The<unk>  is the<unk>  between early (E) and late (atrial - A) ventricular filling velocity.
The early (E) diastolic wave is caused by accumulation of blood in the atria during previous systole, and second, a late one created by atrial contraction (A).
In a young and compliant heart, early ventricular filling accounts for ~80% of ventricular end diastolic volume (with atrial systole pushing the last ~20% of blood into the ventricle). Thus, the 'E' component of the<unk>  is greater than the 'A' component. In an aging, less compliant heart, a greater proportion of this blood is pushed into the ventricles during atrial systole. In this scenario, the emphasis of ventricular filling during late diastole increases the 'A' component of the<unk>  causing a reversal of the<unk> .
The reversal of the<unk>  is widely accepted as a clinical marker of diastolic heart failure.
It can be estimated by doppler echocardiography. 
In diastolic dysfunction, a greater portion of end-diastolic volume results from late filling rather than early filling. Therefore, the<unk>  is reduced in diastolic dysfunction. 
The late phase is dependent upon atrial contraction and is therefore absent in patients with atrial fibrillation, making the<unk>  very large.
The<unk>  is a first generation test for diastolic performance of the heart.
Diastolic relaxation is divided into four distinct phases during the cardiac cycle:
In short, there are a number of factors that influence ventricular filling during each of these phases, but remember that the main factor is the driving gradient between the atrial and ventricular pressure.
The<unk>  is measured by placing a pulse wave doppler across the mitral valve, and measuring the velocities across the valve. Hence the other names for the test - transmitral velocity profile or transmitral doppler waveforms.
Pulse wave doppler allows measurement of velocities at a specific point, but has the disadvantage of aliasing, so often has to be adjusted (baseline shifted etc.) to best fit the individual point of measurement.
IVRT is measured as the time between the closure of the aortic valve and the opening of the mitral valve.
The normal transmitral flow profile has two peaks - an E and an A wave.
The E peak arises due to early diastolic filling. Most filling (70-75%) of the ventricle occurs during this phase.
The A peak arises due to atrial contraction, forcing approximately 20-25% of stroke volume into the ventricle.
The deceleration time (DT) is the time taken from the maximum E point to baseline. Normally in adults it is less than 220 milliseconds.
Below is a transthoracic image - for those with TEE/TOE experience, simply invert the image mentally, but the concept is the same.
On the left is a heart with normal diastolic function, and on the right is a heart of impaired relaxation (note the different height of the E and A waves). Note the DT is prolonged - another hallmark of impaired relaxation.
Note too the timing according to the ECG - the waves are being measured prior to the commencement of the QRS complex (the start of systole). The A wave corresponds to the mechanical action of the electrical P wave on the ECG.
E/a ratio
Polarization density
QRS complex
RR interval
ST interval
PR interval
V wave
P wave