Cardiac cycle: ECG

Diastole is divided into four phases and comprises 2/3 of the cardiac cycle. It begins when the aortic valve closes. This is the start of Isovolumic Relaxation (volume remains constant but the pressure in the ventricles fall). The next phase of diastole occurs when the mitral valve opens and allows for rapid ventricular filling. This accounts for the majority of ventricular filling (70-75%). The third phase called diastasis occurs next describing the decrease in passive filling of the ventricles (accounts for 5% of ventricular filling). The final phase is atrial contraction. This typically contributes approximately 20-25% of ventricular filling.

The isovolumic relaxation phase can be used to access diastolic function. This is performed by calculating the instantaneous rate of decline of the LV pressure (-dP/dt) or the time constant of isovolumic decline in the LV pressure (Tau). Many different factors contribute to diastolic function. These include: magnitude of systolic volume, chamber stiffness, elastic recoil of the ventricle, diastolic interaction between the two ventricles, atrial properties and catecholamines. All of these interactions can impair the ability of the heart to fill.