High-resolution SQUID imaging of octupolar currents in anisotropic cardiac tissue

A monopolar stimulus electrode triggered depolarization in a 1-mm-thick slice of canine ventricular myocardium, maintained in a thermally regulated, oxygenated chamber. Five hundred milliseconds of data at 2 kHz were recorded with a MicroSQUID (superconducting quantum interference device) magnetometer at 1-mm intervals over a 23-mm*23-mm area, centered on the stimulus site. Without averaging, a signal-to-noise ratio of better than 100:1 was achieved, and the field maps provided evidence of a propagating wavefront of activity. Application of an inverse Fourier filter yielded current density images that consisted of four expanding circular current paths, in agreement with predictions of the bidomain model. The ability to image action currents in a DC to 2-kHz bandwidth should prove useful for understanding the complex anisotropy of cardiac tissue and how it is altered by pharmacological interventions.<>