The design of accurate 3D heart function simulations constitutes a big challenge and an extremely difficult task for computer scientists and cardiologists because of the enormous computational power and calculation time involved in this process. An integrated heart model encompasses about 1.000.000 cardiac cells, which hardly can be simulated using the currently available systems. A group of researchers, consisting of representatives from the Signal Processing Laboratory at the Swiss Federal Institute of Technology (EPFL), Medtronic Europe, Oasya Company and the Division of Cardiology at Lausanne University Hospital has therefore limited the virtual reconstruction of the human heart to the atria. As a result, the Swiss model includes some 250.000 cardiac cells.
In contrast with the powerfully pumping ventricles, the blood receiving atria are constituted of thin walls and only have a limited surface. This enabled the team to build an anatomical model, based on a 2D heterogeneous and anisotropic model of cardiac tissue, which subsequently could be integrated into a 3D structure of one layer of cells with a total area of 100 cm². In addition, the scientists have introduced holes in order to simulate veins and valves. The model has been implemented in C++ on a Silicon Graphics Onyx supercomputer, equipped with 8 R10.000 processors, each having an internal frequency of 195 MHz.
Atrial fibrillation symptoms are generated artificially via electrical activation by introducing an intracellular current into the model's cells. The software provides a colour-coded video, showing the electrical activation in the tissue at a speed of 5 frames per second. The normal heartbeat is characterised by the simultaneous contraction of the cells in the atria followed by those in the ventricles. Fibrillation occurs whenever spiral-like wavefronts of an anarchic excitation appear in the cardiac tissue and start to travel randomly. In this case, the heart rapidly loses its efficiency as a pumping muscle. Whereas the symptoms of ventricular fibrillation cause sudden cardiac death, atrial forms of arrhythmia most often lead to heart failure and stroke.
The Swiss 3D cardiac model is utilised especially to focus on the detection of atrial flutter, easily recognisable because of its typical periodic pattern. The research team is able to analyse in detail the temporal and spatial evolution of the excitation as well as to record the activation potential in specific cells. It is also possible to generate an electrocardiogram (ECG) which corresponds to the global activation of the cardiac tissue. Future work will consist in the simulation of therapeutic treatments, including defibrillation via an electric shock to the heart, and ablation which involves the creation of a zone of non-conducting tissue to block any onset of arrhythmia.
You can find all technical details about the cardiac computer simulation at the Web site of the Swiss Federal Institute of Technology. Further news on the use of supercomputers for heart function simulation is available in the January 2000 VMW article Tera Computer's multithreaded architecture.