  A parasystole from a heart-transplant patient is analysed using a
beat-to-beat RR interval time series obtained from an electrocardiogram (ECG).
The dysrhythmia, resulting from the co-existence of two pacemakers, the sinus
node and an ectopic focus, presents distinctive regular patterns, with
transitions from one pattern to another occurring abruptly. It is shown that
the parasystolic rhythm can be simulated by a model involving two oscillators
firing at fixed rates, under the restriction that neither is allowed to fire
during the other's refractory period. It is found that the structure of the
generated RR time series is essentially determined by the ratio of the period
of the two oscillators. In the case of a heart-transplant patient with a small
heart-rate variability as a result of heart denervation, the model predicts the
RR intervals with an error of less than 6% for an 80-beat sequence. From a
physiological point of view, the results imply that the interaction between the
two pacemakers in the heart is fairly weak, and hence the parasystole observed
in the heart-transplant patient can be modelled as a pure parasystole.
