Theoretical analysis of the relationship between the ratio of ventricular systolic elastance to diastolic stiffness and stroke volume.

The maintenance of adequate blood circulation requires a sufficient ventricular contractility; in addition, to eject blood, the ventricles must first receive a sufficient volume, requiring a low diastolic stiffness. A simplified cardiovascular model was used to derive formulae for stroke volume (SV) as a function of atrial pressure and the ratio of ventricular end-systolic elastance to end-diastolic stiffness. A more complex cardiovascular model was used to assess the ability of the expressions to predict stroke volume under various steady-state conditions. The predicted SV correlated linearly with the model SV over a wide range of diastolic stiffnesses and systolic elastance. The formulae predict that with fixed right atrial pressure the SV is proportional to the ratio of end-systolic elastance to end-diastolic stiffness (GR) for the right ventricle, but relatively insensitive to the ratio (GL) for the left ventricle provided that GL is greater than GR. Model simulations confirmed this. When the right atrial pressure was not fixed increases in GR with fixed GL reduced the right atrial pressure with little change in SV. Similarly, varying GL with fixed GR produced little change in SV. The ratios highlight the importance of diastole to cardiac function.