  This work addresses the topic of knotted stable structures in excitable
media. These structures appear in a wide variety of situations, such as cardiac
fibrillation, chemical reactions, etc. Entangled curves have been found in
numerical computations of the equations that describe excitable media. They
present an unusual stability. An explanation for this behaviour has been an
open question. In the present work we introduce for the first time the meaning
of the helicity in an excitable media as a new tool to study the stability of
these systems. The helicity is related to the total entanglement of the system.
We have studied how the helicity is conserved or lost through the walls of the
medium and shown that these behaviours are dominated by the boundary
conditions, so the distortion of these conditions could lead to the
disappearance of the structures.
