For periodic wavetrains the situation is completely different, as
the spatial period measured in terms of the variable 
(the period in the direction of the flow)
is fixed. In physical
space, if measured in the instantaneous direction of propagation,
the spatial period changes according to (6).
But in excitable media, there is always a minimal wavelength, 
at which a periodic wavetrain can propagate,
and so 
decreasing below this value is a sufficient condition for the
propagation block. From (6) we obtain
the following estimate of the time and the
angle for which the break will occur, for any value of the shear
however small:
where
