In a functional heart, the unstressed original length is difficult to measure, therefore end-diastolic length is most often used (Urheim, 2000). Strain is a dimensionless quantity, therefore the deformation of the material is generally expressed as a percentage (%). Strain rate (SR) is the rate at which deformation changes, i.e. change of strain per unit of time (dt).
This derivative of strain describes the rate of shortening or lengthening of the heart. Strain rate describes the velocity of deformation, therefore the measurement unit is (1/s).Rather than measuring instantaneous local velocities we take Lo and L in the myocardial wall, the spatial gradient of velocities in a predefined length (d) is determined by the Strain Rate of that region. Thus, instead of measuring just the velocities (Lo and L) at the ends of offset distance d, the strain rate is calculated from the slope of the regression line of all velocities along that distance (d) (Fleming, 1994). It is the temporal derivative of strain and describes the rate of shortening or lengthening of an object or a part of the heart. As strain rate describes the speed of deformation, its measurement unit is (1/s). Myocardial fibers shorten in the longitudinal and circumferential directions and thicken in the radial direction.
Therefore, shortening is indicated by negative SR and lengthening by positive SR values. SR measures the time course of deformation, and is the primary parameter of deformation derived from tissue doppler. Strain is an analog of regional ejection fraction while strain rate seems to measure the change in pressure (Weidemeann, 2002).
The measurement of peak values in strain and strain rate curves is often used in data analysis. Therefore the sample rate of interrogation is determined by the Frame Rate setting, which defines the temporal resolution of the technique. The number of scan lines is another important parameter and regulates the lateral resolution of the measurements (Hoskins, 2003). In order to have a reasonable line density in each frame, the field of view width, the frame rate or the depth of penetration must be justified and as low as possible.Longitudinal function, as exhibited by Doppler strain and SR data can be measured in every segment of the ventricular wall. Shortening is indicated by negative strain or strain rate, so that systolic strain or strain rate curve is expected to be negative in the longitudinal direction (Marwick, 2006). The Radial function can be estimated from the posterior wall in parasternal long or short axis views, or thickened in the systolic phase of the cardiac cycle, therefore the strain and strain rate curve is usuallty positive. Estimation of radial LV deformation parameters at the septum is limited, as myocardium contains both right and left cardiac fibres (Marwick, 2006).
The quality of images needed for strain analysis should be above average. In terms of Doppler strain, the noise incorporation, reverberation artifacts, dropouts, aliasing, lateral resolution limitations, insufficient tracking and inadequate frame rate constitute the main challenges of the technique (Santos, 2001).