Amount of distance which a predefined region of the heart moves during a predefined time interval. Typically, the displacement of the annular plane (either the mitral or the tricuspid) during systole in the longitudinal direction (towards the transducer). Value is expressed in mm or cm.
The (maximum) speed of a predefined region of the heart. Typically, the maximal speed is measured at the annular plane (either the mitral or the tricuspid) in the longitudinal direction (towards the transducer). The maximal velocity during systole is termed S`. During diastole, both the early relaxation (E`) and late atrial contraction (A`) peak velocities can be measured. Value is expressed in cm/sec.
The amount of deformation of a predefined region of the myocardium in respect to its original length. The original length is not measured, but expressed as the 0% point, typically set at the onset of the QRS complex, when ventricular contraction starts. The amount of deformation is expressed as %. During systole, the myocardium deforms due to myocardial cell shortening. This deformation reaches its peak at the time of the aortic valve closure of shortly afterwards. In the longitudinal and the circumferential direction, the myocardium shortens, thus is represented by a negative percentage value. In the radial (or transverse) direction, the myocardium “thickens” and is therefore expressed by a positive percentage value. During diastole, the myocardium deforms back to its original length/thickness and should have a value of 0% at the beginning of the next cardiac cycle. Deformation can be measured/derived form a pre-specified region of interest (typically a segment according to the ASE/EAE). This calculates regional strain. When the average of all segments is used, the global strain is measured. Typically, the global longitudinal strain (GLS) of the left ventricle is measured.
During the cardiac cycle, different “events” on the strain graph can be identified and are shown in the figure.
Finally, the following definitions should be known:
- Onset of strain: the time the myocardium starts with systolic deformation (e.g. longitudinal shortening). This is measured from the onset of the QRS complex on the ECG to the onset of negative deflection (see figure)
- Peak systolic strain: The maximal/minimal value of strain occurring between the onset of the QRS complex and the time of aortic valve closure
- Peak strain: The maximal/minimal value of strain in the cardiac cycle (this could therefore be during systole, but also after the aortic valve closure
- Post systolic strain / PSI: The amount of additional shortening occurring after the aortic valve closure (i.e. peak strain minus peak systolic strain). From this the post systolic index (PSI) can be calculated: PSI = (post-systolic strain / peak strain) *100 and is expressed as %
- Time to peak strain: Defined as the time in msec from the onset of the QRS complex on the ECG to the peak strain value.
Strain-rate is the first derivative of strain and indicates the speed at which deformation is occurring. When there is longitudinal shortening (during diastole), the strain- rate is negative, when there is longitudinal lengthening / elongation (during both the early and late filling phase during diastole), the strain-rate is positive. When there is no deformation occurring, the strain-rate value is zero. The steepness of the strain curve indicates the speed at which deformation is occurring, the faster this is taking place, the higher (or lower) the strain-rate value is. This is further explained in figure.
Strain rate is expressed as s-1. The graph generally shows multiple peaks during one cycle due to the several cardiac events occurring and due to artefacts, since is generally more susceptible to noise. This consequently implies 1) that the signal to noise ratio is higher (and absolute values are less reliable) and 2) the strain rate graph is more difficult to interpret. Nevertheless, several parameters / events can be clearly identified in most patients and are shown in the figure
- Peak systolic strain rate, or SR-S. The maximal value (negative for longitudinal and circumferential SR and positive for radial/transverse SR) between the onset of the QRS complex and the AVC. This parameter has been found to correlate to regional myocardial function and is less influenced by loading conditions (in other words, it might reflect contractility better than strain values).
- Peak early (active) diastolic strain rate, or SR-E: The maximal value after the AVC and before the P-wave on the ECG. Often, multiple peaks can be identified due to the complex relaxation sequence of the LV.
- peak late (passive) diastolic strain rate, or SR-A: The maximal value after the P-wave on the ECG and the AVO. Due to active contraction of the atrium, blood is pumped into the ventricle resulting in passive stretching of the LV myocardium. As a consequence, the peak SR-A is occurring almost simultaneously in all LV segments (with a base to apex gradient)
Torsion / Twist
Rotation: During systole there is an apical counterclockwise systolic rotation (which can be measured at the LV apical short-axis cross section) and there is a clockwise systolic rotation of the
LV basal short-axis cross section level. This rotation is expressed in degrees (°). The peak systolic rotation can be measured as well as the peak diastolic rotation. The rotation rate expresses the velocity of (counter)clockwise rotation during either systole or diastole (see figure). This is expressed in °/s
Twist: Peak difference in systolic rotations of LV apex and base as viewed from the apex during the systolic period. LV untwist (°) Difference in diastolic reverse rotations of LV apex and base as viewed from the apex, measured as percentage of untwist from aortic valve closure to mitral valve opening
LV torsion: This is the normalized twist; the twist angle divided by the distance between the measured locations of base and apex. This value is expressed as °/cm. Since this distance cannot be exactly measured on ultrasound , I encourage the use of LV-twist. An exception would be the calculation of torsion from a full volume 3D dataset