Despite extensive experimental and computational investigation, the mechanism by which the sinoatrial node is able to drive the atrium is not completely understood. Current knowledge considers an insulating fibrous-fatty border, discrete sinoatrial exit pathways and gradients in cellular coupling (and possibly in cellular ionic properties) as key elements in determining atrial excitation by the sinoatrial node. However, it is not known as other aspects - such as cellular heterogeneity - affect this phenomenon.
In this work, we developed a 2D discrete computational model of the rabbit sinoatrial node and the surrounding atrium, with a total of 40000 cells. The simulation of such a high number of cells in reasonable times was made possible by the use of high performance computing techniques: the model was developed in CUDA/C language, which allowed the code to be executed very efficiently on parallel hardware (GPU). The geometry of the model was represented by an ellipsoidal sinoatrial node encircled by an insulating border, opened in five different locations to form exit pathways through which the electrical impulse can travel from the sinoatrial node to the atrium. A gradient in cellular coupling inside the exit pathways was necessary to obtain propagation to the atrium.
In this setting, simulations with homogeneous cells show simultaneous excitation of the atrium at the five exit pathways, with a resulting cycle length that is identical to the single cell value (355 ms). When cellular heterogeneity is considered, phase synchronization between cells is lost in the central part of the sinoatrial node. However, the atrium is still effectively stimulated, with one exit pathway delivering the stimulus earlier than the others. This results in a slightly shorter cycle length (345 ms).
In conclusion, the model shows that cellular heterogeneity can play a role in determining atrial excitation and thus the heart rate.