscope. Based on a simple model of the spiral structure of the apex, Vm was expected to exhibit circular wave front patterns and Bz to reflect the circular component of the action currents. Although the circular Vm wave fronts were detected, the Bz maps were not as simple as expected. However, we observed a pattern consistent with a tilted axis for the apex spiral fiber geometry. We were able to simulate similar patterns in both a monodomain model of a tilted stack of rings of dipole current and a bidomain model of a tilted stack of spiraled cardiac tissue that was stimulated at the apex. The fact that the spatial pattern of the magnetic data was more complex than the simple circles observed for Vm suggests that the magnetic data contain information that cannot be found electrically.