Assessment of left ventricular (LV) wall motion traditionally has been performed through visual inspection of endocardial thickening and excursion in two-dimensional (2D) images. This subjective method has a high degree of interobserver and intraobserver variability among experienced cardiologists and anesthesiologists.1,2 Over the past few years, the introduction of the concepts of strain and strain rate and the development of new technologies have allowed for the objective quantification of myocardial deformation during the cardiac cycle. These techniques are grouped under the name myocardial deformation imaging and comprise tissue Doppler strain (TDS) and speckle tracking (ST) using either two-dimensional (2DST) or three-dimensional (3DST) echocardiography.
Strain is a measure for the change in length of an object and is calculated as:
is strain, L0
is the baseline length, and L1
is the final length of the object. An object that lengthens (L1
) will have positive strain; an object that shortens (L1
) will have negative strain. Strain rate is the rate of change in length over time and is calculated as:
where SR is strain rate, ε
is strain, and Δt
is the time from baseline to final length.
Myocardial strain can be measured via tissue Doppler strain and 2D speckle tracking in the radial (transmural), circumferential, and longitudinal directions, with myocardial fibers shortening in the longitudinal and circumferential directions and thickening in the radial direction. 3D speckle tracking can measure radial, circumferential, and longitudinal strain, as well as area strain, which is a measure of the relative reduction in left ventricular area during systole and incorporates both longitudinal and circumferential strain changes. Strain can be measured in specific myocardial regions as regional strain, and the average strain of the left or right ventricle can also be measured as global strain (Table 24-1). Global strain is representative of the overall function of the left ventricle, whereas regional strain provides information on discrete myocardial regional function. Regional strain is displayed according to each operating platform using a 16- to 18-segment cardiac anatomical model. Longitudinal strain shows a base-to-apex gradient, with higher strains measured at the level of the apex. Radial strain rates are commonly twice the longitudinal values, and higher strain rates are detected in the subendocardial layers as opposed to the epicardium. Strain rates are distributed more homogeneously from apex to base.3
Table 24–1.Strain definitions and measurements ||Download (.pdf) Table 24–1. Strain definitions and measurements
|Strain (ε) ||Measure of change in myocardial length [ε = (L1 – L0) / L0] |
|2-Dimensional strain ||Longitudinal, circumferential, and radial strain |
|3-Dimensional strain ||Longitudinal, circumferential, radial, and area strain |