Modelling a parasystolic rhythm in a heart-transplant patient

A parasystole from a heart-transplant patient is analysed using a beat-to-b eat RR interval time series obtained from an electrocardiogram (ECG). The d ysrhythmia, resulting from the co-existence of two pacemakers, the sinus no de and an ectopic focus, presents distinctive regular patterns, with transi tions from one pattern to another occurring abruptly. It is shown that the parasystolic rhythm can be simulated by a model involving two oscillators f iring 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 th e generated RR time series is essentially determined by the ratio of the pe riods of the two oscillators. In the case of a heart-transplant patient wit h a small heart-rate variability as a result of heart denervation, the mode l predicts the RR intervals with an error of less than 6% for an 80-beat se quence. From a physiological point of view, the results imply that the inte raction between the two pacemakers in the heart is fairly weak, and hence t he parasystole observed in the heart-transplant patient can be modelled as pure parasystole.