  We discuss the forward and inverse problems between the potential V(x)
measured in a heart chamber and its sources represented by a dipole density
d(y) located on the heart wall. We show that the mapping from d(y) to V(x) is a
compact integral operator. Its inverse is unbounded which makes the inverse
problem ill-posed in the mathematical sense. We investigate methods to solve
the inverse problem approximately in view of the mapping of complicated cardiac
arrhythmias. We point out an analogy between phase mapping and 2-dimensional
hydrodynamics.
