Cardiac strain
Cardiac strain or myocardial strain describes the deformation of the cardiac wall or chamber from a relaxed to a contracted condition more precisely the alteration of length in one dimension or spatial orientation.
It can be expressed by a mathematical principle with the following formula :
ɛ = (L-L0)/L0
ɛ is strain, L0 is the baseline length and L is the length in systole
Usage
Cardiac strain can be assessed with cardiac strain imaging which can be performed with different imaging modalities, for which different acquisition methods and software algorithms exist .
Cardiac strain parameters can be used for the assessment or description of myocardial contractility on a global or segmental level.
The best-evaluated strain parameter global longitudinal strain (GLS) is considered to be more sensitive than left ventricular ejection fraction for functional assessment of the heart.
Measurement
The general workflow of cardiac strain imaging techniques includes the following steps :
- identification of end-diastole and end-systole
- definition of endocardial, epicardial and (mid-)myocardial contours/lines
- definition of the points or segment to be tracked (segmentation)
- tissue tracking and computing of the respective strain curves
The different strain values could be calculated at every time point during the cardiac cycle and for every point along the defined myocardial lines, but typically length values are determined in peak systole and early diastole and are computed on a global and/or segmental level .
Lengthening relates to positive strain values, shortening to negative strain values .
Segmental strain values are calculated by the average values of the segment. Global strain parameters can be determined by using the whole myocardial line length for calculation or averaging a number of points of the same frame .
The different cardiac strain parameters are broken down by the spatial orientation or direction of the myocardial deformation and differ according to whether they are calculated for a specific segment (segmental strain) or the whole cardiac chamber (global strain). They include the following :
- longitudinal strain
- measures chamber deformation in a tangential z-direction (base to apex) of the cardiac wall
- describes a systolic shortening of ventricular chamber length under normal conditions
- systolic strain values are negative
- can be calculated as endocardial, epicardial, myocardial/midline strain or averaged
- radial strain
- measures the deformation along the chamber wall in a radial or perpendicular direction to the wall
- describes a systolic thickening of the ventricular wall
- requires the simultaneous acquisition of endocardial and epicardial border points for strain calculation
- can be measured in all center planes
- systolic strain values are positive
- features only one and no separate endocardial or epicardial strain values
- circumferential strain
- measures chamber deformation along the circumference of the cardiac wall in a tangential xy-direction
- describes a systolic shortening of ventricular chamber circumference under normal conditions
- systolic strain values are negative
- can be calculated as endocardial, epicardial, myocardial/midline strain or averaged
- ventricular twist and torsion
- the basal myocardial segments rotate clockwise, the apical segments counterclockwise in relation to the equatorial plane
- the difference is expressed in degrees [°]
- twist refers to the difference in systolic rotation between apex and base measured along the circumference
- torsion is twist normalized by the distance between apex and base or left ventricular length
- strain rate
- is the amount of strain within a certain time of a specific orientation or dimension
- is expressed in [s]
- other parameters include displacement [mm] velocity [cm/s] and rotation rate [°/s]
Interpretation
Normal values still differ substantially between imaging modalities, acquisition methods and postprocessing algorithm . Block matching algorithms as speckle tracking echocardiography and MR feature tracking suffer from reduced reliability in the assessment segmental strain values .
Other influences include patient age on circumferential and radial strain values or longitudinal strain which shows gender differences .
Due to the above-mentioned difficulties and influences, normal reference ranges are related to modalities and acquisition methods. Additionally, they seem yet to vary between different software version and therefore software specific cut off values are recommended to be used .
A global longitudinal strain <12% has been suggested to indicate severe systolic dysfunction and a value <15-16% seems to pose a risk in patients with preserved ejection fraction .
History and etymology
Cardiac strain was first described in 1973 as the systolic deformation after stress .
See also
