The left ventricle is not homogenous and is composed of 3 myocardial layers. Until recently, magnetic resonance imaging has been the only noninvasive technique for detailed evaluation of the left ventricular (LV) wall. The aim of this study was to analyze strain in 3 myocardial layers using speckle-tracking echocardiography.
Twenty normal subjects and 21 patients with LV dysfunction underwent echocardiography. Short-axis (for circumferential) and apical (for longitudinal strain) views were analyzed using modified speckle-tracking software enabling the analysis of strain in 3 myocardial layers.
In normal subjects, longitudinal and circumferential strain was highest in the endocardium and lowest in the epicardium. Longitudinal endocardial and mid layer strain was highest in the apex and lowest in the base. Epicardial longitudinal strain was homogenous over the left ventricle. Circumferential 3-layer strain was highest in the apex and lowest in the base. In patients with LV dysfunction, strain was lower, with late diastolic or double peak.
Three-layer analysis of circumferential and longitudinal strain using speckle-tracking imaging can be performed on a clinical basis and may become an important method for the assessment of real-time, quantitative global and regional LV function.
The left ventricular (LV) wall is not homogenous and is composed of 3 layers of fibers. Opposite orientation of the myocardial fibers in the subendocardial and subepicardial layers is important for the equal redistribution of stress and strain in the heart. During morphogenesis, the heart undergoes structural changes. The myocardial wall matures from a single-layered epithelium to a complex, multilayered structure. Successive contraction and relaxation of the ventricular myocardial band produces several fundamental movements of the left ventricle: narrowing, shortening (with torsion or twisting), lengthening (with untorsion or untwisting) and widening, and thickening and thinning.
Nonhomogenous deformation of the basal, mid, and apical ventricular segments provides coordinated LV contraction. The long axis of the left ventricle is directed from the apex to the base; the radial axis is perpendicular to the epicardium, away from the cavity; and the circumferential axis is perpendicular to the radial and longitudinal directions and is oriented counterclockwise around the short-axis image. The base and apex of the left ventricle rotate in opposite directions: the apex rotates counterclockwise throughout systole, and the base rotates first counterclockwise and then clockwise later in systole. The myofiber helix angle changes from a right-handed to a left-handed helix from +60° at the subendocardium to −75° at the subepicardium.
Until recently, magnetic resonance imaging (MRI) has been the only noninvasive technique for the detailed evaluation of 3-layer myocardial structure and function. Recently, two-dimensional (2D) strain has become a viable alternative for the evaluation of myocardial rotation, torsion, and radial and longitudinal strain. Adamu et al were the first to measure 3-layer circumferential strain using advanced 2-dimensional strain (EchoPAC; GE Healthcare, Haifa, Israel) in normal subjects and in patients with coronary artery disease. In the current study, using the same software package, we analyzed not only circumferential but also longitudinal strain in 3 myocardial layers. A new, original 3-level × 3-layer × 6-segment color-coded map of the left ventricle specially designed for the presentation of a large volume of complicated 3-dimensional data was applied.
Twenty of 22 subjects with normal LV function and no risk factors for coronary disease (mean age, 36 years; range, 21-70 years; mean ejection fraction, 60%) and 21of 25 patients with regional LV dysfunction due to ischemic heart disease (mean age, 63 years; range, 40-93 years; mean ejection fraction, 38%; range, 25%-55%) were studied using a Vivid 7 system (GE Vingmed Ultrasound AS, Horten, Norway) and a Vivid I system (GE Healthcare, Haifa, Israel). Images were stored in digital format for offline analysis. Two normal subjects and 4 patients with LV dysfunction were studied but not included because of inadequate image quality. In the group of patients with ischemic regional LV dysfunction, 10 had anterior, 10 inferoposterior, and 1 posterolateral wall motion abnormalities. The frame rate was ≥40 frames/s. Short-axis views at the basal, mid (papillary muscle level), and apical levels and apical 4-chamber, 3-chamber, and 2-chamber views were analyzed using the EchoPAC software package. Peak systolic myocardial strain was obtained in 3 myocardial layers from short-axis views (circumferential strain) and from apical views (longitudinal strain). The left ventricle was divided into 18 cardiac segments: 3 levels (basal, mid, and apical), each further divided into 6 segments (anterior, posterior, lateral, inferior, septal, and anteroseptal). Longitudinal and circumferential strains were calculated in normal segments (healthy subjects), abnormally contracting segments of the patients, and tethered segments (visually normal segments of the patients with wall motion abnormalities).
Modified 2-Dimensional Strain Software
The traditional 2D strain speckle-tracking method of EchoPAC has been described in detail. Briefly, the initial contour of the endocardial border (a single chain of nodes) is delineated semimanually at an end-systolic frame and is tracked with the myocardial wall frame by frame for all frames automatically. This 2D strain technique is based on the assumption of the transmural uniformity of displacements across the myocardial wall. The new, modified 2D strain speckle tracking starts, similarly to traditional 2D strain, by delineating the endocardial border, but instead of a single chain of nodes, the myocardial wall is automatically defined with multiple chains of nodes, allowing investigation of 3 myocardial layers: endocardial, mid, and epicardial. Recently, this modified speckle-tracking imaging was successfully applied in the assessment of 3-layer circumferential strain in normal subjects and in patients with coronary artery disease.
All data are presented as mean ± SD. Student’s t test was used. P values < .05 were considered statistically significant.
Among 378 normal segments, 369 apical long-axis and 357 short-axis segments were successfully analyzed using modified speckle-tracking imaging. Among 378 cardiac segments with wall motion abnormalities, 365 long-axis and 359 short-axis segments were successfully analyzed. Wall thickness in abnormally contracting segments varied from 6 to 11 mm. Intraobserver and interobserver variability ranged up to 5%. Heart rates ranged from 42 to 100 beats/min (mean, 71 beats/min).
Color-Coded Map of the Left Ventricle
For the presentation of the large volume of complicated data obtained during the analysis, we designed an original 3-level × 3-layer × 6-segment color-coded map of the left ventricle ( Figures 1 and 2 ). The left ventricle is represented as a set of 3 rings corresponding to the 3 levels of the LV: basal (the biggest, outer ring), mid (the middle ring), and apical (the inner, central ring). Each of these rings is further divided into 3 layers: epicardium (the outer layer), mid, and endocardium (the inner layer) and 6 segments: anterior, lateral, posterior, inferior, septal, and anteroseptal. Strain value of each segment is colored according the color scale.
Longitudinal Strain in Normal Subjects
Longitudinal strain in normal subjects is depicted in Table 1 and Figures 1 A and 3 A . At the basal level, there was a trend toward an epicardial-to-endocardial gradient: endocardial strain was significantly higher than epicardial strain ( P = .03), but there was no significant difference between adjacent layers, mostly because of the lack of an epicardial-to-endocardial gradient in inferobasal and basal septal segments ( Figure 1 A). At the mid and apical levels, endocardial strain was higher than at the mid, and midlayer strain was higher than epicardial strain. An epicardial-to-endocardial gradient ( Figures 1 A and 3 A) existed at each level and was more prominent toward the apex. Endocardial and mid layer strain at the apical level was significantly higher than at the mid ventricle and at the mid ventricle was higher than at the base (a basal-to-apical gradient). Epicardial strain was homogenous over the left ventricle, without a significant difference between apical, mid, and basal segments. As illustrated on the color-coded map ( Figure 1 A) longitudinal strain was lowest at the basal septal and posterior segments and highest at the apex.
|Base||−20.3 ± 4.8||NS||−19.8 ± 3.2||NS||−19.2 ± 3.2|
|Mid ventricle||−23.5 ± 3.6||<.0001||−21.3 ± 3.1||<.0001||−19.4 ± 3.0|
|Apex||−33.5 ± 5.3||<.0001||−25.0 ± 4.5||<.0001||−18.9 ± 3.9|
Circumferential Strain in Normal Subjects
Circumferential strain in normal subjects is depicted in Table 2 and Figures 2 A and 3 B to 3 D. At each myocardial level, endocardial strain was highest and epicardial strain was lowest (an epicardial-to-endocardial gradient). At all myocardial layers, apical strain was highest and basal strain was lowest (a basal-to-apical gradient). Circumferential strain was lowest at the basal segments of the inferior, posterior, and lateral wall and highest at the apex. As illustrated on the color-coded map, in normal subjects, circumferential strain was higher than longitudinal strain at all levels and layers ( Figures 1 A and 2 A).
|Base||−32.4 ± 8.4||<.0001||−22.2 ± 8.0||<.0001||−15.6 ± 6.7|
|Mid ventricle||−35.5 ± 9.1||<.0001||−26.8 ± 6.1||<.0001||−19.9 ± 6.4|
|Apex||−44.2 ± 11.2||<.0001||−34.0 ± 4.9||3 × 10 −14||−26.1 ± 7.6|
Longitudinal Strain in Patients With Regional LV Dysfunction
Longitudinal strain in patients with regional LV dysfunction is depicted in Table 3 and Figures 1 B and 4 A . In abnormally contracting hypokinetic and akinetic segments, longitudinal strain was significantly lower than in corresponding intact (normally contracting) segments at each myocardial level (basal, mid, and apical) in all 3 layers.
|Abnormal||−10 ± 5.6||NS||−9.6 ± 5.3||NS||−9.3 ± 5.2|
|Intact||−14.4 ± 7.7||NS||−13.4 ± 7.8||NS||−9.3 ± 7.3|
|Abnormal||−8.6 ± 6.8||NS||−7.9 ± 5.8||NS||−7.1 ± 5.5|
|Intact||−17.5 ± 5.6||NS||−15.8 ± 4.6||NS||−14.3 ± 4.5|
|Abnormal||−43.2 ± 5.3||<.0001||−33.0 ± 4.5||<.00001||−25.2 ± 3.9|
|Intact||−28.1 ± 6.8||<.00001||−20.1 ± 5.1||<.00001||−14.5 ± 4.3|