A time-variyng elastance [elv(t)]  model describes the behavior of the left native ventricle. The mechanical  properties of the ventricle are related to the ECG signal. elv(t) is based on the electro-mechanical interaction by synnchronising the different phases of the left ventricle with the ECG signal. elv(t) is a function of the left ventricular systolic elastance Elvs, left ventricular diastolic elastance Elvd and finally, left ventricular activation function alv(t) written according to ECG schematisation. Plv(t) and Vlv(t) are the instantaneous left ventricila pressure and volume, respectively. V*lv(t) is the instantaneous left ventricular free wall volume and Vlo is the rest volume of the left ventricle. In CARDIOSIM^© the same representation is used to describe the behavior of the right native ventricle.

Ed_Vspt is the interventricular septum diastolic elastance, Es_Vspt is the interventricular septum systolic elastance and a_Vspt is the activation function described according the ECG signal. V_Vspt(t) is the instantaneous interventricular septum volume. When Plv(t) and Prv(t) are different, V_Vspt(t) becomes the volume of shift of the interventricular septum from the neutral position to the left/right ventricular lumen.

The left atrial time variyng elastance ela(t) is based on the electro-mechanical interaction by synchronising phases of the atrial cycle with ECG signal. ela(t) is function of left atrial systolic elastance Elas, left atrial diastolic elastance Elad and left atrial activation function ala(t). V*la(t) is the instantaneous left atrial free wall. P₀la is constant and V₀la is the rest atrial volume (constant).

The concept of atrial interdipendence considers the properties of one atrium to be a function of the properties of the contro-lateral one. The time-varying interatrial septum e_Aspt(t) is described as a function of interatrial septum diastolic elastance (Ed_Aspt), interatrial septum systolic elastance (Es_Aspt) and interatrial septum systolic elastance  (a_Aspt). When Pla(t) and Pra(t) are different, V_Aspt(t) becomes the volume of shift of the interatrial septum from the neutral position to the left/right atrial lumen.

The equations describing the heart numerical model are used to simulate the interaction between the two ventricular chambers and between the two atrial chambers. This numerical model representation is used to simulate the atrioventricular delay that occurs in atrioventricular block and atrial fibrillation. Also interventricular delay can be simulated.