We analyse time series of meandering spiral waves in a two-dimensional
model of excitable tissue. Time series of the spiral tip studied are
quasiperiodic and erratic. In the case of erratic motion an intrinsic
non-stationarity of the tip position is present due to the possibility
of arbitrary large excursions. The correlation dimension D-2 and corr
elation entropy K-2 are then not well defined. However, low-dimensiona
l behaviour is found in the spiral tip velocity for the erratic series
. Non-stationarity of the tip position can be explained from chaotic b
ehaviour of the tip velocity.