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Speckle tracking echocardiography to assess regional ventricular function in patients with apical hypertrophic cardiomyopathy

AIM To explore regional systolic strain of midwall and endocardial segments using speckle tracking echocardiography in patients with apical hypertrophic cardiomyopathy (HCM). METHODS We prospectively assessed 20 patients (mean age 53 ± 16 years, range: 18-81 years, 10 were male), with apical HCM. We measured global longitudinal peak systolic strain (GLPSS) in the midwall and endocardium of the left ventricle. RESULTS The diastolic thickness of the 4 apical segments was 16.25 ± 2.75 mm. All patients had a normal global systolic function with a fractional shortening of 50% ± 8%. In spite of supernormal left ventricular (LV) systolic function, midwall GLPSS was decreased in all patients, more in the apical (-7.3% ± -8.8%) than in basal segments (-15.5% ± -6.93%), while endocardial GLPPS was significantly greater and reached normal values (apical: -22.8% ± -7.8%, basal: -17.9% ± -7.5%). CONCLUSION This study shows that two-dimensional strain was decreased mainly confined to the mesocardium, while endocardium myocardial deformation was preserved in HCM and allowed to identify subclinical LV dysfunction. This transmural heterogeneity in systolic strain had not been previously described in HCM and could be explained by the distribution of myofibrillar disarray in deep myocardial areas. The clinical application of this novel finding may help further understanding of the pathophysiology of HCM.


INTRODUCTION
Hypertrophic cardiomyopathy (HCM) is a genetic disease transmitted with an autosomal dominant pattern, where by the direct relatives of affected subjects carry 50% chances of having the disease [1] . Its prevalence in the general population is 0.2% and it is frequent cause of sudden cardiac death in patients younger than age 30, including athletes. The annual mortality rate is 1%, but may be as high as 6% during childhood and adolescence; of note, sudden death may be the first symptom of dis ease [2] . It is a heterogeneous disease in its clinical as well as genetic aspects, characterized by the presence of primary left ventricular hypertrophy (LVH), with variable clinical expression and outcome [3] , and caused by genetic mutations that lead to abnormal sarcomeric proteins [4] .
In patients with complete phenotypic expression, characteristic findings are: Hypertrophy, myofibrillar dis array, interstitial fibrosis and microvascular dysfunction, all of which contribute to the progression to heart failure, ventricular arrhythmias and sudden death. Recent studies with MRI have shown that many patients with HCM have multiple areas of myocardial fibrosis, even with normal LV ejection fraction [5] . Epicardial coronary arteries in patients with HCM are usually normal, but coronary flow reserve is diminished due to narrowing of the small intramyocardial arteries [6] . This microvascular ischemia is one of the factors resulting in LV diastolic dysfunction, which in turn is the main functional consequence of this disease.
Although patients with HCM have a normal ejection fraction, studies with Doppler tissue imaging have documented a regional systolic dysfunction in the long itudinal fibers of the LV [710] .
Regional LV function may be assessed noninvasively by measuring strain or systolic deformation. Initially, strain calculated with colour tissue Doppler proved to be a useful and sensitive tool to detect early systolic function abnormalities in patients with HCM [11] . However, its clinical application proved to be hindered by the complexity of data collection and limited reproducibility.
Recently, a method derived from the twodimensional (2D) echocardiogram, called "speckle tracking" of 2D strain, has been developed to measure systolic strain [12] . The goal of this study was to assess the abnormalities of global and regional systolic LV function using 2D strain in patients with apical HCM.

Population
The study has a crosssectional design and included 20 patients with apical HCM who were being followed at our tertiary referral center. Using a retrospective methodology, 2D strain was measured in 340 myocardial segments.
The diagnostic criteria for apical HCM included demon stration of asymmetric left ventricular hypertrophy (LVH), confined predominantly to the LV apex with an apical wall thickness > 15 mm, a ratio of maximal apical to posterior wall thickness >1.5 [13] , and a "spade shape" deformity of the left ventricle with apical cavity obliteration in end systole based on 2D echocardiography.
Inclusion criteria were: HCM with apical involvement, nondilated LV, normal global and regional systolic LV function, normal blood pressure, sinus rhythm, absence of comorbidities and without history of hypertension.
Noninvasive evaluation of global and regional systolic LV function was done by calculation of LV ejection fraction and visual judgement of segmental function from 2D echocardiographic images.
Exclusion criteria were obesity, poor echocar diographic window, concomitant diseases that could cause ventricular hypertrophy or abnormal systolic or diastolic function (hypertension, diabetes, coronary heart disease, valve disease, cardiomyopathy, pericardial disease, congenital heart disease or systemic disease).
The study was approved by the Education and Research Committee and the Ethics Committee of the "Dr. Cosme Argerich" Hospital. All patients signed the informed consent form, including the authorization to use the data collected for future studies.

Echocardiographic measurements
Standard echocardiographic examinations were performed in all patients using a Vivid Seven digital ultrasound system (GE Medical System, Hotern, Norway). Cardiac cycles were stored in digital, cineloop format for offline analysis performed by two independent observers (TFC and JAL) with a dedicated software package (EchoPAC PC, version 108.1.5).
Both parasternal long and shortaxis views were analyzed. The Mmode echo was derived from the parasternal shortaxis at the papillary muscle level, and the following measurements were obtained according to the American and European Societies of Echocardiography [14] : 365 April 26, 2017|Volume 9|Issue 4| WJC|www.wjgnet.com LV enddiastolic diameter (EDD), LV endsystolic diameter (ESD), interventricular septum and posterior LV wall thickness, and endsystolic left atrial diameter. Ejection fraction was measured by Simpson method. Continuous Doppler from the apical 5chamber view was used to rule out the presence of a dynamic subaortic gradient.

Measurement of 2-D strain
2D strain is a novel nonDopplerbased method to evaluate strain from standard 2D acquisitions [15] . By tracing the endocardial contour on an endsystolic frame, the software will automatically track the contour on subsequent frames. Adequate tracking can be verified in realtime and corrected by adjusting the region of interest (ROI) or manually correcting the contour to ensure optimal tracking. A minimum frame rate of 30 Hz was required for reliable operation of this program and frame rates of 30 to 80 Hz were used for routine gray scale imaging. 2D longitudinal strains were assessed in 2 orthogonal apical views (4 3 and 2chambers, 17 segments) starting from the septal, posterior and the inferior atrioventricular wall junction, respectively. The 2D strain software adequately tracked > 85% of the attempted segments.
The ROI was reduced and shifted to the meso cardium to obtain the parametric image, which allowed quantifying strain in each of the 17 segments of the LV as a "bull's eye" ( Figure 1A and B), the mean value of peak global systolic strain and strain in the 3 apical views. Later, the ROI was shifted to the endocardium to obtain

RESULTS
The clinical and echocardiographic characteristics of patients with apical HCM are summarized in Table 1. No patient was receiving medication at the time of inclusion in this study.
All patients exhibited apical hypertrophy, (the dia stolic thickness of the 4 apical segments is described in Table 1). All patients had a normal ejection fraction (69% ± 5%).
A total of 20 patients with apical HCM were assessed and 340 myocardial segments were analyzed; midwall longitudinal peak systolic strain (LPSS) was measured and compared to endocardial LPSS (Table 2 and Figure 4). We confirmed that, in spite of a supernormal systolic LV function, midwall GLPSS exhibited a diminished percent of strain, which was more marked in the apical than in basal segments. By contrast, endocardial GLPSS was significantly higher and reached normal values.
Midwall GLPSS in the basal segments (Table 3) was lower than the endocardial GLPSS, but without significant differences (15.5% ± 6.93% vs 17.9% ± 7.5%, P the endocardial strain of the 17 segments represented as a bull's eye ( Figure 1C and D).
In the present study we only used global longitudinal peak systolic strain (GLPSS), which was plotted as a negative curve with a peak close to the aortic closure ( Figure 2). These GLPSS curves represent the maximum myocardial longitudinal shortening during contraction in each of the 17 segments. In a normal subject ( Figure 3) GLPSS varies between 15% and 20% [15] .

Reproducibility
The first 10 studies were analyzed blindly by a second operator who measured longitudinal 2D strain in 170 myocardial segments. Intraobserver variability was cal culated from the mean of the differences obtained in the 170 segments. Interobserver variability was calculated as the absolute difference divided by the mean of the 2 observations for all segments measured [16] .

Statistical analysis
Quantitative data with a normal distribution were expr essed as the mean ± SD and data without a Gaussian distribution were expressed as medians (interquartile interval).
For the comparison of quantitative variables with a normal distribution we used the Student's t test for paired data; for variables without a normal distribution   All patients had hypertrophy of the LV apex with normal apical wall motion, but they exhibited a decreased midwall 2D strain, predominantly in the apex. One might postulate that this finding expresses myofibrillar disarray with microvascular ischemia, which contributes to increased myocardial fibrosis in those segments with greater hypertrophy.
In patients with HCM, Popović et al [17] have shown that 2D strain was lower in patients whose MRI showed myocardial fibrosis than in patients without fibrosis, but they did not analyze whether longitudinal strain had = NS). Midwall GLPSS was significantly decreased in the medial segments (12.4% ± 7.3% vs 19.7% ± 7.6%, P < 0.001), with a 52% increase in endocardium strain. But the largest difference between midwall and endocardial strain was found in the apical segments, with a 168% increase in endocardial strain (7.3% ± 8.8% vs 22.8% ± 7.8%, P < 0.001). The increase of the GLPSS from basal to apex segments can be seen in the dotted line ( Figure 4). Using 2Dbased method for myocardial velocity strain (XStrain) that allows analyse the endocardial and epicardial border, this transmural gradient between the midwall and endocardial of global longitudinal peak systolic strain were seen in normal subjects, but without significant differences.

Reproducibility
In our laboratory, intraobserver and interobserver varia bility of 2D strain was low and varied between 3.6% and 5.3% and 7% and 11.8% respectively.

DISCUSSION
To our knowledge, this is the first study to show that in a selected population of patients with apical HCM and normal LV ejection fraction, the regional systolic strain is decreased in the mesocardium, with a compensatory effect in the endocardium. The clinical application of this new finding may help to further understanding the pathophysiology of apical HCM.
Mutations of genes that code for contractile proteins of the sarcomere are responsible for the structural and functional changes seen in patients with HCM, and cause ventricular hypertrophy, myofibrillar disarray and interstitial fibrosis. In spite of the hyperdynamic systolic    is preserved. An explanation for this phenomenon could be that myofiber disarray [5,20] and interstitial fibrosis [2123] are mostly located in the mid third of the ventricular wall. This particular distribution of histological abnormalities in apical HCM also explains why endomyocardial biopsy is not useful in the diagnosis of HCM, since the biotome does not reach the myocardium with fiber disarray and interstitial fibrosis [24] .

Study limitations
One limitation of this study is that we only measured longitudinal strain. It is possible that measurement of radial and circumferential strain will add useful infor mation to the data obtained in this work. 2D strain is a sensitive method to measure myocardial strain, but it is very much dependent on echo image quality, and in patients with necrotic scars strain may be measured in 80% of segments analyzed [12] . Such limitation is not applicable to our population, since the presence of LVH helped in obtaining a good quality image.
Another limitation is that the ROI of speckle tracking method cannot be diminished more than 10 mm. The most patients did not exhibit hypertrophy of the basal segments; hence, further midwall strain overlapped with endocardial strain, which might explain the smaller difference between them.
In conclusion, this study shows that 2D strain assessed by "speckle tracking" is a sensitive method to detect subclinical systolic LV dysfunction. When transmural heterogeneity, as shown in our study.
The normal LV contracts longitudinally in systole and also radially. The array of myocardial fibers in the ventricular wall is quite unique; endocardial and sub epicardial fibers align longitudinally, in a spiral shape, and midwall fibers are aligned circumferentially. This latter group is responsible for the radial contraction in the minor axis of the LV (similar to the movement of the bellows of an accordion), while the former cause longitudinal contraction similar to the movement of a piston. This fiber orientation is so efficient that a 15%20% reduction in the myocite's length can result in a 40%60% radial wall thickening, thus allowing the LV to achieve an ejection fraction of 60%.
In patients with apical HCM, longitudinal midwall strain allowed to identify subclinical global systolic dysfunction, with a lower intra and interobserver variability than for strain derived from colour tissue Doppler [18] . In our study of 20 patients with apical HCM, we analyzed 340 myocardial segments with midwall LPSS and compared it to endocardial LPSS. We confirmed that although systolic ventricular function was supernormal, midwall GLPSS exhibited a decrease in the percent of strain, more evident in apical than in medial segments, whereas endocardial GLPSS was significantly greater, and reached normal values [19] . These findings indicate that in spite of the apical ventricular hypertrophy with excellent ejection fraction parameters, there is subclinical abnormality in midwall strain, while endocardial function   Future studies, with more subjects, will allow assessing whether patients with greater change in midwall strain may be at higher risk for ventricular arrhythmias, sudden death or progression to heart failure due to systolic dysfunction. Additionally, the method could help in evaluating the benefit of conventional treatment and new therapeutic strategies.

ACKNOWLEDGMENTS
We would like to thank Ing. Ariel Desseno (General Electric) for his technical assistance.

Background
Hypertrophic cardiomyopathy (HCM) is associated with normal left ventricular (LV) ejection fraction and impaired LV strain, but there are no studies so far comparing midwall and endocardial strain.

Research frontiers
The diagnostic criteria for apical HCM included demonstration of asymmetric left ventricular hypertrophy (LVH), confined predominantly to the LV apex with an apical wall thickness > 15 mm, a ratio of maximal apical to posterior wall thickness > 1.5, and a "spade shape" deformity of the left ventricle with apical cavity obliteration in end-systole based on 2-D echocardiography.

Innovations and breakthroughs
This study shows that two-dimensional strain assessed by "speckle tracking" is a sensitive method to detect subclinical systolic LV dysfunction. When midwall and endocardial strain values were compared, the authors confirmed that the decrease in strain was confined to the midwall, while endocardial myocardial function was preserved. This transmural heterogeneity of systolic deformity in apical HCM has not been previously described. A possible explanation could be that myofibrillar disarray and interstitial fibrosis are distributed in deeper areas of the myocardium. The clinical application of this new finding may help in the pathophysiological interpretation of HCM.

Applications
Two-dimensional strain is a novel non-Doppler-based method to evaluate strain from standard two-dimensional acquisitions. By tracing the endocardial contour on an end-systolic frame, the software will automatically track the contour on subsequent frames. Two-dimensional longitudinal strains were assessed in 2 orthogonal apical views (4-3 and 2-chambers, 17 segments). The region of interest (ROI) was reduced and shifted to the mesocardium to obtain the parametric image, which allowed quantifying strain in each of the 17 segments of the LV. Later, the ROI was shifted to the endocardium to obtain the endocardial strain of the 17 segments.

Peer-review
The study by Saccheri et al reports the data obtained by speckle tracking echocardiography in patients with apical hypertrophic cardiomyopathy. The authors show that two-dimensional strain is able to identify subclinical systolic COMMENTS Saccheri MC et al . Speckle tracking echocardiography in HCM