Measuring longitudinal left ventricular function and strain using cardiovascular magnetic resonance imaging

This editorial refers to ‘Fast manual long-axis strain assessment provides optimized cardiovascular event prediction following myocardial infarction’, by A. Schuster et al. pp. 1262--1270. It was Leonardo Da Vinci who first described the importance of longitudinal ventricular function in cardiac mechanics.1 In a well-known experiment, he observed peasants slaughtering pigs in Tuscany using an instrument called a ‘spillo’ when it penetrated the chest wall and heart. By noting the direction of movement of the ‘spillo’ handle, he concluded that the ventricles shorten in systole and lengthen in diastole; and that this movement in the long axis was approximately the thickness of a finger.1 Subsequently in the 1930s, Hamilton and Rompf2 made a number of fundamental observations regarding the pumping action of the heart based on experiments in frogs, turtles, and dogs. They concluded that the outer contour volume of the heart remains relatively constant throughout the cardiac cycle with the apex remaining still, while the mitral annulus moves longitudinally. This results in reciprocal filling and emptying of the ventricles and atria—such that filling of one chamber occurs at the expense of emptying of the other. These seminal observations have largely been validated by modern imaging techniques such as cardiac magnetic resonance (CMR) imaging (Figure 1). CMR imaging shows that the outer contour of the heart remains relatively constant throughout the cardiac cycle with the apex remaining still, while the mitral annulus moves longitudinally. There is reciprocal filling and emptying of the ventricles and atria—such that filling of one chamber occurs at the expense of emptying of the other. We now know that long-axis function plays a fundamental role in cardiac mechanics. Longitudinal movement of the mitral annulus is the major driver of ventricular ejection and atrial filling. Since the outer contour of the heart remains relatively constant, movement of the annulus in systole results not just in shortening of ventricular length but also increase in wall thickness due to conservation of myocardial volume.3 CMR has been used to measure the contribution of longitudinal mitral plane displacement to overall stroke volume in normal subjects, elite athletes, and patients with dilated cardiomyopathy.3 These studies suggest that as much as 60% of stroke volume maybe explained by longitudinal mitral annular motion. In diastole, the mitral annulus springs back to its equilibrium position moving around the column of blood passing through the mitral valve, thus aiding ventricular filling. Possibly because of their subendocardial location, the more longitudinal myocardial fibres seem to be exquisitely sensitive to disturbance by various pathologies, and mitral annular motion is very rapidly reduced by ischaemia in experimental models.4 Since the cardiac apex is fixed with respect to the chest wall, long-axis function was initially assessed by measuring changes in the position of the mitral annulus. These studies used M-mode echocardiography, and more recently CMR, to directly follow the position of the mitral annulus and measure mitral annular plane systolic excursion (MAPSE).5–8 However, over the last few years, enormous interest has been generated by development of echocardiographic 2D speckle tracking techniques to assess long-axis function by measuring global longitudinal strain (GLS).9 There is now a large body of literature demonstrating the diagnostic and prognostic utility of echo derived GLS in various cardiovascular disorders.9 However, these echo strain techniques are highly dependent on attainment of good quality imaging and there are significant inter-vendor measurement differences.9 More recently, development of CMR feature tracking technology allows measurement of GLS with proprietary software by recognition of ‘patterns of features’ or ‘irregularities’ in the endocardial border that are tracked and followed in successive frames. Importantly, this approach can be applied to routine cine CMR acquisitions thus avoiding the need for dedicated pulse sequences, which are required for other specialized CMR strain techniques such as tagging, HARP (Harmonic Phase), DENSE (Displacement Encoding with Stimulated Echos), and SENC (Strain encoded CMR). Feature tracking derived GLS has been shown to be an independent predictor of adverse events in many patient groups including ischaemic and non-ischaemic cardiomyopathy, hypertension, post-myocardial infarction (MI), and during stress perfusion imaging.10–13 It is in this context that in the current issue of the journal, Schuster et al.14 examine the prognostic value of a simplified measure of global long-axis strain (LAS) on CMR scans performed approximately 3 days after MI. They retrospectively measured LAS in 1235 patients (ST-elevation myocardial infarction = 760, non-ST-elevation myocardial infarction = 347) from multiple sites across Germany. LAS was defined as the change in length of a line drawn between the apex and the middle of the mitral annulus corrected to its end-diastolic length (Figure 2). The mean values from standard two- and four-chamber cine SSFP views were used. Over a follow-up of 1 year, there were 76 major adverse cardiac events (MACE) (all cause death = 34, MI = 26, heart failure hospitalization = 36). They found that LAS is independently associated with MACE incremental to clinical and imaging variables. Finally, they reported no significant difference between the incremental prognostic value of LAS compared with GLS when added to clinical and imaging predictors. Comparison of CMR methods for assessment of global longitudinal function. The authors are to be congratulated for demonstrating the prognostic value of this simple marker of longitudinal left ventricular (LV) function post-MI in a large multicentre population. However, a few points are deserving of discussion. First, given the dynamic nature of post-MI remodelling, the optimal timing for measurement of longitudinal function is unclear from this study. CMRs were performed a median of 3 days post infarct but its possible that sicker patient were scanned later which may introduce additional confounding from changes in infarct size and other variables due to remodelling. Second, infarct size has long been known to be a very powerful predictor of adverse events post-MI.15 Yet, in this study, it appears not to be associated with MACE in univariable analysis and was not included in the multivariable models. This is despite the presence of significantly larger infarct size in patients with MACE vs. those without (20% vs. 13.1%, P < 0.001). Finally, compared with GLS, LAS is faster to measure and requires no proprietary software. However, as pointed out by the authors, MAPSE is an even simpler measure of longitudinal function and has a much larger body of prognostic literature both by CMR and echocardiography.5–8 Moreover, there is strong evidence that MAPSE assessment can be simplified even further by measuring only at the lateral annulus in the four-chamber view.5–7 Lateral MAPSE has shown powerful independent prognostic value in large multicentre CMR studies of patients with reduced ejection fraction as well as hypertension, incremental to late gadolinium enhancement, and ejection fraction.6,7 It has been shown that the mitral annulus moves as a unified structure such that a similar global MAPSE decrease is seen regardless of MI location—explaining the strong prognostic value of simply measuring MAPSE at the lateral position.16 Moreover, in a recent echocardiographic study of normal subjects, Stoylen et al.17 found that normalizing LV shortening to ventricular length (as in GLS or LAS) results in a systematic bias, increasing dependency of the measurement to body size, which is not the case for MAPSE. Given these and other advantages of MAPSE (Figure 2), it remains to be seen if LAS is able to provide additive diagnostic or prognostic value beyond this simple measure. The paper by Schuster et al. and other studies of longitudinal function using CMR show that these techniques allow better identification of high-risk patients than current clinical approaches, in many different patient groups. Of particular future interest is the potential value of long-axis function for better risk stratification of sudden cardiac death in primary prevention, which currently relies exclusively on ejection fraction to decide defibrillator placement. In addition, long-axis function assessment may hold promise in early identification of patients at risk of cancer related cardiotoxicity. However, the manner in which measurement of long-axis function will affect patient care requires careful further investigation. Future studies will need to demonstrate that such imaging driven strategies improve outcomes before these techniques can be advocated and included in guideline recommendations. Conflict of interest: none declared. The opinions expressed in this article are not necessarily those of the Editors of EHJCI, the European Heart Rhythm Association or the European Society of Cardiology.