  The human heart is enclosed in the pericardial cavity. The pericardium
consists of a layered thin sac and is separated from the myocardium by a thin
film of fluid. It provides a fixture in space and frictionless sliding of the
myocardium. The influence of the pericardium is essential for predictive
mechanical simulations of the heart. However, there is no consensus on
physiologically correct and computationally tractable pericardial boundary
conditions. Here we propose to model the pericardial influence as a parallel
spring and dashpot acting in normal direction to the epicardium. Using a
four-chamber geometry, we compare a model with pericardial boundary conditions
to a model with fixated apex. The influence of pericardial stiffness is
demonstrated in a parametric study. Comparing simulation results to
measurements from cine magnetic resonance imaging reveals that adding
pericardial boundary conditions yields a better approximation with respect to
atrioventricular plane displacement, atrial filling, and overall spatial
approximation error. We demonstrate that this simple model of
pericardial-myocardial interaction can correctly predict the pumping mechanisms
of the heart as previously assessed in clinical studies. Utilizing a
pericardial model can not only provide much more realistic cardiac mechanics
simulations but also allows new insights into pericardial-myocardial
interaction which cannot be assessed in clinical measurements yet.
