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Why GLS?

The advent of feature tracking technology for routinely acquired cardiovascular magnetic resonance (CMR) cine images has enhanced the ability to describe cardiac function. Nowadays parameters describing myocardial deformation are increasingly used in both clinical research and clinical practice.

We must consider that whenever we measure:

  • ejection fraction,
  • fractional shortening,
  • annulus displacement,
  • radial displacement
  • longitudinal displacement,
  • global longitudinal strain or strain rate,

we estimate the deformation of the LV. To an active myocardial wall contraction corresponds a systolic chamber deformation (chamber length shortening, left ventricle upward base displacement, radius reduction, left ventricle twisting and volume reduction).  We also have to consider that the LV deformation during active contraction and the ejection of blood are the result of interaction between contractility (developed force) and loading conditions.

In the last ten years, the measurement of Global Longitudinal Strain (GLS) has gained an increasing importance in the evaluation of LV mechanics. GLS measures the entity of ventricular longitudinal shortening. It has gained increasing consensus in cardiology thanks to its accuracy and repeatability and has become commonly used in the echocardiographic evaluation of LV systolic performance.

Why such a success of GLS?

First of all, from a technical standpoint the wall tracking technology has numerous sources of inaccuracies. They range from the through-plane motion (the tissue shown in a 2D image may vary when it moves across the slice) to the spatial resolution (tissue motion is invisible as long as it remains inside a same pixel) and temporalresolution (wide displacements between two frames are difficult to follow), to cite a few. Therefore, parameters become increasingly reliable when they represent average values where such inaccuracies are smeared out; this make “global” parameters more successful than segmental or regional values. However, due to these limitations, the reliability of the different solutions available in the CMR market can vary substantially [1].  The reliability of GLS was validated during the years cross different imaging technologies (CMR, Echocardiography, CT) showing an underlying robustness that is partly independent on imaging modalities [2,3].

Secondly, GLS does not require drastic conceptual changes to the existing picture of cardiac contraction. The time profile of a GLS curve is similar to the familiar volume curve and the percentage of endocardial border shortening, from end-diastole to end-systole, normalized for the end-diastolic length, plays a role that is analogous to that played by ejection fraction (EF) that is the normalized percentage value of volume reduction from end-diastole to end-systole. Therefore, end-systolic GLS provides additional information that well integrates on top of the EF without the need of any novel abstraction.

The third, and most important, reason for the success of GLS is that this parameter has an effective clinical value. In general, the GLS provides information about the entity of contraction, thus it is similar to EF and in most of the cases GLS correlates well with EF: if the overall contraction is reduced (EF decreases), then the contraction in the base-apex direction is reduced as well (GLS decreases, in absolute value[1]). However, in some clinical conditions, discrepancies between EF and GLS are frequently observed. For example, a preserved EF, despite a reduction of contraction along the base-apex direction, is the diagnostic connotation of pathological condition grouped under the definition of “Heart Failure with Preserved Ejection Fraction” (HFPEF). These conditions often present an increase of ventricular wall thickness associated with a slight reduction of LV volume. Here, a reduced GLS is combined with a relatively large thickening of the LV walls that, particularly in a smaller cavity, gives rise to a normal EF, but with a reduced capability to increase cardiac output during effort.  Preserved ejection fraction in heart failure does not reflect “diastolic heart failure”.

A clinical point comes to light,that EF does not measure systolic function in concentric geometry, while GLS easily detects this dysfunctional condition and can be reduced, while the EF may still be preserved in small cavities. On the contrary, in enlarged ventricles, for the same value of EF, the GLS decreases while the volume increases: in a large ventricle, a lower degree of length reduction is needed to obtain the same volume change (systolic output). Consequently, reduction of GLS with unmodified EF means negative LV remodeling.

The clinical success of GLS is that these conditions are much more frequent than the opposite (preservation of GLS and reduction of EF), therefore GLS can be provocatively claimed as a more sensitive parameter than EF is. Although, evidently, the best option is to use both information for an integrated assessment.

In conclusion, GLS is a clinical useful measure that is applicableacross different multiple imaging modalities for integration to EF. The Medis’ QStrain solution, that is based on all these fundamental and extensively validated principles, has now been made available for heart MRI, as well as for CT function.

[1]Accidentally, the end-systolic strain values were introduced since its introduction with opposite sign than EF (difference of minimum to maximum and not vice versa); therefore, a reduction of GLS must be read that it becomes less negative, or it is reduced in absolute value.

References:

[1] Palumbo P, Symons R, Barreiro‑Pérez M, Curione D, Dresselaers T, Claus P, Bogaert J. Left ventricular global myocardial strain assessment: Are CMR feature‑tracking algorithms useful in the clinical setting? La radiologia medica 2020; 125:444-450. DOI:10.1007/s1154 7-020-01159 -1

[2] Amaki M, Savino J, Ain DL, Sanz J, Pedrizzetti G, Kulkarni H, Narula J, Sengupta PP. Diagnostic Concordance of Echocardiography and CMR-based Feature Tracking for Differentiating Constrictive Pericarditis from Restrictive Cardiomyopathy. Circ Cardiovasc Imaging 2014;7:819-827. DOI: 10.1161/CIRCIMAGING.114.002103

[3] van den Hoven AT, Yilmazer S, Chelu RG, van Grootel RWJ, Minderhoud SCS, Bons LR, van Berendoncks AM, Duijnhouwer AL, Siebelink HMJ, van den Bosch AE, Budde RPJ, Roos‑Hesselink JW, Hirsch A.  Left ventricular global longitudinal strain in bicuspid aortic valve patients: head‑to‑head comparison between computed tomography, 4D flow cardiovascular magnetic resonance and speckle‑tracking echocardiography. Int J Cardiovasc Imaging 2020. DOI:10.1007/s10554-020-01883-9

Gianni Pedrizzetti, PhD
Giovanni Tonti, MD, PhD