1.9.3 Diastole Definition

Diastole Definition is the phase of the cardiac cycle during which the heart muscle relaxes following contraction, allowing the chambers of the heart, primarily the ventricles, to fill with blood in preparation for the next contraction. Diastole encompasses the interval from the closure of the semilunar valves to the closure of the atrioventricular valves and represents the period of lowest intraventricular pressure within the cardiac cycle, during which the myocardium undergoes active relaxation and the chambers passively and then actively accommodate incoming venous return.

Subphases of Diastole

Diastole is not a single uniform event but a sequence of distinct mechanical stages, each characterized by specific pressure and volume changes.

Isovolumetric Relaxation

Isovolumetric relaxation begins immediately after the semilunar valves close and continues until the AV valves open. During this brief interval, the ventricular myocardium relaxes and pressure falls steeply while chamber volume remains constant, since all four valves are closed.

Rapid Filling

Once ventricular pressure drops below atrial pressure, the AV valves open and blood that has accumulated in the atria rushes rapidly into the ventricles, producing the phase of rapid, largely passive ventricular filling.

Diastasis

Diastasis is a period of slow, gradual filling that occurs as atrial and ventricular pressures approach equilibrium. It is the longest subphase at normal heart rates but contributes comparatively little additional volume.

Atrial Systole

Atrial systole, sometimes described as the "atrial kick," occurs at the end of diastole when the atria contract and actively propel the remaining blood into the ventricles, completing ventricular filling just before the onset of the next ventricular systole.

Hemodynamic Characteristics

Diastole is defined by specific pressure, volume, and flow relationships that distinguish it from systole.

Ventricular Pressure

Ventricular pressure remains low throughout most of diastole, rising only modestly toward the end as atrial contraction contributes additional volume.

Ventricular Volume

Ventricular volume increases progressively throughout diastole, reaching its maximum, the end-diastolic volume, at the moment the AV valves close and systole begins.

Coronary Perfusion

Diastole is also the period during which the majority of coronary artery blood flow to the left ventricular myocardium occurs, since the compressive forces of ventricular contraction are absent, allowing coronary vessels to fill more readily.

Duration and Heart Rate Dependence

The duration of diastole is inversely related to heart rate: as heart rate increases, diastole shortens disproportionately compared to systole.

Tachycardia Effects

At elevated heart rates, the shortened diastolic period can reduce both ventricular filling time and coronary perfusion time, with important implications for cardiac output and myocardial oxygen supply.

Bradycardia Effects

At slower heart rates, diastole lengthens, allowing more complete ventricular filling and greater coronary perfusion time.

Diagrammatic Summary

Physiological Importance

Adequate diastolic function is essential for normal cardiac performance, since insufficient filling time or impaired ventricular relaxation directly limits stroke volume and cardiac output. Diastolic dysfunction, in which the ventricle fails to relax or fill normally despite preserved contractile strength, is a recognized clinical entity distinct from systolic dysfunction and underlies a significant proportion of heart failure presentations.