The magnetic field detectable outside the chest arising from the heart is analyzed using Maxwellrsquo;s equations. It is shown to consist of two contributions: (i) the field arising from the heartrsquo;s electrical activity, commonly known as the magnetocardiogram, and (ii) a contribution occurring only in the presence of an external magnetic field arising from the motion of diamagnetic material, principally blood. The latter contribution has been studied by means of a superconducting gradiometer in model circulatory systems consisting of plastic or rubber ventricles filled and emptied by pumps and tubing, and has been shown to be readily detectable in magnetic fields comparable to that of the earth. The effect has also been measured with human subjects, suggesting the possibility of clinical application of our results.
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