  The final state interaction of pions in $K_{e4}$ decay allows one to obtain
the value of the isospin and angular momentum zero $\pi\pi$ scattering length
$a_0^0$.We take into account the electromagnetic interaction of pions and
isospin symmetry breaking effects caused by different masses of neutral and
charged pions and estimate the impact of these effects on the procedure of
scattering length extraction from $K_{e4}$ decay.
