Compliance mismatch between the elastic natural aorta and the nonelastic artificial graft is considered as a possible reason.

Computer simulation, an in-vitro study and animal experiments were performed to investigate the changes of pressure load caused by stiffened seg\-ments at the aortic root. In the numerical simulation, a lumped-parameter model of the whole circulatory system was used, in which the aorta was simulated by 10 segments of varying elasticity. In the in-vitro model, an electrically driven piston pump was co upled to porcine aortae from the slaughterhouse (n=3). Pressure- and flow-monitoring at different locations and ultrasound cross section measurement were applied in native and stiffened conditions. Finally, the observed phenomenons were validated in anim al experiments in mini-pigs (n=4). In the simulation, a compliance reduction of the ascending aorta (to 1/10 of natural compliance) resulted in an increase of systolic-diastolic pressure relationship in the aortic root at unchanged cardiac output (e.g. Psys-Pdia from 45.3 to 57.4 mmHg at an CO of 4.53 to 4.51l/min.) and in pressure oscillations after valve closure at frequencies 10-20 Hz at an ampli\- tude of about 10 mmHg). These simulations were proven in the in-vitro and in vivo (statistical analysis under the way). We conclude that non-compliant prostheses at the ascending aorta cause high frequency pressure oscillations and correlated strain in the aortic root, which may be an important cause for reiterating aneurisms.