The main finding from this study was an increased basal rotation (Rotmax-B) as well as twist (Twistmax) that compensates for the reduction in LV longitudinal and circumferential deformation in patients with severe AS, thus allowing the ventricle to maintain LVEF. The counter-coiled helix, which is composed of subepicardial and subendocardial fibers, generates an LV twist that has been proven to be fundamental to LV systole and, therefore, EF (22,23). The direction of LV twist is governed by the larger radial fibers at the subepicardium. Several previous studies have reported good correlation between LV twist derived from 2D–speckle-tracking echocardiography and magnetic resonance imaging (24,25). In our study, Rotmax-B and Twistmax in 2D images dramatically increased, a finding consistent with other reports (21,26). Possibly, subendocardial ischemia leads to a reduction of the opposing rotational forces of the subendocardial fibers, which would increase the difference in radius between the subepicardium and subendocardium. Such alterations would increase the arm of movement governed by the fibers of the subepicardium. In addition, LV hypertrophy might increase the arm force. More importantly, increased rotation and Twistmax may be compensating for the reduction of LV deformation in the other directions in patients with AS (26). All of these potential mechanisms may theoretically also lead to increased Rotmax-B.
Furthermore, twist was also significantly increased in AS study patients compared with normal values. Our results confirm that LV twist has an important role in LV ejection, which could explain why LVEF and cardiac output are preserved in patients with severe AS, but LV systolic function is impaired. A recent study by Musa et al (27) showed that transcatheter aortic valve implantation and surgical aortic valve replacement procedures were associated with comparable declines in rotational LV mechanics.
It is widely acknowledged that myocardial deformation on echocardiography can be described by 3 directions: longitudinal (LS), circumferential (CS), and radial (RS). LS denotes contraction of the longitudinally arranged endocardial fibers; CS denotes contraction of the circumferentially arranged mid-layer fibers; RS is defined as contraction of all the wall thickness (8,28). In patients with severe AS, the increasing afterload may lead to hypertrophy, decreased coronary perfusion, myocardial ischemia, and fibrosis. The endocardium is usually the most vulnerable to increased wall stress and stress-induced ischemia with LV pressure overload (29). As a result, impairment of GLS usually occurs first among other strains.
The finding of decreased GLS in 2D and 3D is consistent with other reports (4,5,30). We also found that GCS on 3D echocardiography was not significantly different when compared with 2D measurements (3). In our study, 2D echocardiography–derived GCS decreased significantly, and GRS had no significant change. Delgado et al (17) observed a significant decrease in all strain directions in 2D images. This variability in studies may be due to differences in patient populations, as well as differences in software. The cohorts in the Li et al (3) and Delgado et al (17) studies were also younger, and the sample sizes were smaller. Moreover, GRS does not represent a specific set of muscle fibers, and the variation for this parameter is always greater (31). Our results confirm that GLS is consistently impaired when compared with other parameters of deformation and is compensated by an increase in twist.
In patients with degenerative AS, arterial compliance is frequently reduced, which contributes to increased afterload and decreased LV function. Hence, the left ventricle is often subjected to a double afterload—from valvular obstruction and from the systemic arterial system (1,2,18,32). Zva, a simple index proposed by Briand et al (1), provides an estimate of the global hemodynamic load imposed on the left ventricle and is an important index of AS severity and a predictor of LV dysfunction. We found that Zva was moderately elevated in patients with severe AS (33). A study by Pagel et al (34) showed that aortic valve replacement affected only the valvular component of afterload and had no effect on arterial compliance in elderly patients, which suggests that other comorbidities, such as hypertension and atherosclerosis, may have an important impact on this parameter.
Our study showed a correlation between 2D– and 3D–GLS and Zva but no significant relationship between Zva and other deformation parameters (GRS, GCS). We also found that impaired GLS, both in 2D and 3D, correlated with increasing LVEDV and E/e¢ ratio. Sato et al (35) had similar findings; diastolic dysfunction was present in their patients, although their study patients had low-flow, low-gradient severe AS. The increase in Zva, combined with the increased LVEDV, reflected a large global hemodynamic overload. According to the Laplace law, patients with severe AS are likely to have markedly increased wall stress, which may lead to depressed myocardial contractility. Maréchaux et al (19) reported similar results, confirming that LV longitudinal contraction is, in large part, determined by LV preload and afterload.
Previous studies have reported conflicting results of comparisons of 2D– and 3D–STE measurements, possibly because of major differences in the study populations (sample sizes and the severity of AS) and methodology (ie, software) (3,15,36,37). In a study by Altman et al (16), 3D–STE was not shown to be superior to 2D–STE for any of the 3 components of LV deformation. In our study, we found a modest agreement between 2D–GLS and 3D–GLS assessed with CCC; agreement of other 2D and 3D strain parameters was poor. We also observed a similar correlation between 2D– and 3D–GLS and Zva. Theoretically, 3D–STE should be more accurate than 2D–STE because 3D–STE can overcome well-known limitations of 2D–STE: 3D images can avoid foreshortening of apical views, and 3D images are able to give a more complete picture of myocardial deformation in 3 dimensions and, therefore, eliminate, to an extent, the problem of out-of-plane motion, which may affect the accuracy of LV strain and twist measurements with 2D echocardiography. However, the lower temporal and spatial resolutions of 3D images are potential limitations and could adversely affect the accuracy of 3D–STE measurements in patients with 3D images at the lower frame rates (36,37).