B. Murienne - Master Project Thesis - Infoscience - EPFL

Figure 12. Velocity vector field [1].
Conduction velocity vector fields describe the local speed and direction of propagation of cardiac
activity [1]. Traditionally, the direction of propagation is identified manually and the speed is
computed from ∆t between activation at two points along that direction measured with two
electrodes. However, this procedure is valid only if the electrode used is small enough to
distinguish the local activity and if the temporal resolution is good. If the wavefront is not
perpendicular to line connecting electrodes, two different sites appear to activate nearly
simultaneously as shown in Figure 13.
Figure 13. Schematic diagram of a propagating wavefront observed by two electrodes [1].
(a) The wavefront is perpendicular to the line joining the two electrodes.
(b) The wavefront intersects obliquely the line joining the two electrodes.
If the wavefront is perpendicular to the line joining both electrodes (Figure 13.a), the inter-
electrode distance divided by the difference in activation times gives a good estimation of the
propagation velocity. If the wavefront intersects obliquely the line joining both electrodes (Figure
13.b), an artificially high propagation velocity is deduced from the division.
A new method to estimate velocities of multiple wavefronts at different locations and times and
find vector fields of local conduction was developed [1]. The algorithm consists in fitting
polynomials T(x,y) to a set of “active” points in the (x,y,t)-space and estimating velocity vectors
from partial derivatives of these polynomials.
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