In Eq. 1, P(t) is the instantaneous ventricular pressure, V(t) is the instantaneous ventricular volume and V₀ is the rest volume at zero pressure. In the left upper panel is plotted the ventricular loop on the pressure-volume plane. ESPVR (EDPVR) is the End Systolic (Diastolic) Pressure Volume Relationship. Ved (Ves) is the end diastolic (systolic) ventricular volume and SV is the stroke volume.

Equation 2 is based on the hypothesis tht the relation between the ventricular volume and pressure is linear. The ventricular elastace E(t) changes with the contractility state of the myocardium.

In this first ventricular numerical model implemented into CARDIOSIM^© software simulator, the ventricular contraction and ejection phases was simulated using the time-varying elastance model as reported in Eq. 3. The ventricular filling phase was described by the following Eq 5 (see the next slide). In the pressure volume (P-V) ventricular plain Eq. 4 reproduces the ventricular loop and the ESPVR. The ESPVR slope (togheter V₀) representa an index of the ventricular contractility.

Equation 5 reproduces the ventricular filling phase with the EDPVR. Changing the value of the constants (C, D, E, k and j) it is possible to obtain different EDPVR lines that intersect the volume axis in different V₀ points.

This simple model of the atrial activity is described as a linear capacity characterised by a constat value of compliance and unstressed volume, i.e. the contractile activity of the atrium was neglected.