Concealed conduction effects in the atrium

Here, we present new results obtained from a 1-D model of the atrium, with both the sinoatrial (SA) and atrioventricular (AV) nodes included. This model is able to reproduce forward and backward propagation between the SA and AV node. These nodes were approximated by a 1-D chain of diffusively coupled, modified relaxation oscillators. The atrial muscle was modeled using a chain of modified FitzHugh-Nagumo (FHN) equations. The FHN model captures the key features of excitable media and is widely used as a simple model of cardiac muscle electrical activity. The complete model consists of three segments: the SA node (15 elements), the atrial muscle (90 elements), and the AV node (15 elements) coupled diffusively at the interfaces and was solved numerically using the Euler method. The model is dimensionless, but the parameters were set in such a way that the period of the oscillations was numerically of the order of the length of RR intervals in human heart rate variability recordings. The model is able to reproduce the low-pass filtering properties of the SA node. The interspike intervals (ISIs) of the calculated action potentials of the AV node of our model were compared, with RR intervals obtained from two selected 24-h Holter recordings of patients of the Institute of Cardiology at Warszawa: one recorded in a 11-year-old girl with hypertrophic cardiomyopathy and a second one recorded in a patient with a possible SA block and no ventricular arrhythmia.