Diastole is the portion of the cardiac cycle that spans from isovolumic ventricular relaxation to the completion of antegrade mitral flow.
There are four distinct phases of diastole:
Isovolumic ventricular relaxation: An active adenosine triphosphate (ATP) requiring process that occurs from end-systole until left ventricular pressure falls below left arterial pressure leading to mitral valve opening.
Rapid early ventricular filling: Blood flows from left atrium into the left ventricle during continued active, then passive left ventricular relaxation.
Diastasis: A ventricular relaxation is completed and near equilibration of left atrial and left ventricular pressures occurs with resultant slow left atrial filling from pulmonary venous flow Atrial systole: Increased transmitral pressure gradient from atrial contraction, results in acceleration of blood flow from left atrium to left ventricle.
Traditionally, evaluation of spectral Doppler patterns of mitral inflow has been used to assess left ventricular diastolic function.80 This approach assumes that transmitral flow velocity is an accurate surrogate for volumetric flow. The two major components of normal transmitral flow: the rapid early filling phase, designated the E-wave , and filling associated with atrial contraction, designated the A-wave.
General classification of diastolic function is based predominantly on the pattern of mitral inflow as determined by the relative heights of the E- and A-waves (E:A ratio), their peak velocities and the rate of deceleration of the E-wave as shown in Table 3 in the following chapter.80
Impaired relaxation: The Doppler pattern of impaired relaxation is characterized by E- to A-wave reversal (peak A-wave velocity greater than peak E-wave velocity or E:A <1) and prolongation of E-wave deceleration time more than 200ms. This pattern may be seen more commonly in elderly patients and is not necessarily accompanied by pathophysiological changes, but it generally suggests early abnormalities of diastolic
function if detected in patients less than 60years.80 This pattern occurs because as left ventricular relaxation becomes impaired or left ventricular compliance decreases and left atrial pressure has not become abnormally elevated, there is greater impedance to blood flow from left atrium to left ventricle manifested as a diminution in peak E-wave velocity and a slowing of deceleration.80
Pseudonormal mitral valve inflow: If elevated intracardiac filling pressures are superimposed upon impaired left ventricular relaxation, the Doppler pattern of mitral inflow can appear normal, with an E:A ratio greater than 1 and decreased E-wave deceleration time. This occurs because increased left atrial pressure re-establishes a higher gradient between the left atrium and the left ventricle, providing a larger pressure head to dive left ventricular filling in early diastole. The result is a higher peak E-wave velocity and more rapid filling (decreased E-wave deceleration time). This has been designated as grade 2 diastolic dysfunction.
Restrictive mitral inflow: Further progression of diastolic dysfunction and rise in filling pressures, can lead to the left ventricular filling becoming restrictive with an increase in peak E-velocity (owing to a higher transmitral gradient resulting from increased left atrial pressure), marked shortening of the E-wave deceleration time (owing to rapid equilibration of left atrial and left ventricular diastolic pressures in the noncompliant left ventricle ) and a diminutive A-wave (owing in part to high left ventricle diastolic pressures and co-existent atrial systolic dysfunction). The result is a tall thin E-wave and
a small A-wave with the bulk of the left ventricular filling occurring over a very brief period in early diastole. This pattern has been designated grade 3 diastolic dysfunction (if the pattern is reversible) or grade 4 (if the pattern is irreversible).80
Isovolumic relaxation time (IVRT): The isovolumic relaxation of the left ventricle is the time interval between aortic valve closure to mitral valve opening and the start of
transmitral flow. This is influenced by the rate of left ventricular relaxation and left atrial pressure. Excessive prolongation of IVRT is associated with impaired relaxation, whereas abnormal shortening of IVRT is associated with elevation of left atrial pressure.
Pulmonary vein Doppler flow: There are typically three components of pulmonary venous flow. The S-wave occurs during ventricular systole. Forward flow from the pulmonary veins to the left atrium is driven by atrial relaxation and apical descent of the mitral annulus during ventricular systole. The D-wave occurs during ventricular diastole.
Diastolic flow is largely passive and follows mitral inflow from left atrium to left ventricle. Ar-wave, atrial reversal occurs as the force of atrial contraction forces a small amount of blood retrograde from the left atrium to the pulmonary vein. In adulthood, the normal pattern is S>D.80
Tissue Doppler Imaging(TDI): TDI enables the measurement of the high amplitude low velocity signals of myocardial motion , rather than blood flow velocities as with standard Doppler interrogation. A cardiac cycle is represented by three waveforms. The Sa
systolic myocardial velocity above the baseline, the Ea early diastolic myocardial relaxation velocity below the baseline, Aa myocardial velocity associated with atrial
contraction below the baseline. The subscripts ‘a’ for annulus or ‘m’ for myocardial (Ea
or Em) or the superscript ‘prime’ (E’) are used to differentiate tissue Doppler velocities from the corresponding standard Doppler blood flow velocities.