  Spiral wave propagation in period-2 excitable media is often accompanied by
line-defects, the locus of points with period-1 oscillations. Here we
investigate spiral line-defects in cardiac tissue where period-2 behavior has a
known arrhythmogenic role. We find that the number of line defects, which is
constrained to be an odd integer, is three for a freely rotating spiral, with
and without meander, but one for a spiral anchored around a fixed
heterogeneity. We interpret analytically this finding using a simple theory
where spiral wave unstable modes with different numbers of line-defects
correspond to quantized solutions of a Helmholtz equation. Furthermore, the
slow inward rotation of spiral line-defects is described in different regimes.
