COMPUTATIONAL EVALUATION OF INTRAVENTRICULAR PRESSURE-GRADIENTS BASEDON A FLUID-STRUCTURE APPROACH

The dynamics of intraventricular blood flow, i.e. its rapid evolution, implies the rise of intraventricular pressure gradients (IPGs) charac teristic of the inertia-driven events as experimentally observed by Pa sipoularides (1987, 1990) and by Falsetti et al. (1986). The IPG time course is determined by the wall contraction which, in turn, depends o n the load applied namely the intraventricular pressure which is the s um of the aortic pressure (i,e., the systemic net response) and the IP G. Hence the IPGs account, at least in part, for the wall movement. Th ese considerations suggest the necessity of a comprehensive analysis o f the ventricular mechanics involving both ventricular wall mechanics and intraventricular fluid dynamics as each domain determines the boun dary conditions of the other. This paper presents a computational appr oach to ventricular ejection mechanics based on a fluid-structure inte raction calculation for the evaluation of the IPG timecourse. An axisy mmetric model of the left ventricle is utilized The intraventricular f luid is assumed to be Newtonian. The ventricle wall is thin and is com posed of two sets of counter-rotating fibres which behave according to the modified version of Wong's sarcomere model proposed by Montevecch i and Pietrabissa and Pietrabissa et al. (1987, 1991). The frill Navie r-Stokes equations describing the fluid domain are solved using Galerk in's weighted residual approach in conjunction with finite element app roximation (FIDAP). The wall displacement is solved using the multipla ne quasi-Newton method proposed by Buzzi Ferraris and Tronconi (1985). The interaction procedure is performed by means of an external macro which compares the flow fields and the wall displacement and appropria tely modifies the boundary conditions to reach the simultaneous and co ngruous convergence of the two problems. The results refer to a simula tion of the ventricular ejection with a heart rate of 72 bpm. In this phase the ventricle ejects 61 cm(3) (ejection fraction equal to 54 per cent) and the ventricular pressure varies from 78 mmHg to 140 mmHg. Th e LPG show an oscillating behaviour with two major peaks at the beginn ing (11.09 mmHg) and at the end (4.32 mmHg) of the ejection phase, whe n the flow rate hardly changes, according to the experimental data. Fu rthermore the wall displacement, the wall stress and strain, the press ure and velocity fields are calculated and reported.