Deformation of the myocardium on application of a physical stress is termed myocardial strain. It is measured as the change in distance between two points (L) divided by the initial length (L0).
Myocardial strain: [L – L0)] / L0
Strain rate measured as the rate of deformation and the integral of strain rate over time give the strain. Strain rate has high resolution for evaluation of regional myocardial function and is an early indicator of regional myocardial dysfunction. In strain assessment, tissue Doppler technique is used to determine tissue velocities at two adjacent points along with the relative distance between the points. The instantaneous rate of change in the two velocities divided by the instantaneous distance between the two points is the strain rate. Positive values for strain rate indicate active myocardial contraction and negative strain rate indicates relaxation. Strain rate has a unit of sec-1 while strain is unitless. Strain and strain rate subtracts movement due to tethering effect of adjacent myocardium and hence represents the true regional function. Since they are deformation per unit length, they are also normalized for heart size and hence useful in children with different heart sizes.