Concealed conduction effects in the atrium.

A one-dimensional (1-D) model of the atrium together with the sinoatrial (SA) and atrioventricular (AV) nodes is presented in this article. The two nodes are each modeled by 15-element, diffusively coupled, modified van der Pol oscillator chains, while the atrium tissue is represented by a 90-element chain of diffusively coupled FitzHugh-Nagumo (FHN) equations. The modified van der Pol oscillators are able to reproduce physiologically important properties, such as the refraction period, phase-sensitivity properties, and modes of change of the action potential frequency. The activity of both branches of the autonomous nervous system may be introduced into the model in a simplified way. The model enables the study of the effect of the magnitude of the action potential conduction rate in the nodes on interspike intervals (ISIs; equivalent of RR intervals) and explains the occurrence of RR-interval alternans in certain patients. The effect of breathing modulation of heart rate and of a single deep breath can also be modeled. Finally, concealed conduction effects in the atrium are studied, yielding results comparable with recorded heart rate variability data.