The heart is a very special organ. For example, it has only a limited ability to regenerate itself and is the most important hollow muscle in the human body. Owing to its central role many researchers have also lost their hearts to this blood supply centre. Despite the high level of interest, however, it is still not clear today how exactly cardiac muscle fibres are arranged. Computer tomography and magnetic resonance give only a somewhat fuzzy picture of the inner, microscopic structure of the heart of a living person. Moreover, although tissue sections deliver a higher resolution, owing to their two-dimensionality they only allow limited conclusions to be drawn. This means that scientists have so far only had a sketchy understanding of how the mechanics of the heart function with regard to the forces that work in and on cardiovascular fibres. Peter Niederer, ETH Professor for Biomedical Engineering, has now succeeded in showing in a mathematical model that crosslinking of cardiac muscle fibres exercises an important influence on the stability of the ventricular wall. His work is published in the journal "Biomechanics and Modeling in Mechanobiology“ (1).

Model with solvable equations

For his investigation physicist Peter Niederer, started with a simplified model of the heart by representing the central organ as a thick-walled cylinder. This simplification leads to mathematical equations that are solvable. "Of course we also model the heart in a more realistic way with the help of the method of finite elements," explains the researcher. "But then it is difficult and time-consuming to draw general conclusions." In his cylinder heart, the researcher examined the influence of branching of cardiac fibres. To do this, in a first step he computed the stability of the ventricular wall if all fibres run parallel to one another. In a second step he compared these results with those that resulted when shear forces were introduced between the fibres to simulate the effect of branching. The final analysis of the results demonstrates that branching is vital as far as the stability of the heart is concerned.

The wall of the heart is a thickly woven network of cardiac fibre, although this is not easy to recognise in a cross-section of heart tissue (Picture: P. Lunkenheimer). large

Passing the pig test

Niederer is of course fully aware that a real heart is far more complex than his model. But measurings on pigs'hearts confirm that the fibre branching has precisely the effect on the stability of the ventricular wall that the ETH scientist had predicted. Niederer says, "The congruence of my model with actual measurements in animals doesn't surprise me.“ Because the fundamental mechanical characteristics of branching in a sphere, an ellipsoid or in any heart-shaped structure were comparable to those that occur in a cylinder.

No progress without understanding of the mechanics

But how does this knowledge help? Niederer points out that, in order to make progress, for example in tissue engineering, the mechanics must be understood. And tissue engineering matters in regard to the heart because, despite the sensation surrounding heart transplantations, the future lay in therapeutic methods aimed at preserving the organ. The reason for this was that there are not enough donors by far and xenotransplantation was also still a long way off. What is not such a long way off is Peter Niederer's retirement, planned for next year. He will not be developing the cylinder model any further, as it has been taken practically as far as it can be. But he looks forward to using his know-how on the mechanics of the heart to advise his colleagues and continue to collaborate with them. The researcher jokingly refers to this task as his "last scientific twitches“.