Hypertrophic cardiomyopathy (HCM) is a genetic disorder characterised by left ventricular hypertrophy and myocardial disarray. Conventional echocardiography provides useful morphological information for the diagnosis of HCM,1 although none of the proposed variables are highly sensitive or specific.2^–4 Several studies so far have suggested that myocardial contractility and relaxation are reduced in HCM and that the hypertrophy is compensatory.5^–7 Thus, it has been shown that tissue Doppler imaging (TDI) is a sensitive and accurate method for the identification of HCM, irrespectively of the presence of left ventricular hypertrophy.8 Moreover, it has been suggested that tissue Doppler strain imaging is able to discriminate HCM from left ventricular hypertrophy of different origin,9 as it reflects myocardial contractile and lusitropic properties.10 11 However, clinical application of the technique is limited by complexity of data post-processing and low reproducibility. Thus, a novel method to measure strain from standard bi-dimensional images has been developed.12 13 It has been shown that left ventricular two-dimensional (2D) strain identifies early abnormalities in HCM patients with normal left ventricular systolic function.14 However, little is known about left atrial deformation.

In HCM patients, the left ventricular filling pressure is associated with an increased left atrial size15 16 that seems to be a very powerful determinant of exercise capacity, with documented prognostic value.17 18 Indeed, the important role of the left atrium in HCM, expressed both as exercise capacity and clinical outcome, is well documented.19 Therefore, left atrial function appears to be a marker of the adverse loading conditions and other pathophysiological processes that are likely to be present in HCM.16 20

The aim of this study was to quantify left atrial longitudinal function by both tissue Doppler and two-dimensional strain imaging in HCM patients, in comparison with other forms of left ventricular hypertrophy and normal controls.

METHODS

Study population

Forty-three consecutive patients (28 men, mean age 49 (SD 18) years) with familial HCM were enrolled in the study. The diagnosis of HCM had been based on the presence of a positive family history in combination with a conventional echocardiographic demonstration of non-dilated, hypertrophic left ventricle, in the absence of other cardiac or systemic disorders capable of producing a similar degree of hypertrophy; coronary artery disease had been excluded by coronary angiography. Left ventricular hypertrophy was considered consistent with HCM when an unexplained maximum left ventricular wall thickness on 2D echocardiography greater than 15 mm was present.18 All studied patients were in sinus rhythm and had a normal left ventricular ejection fraction. Twenty seven healthy volunteers (19 men, mean age 42 (13) years) with normal clinical examination, electrocardiogram, treadmill stress testing and conventional echocardiogram were also studied as a control group. Additionally, 21 patients (11 men, mean age 52 (12) years) with left ventricular hypertrophy, defined as a left ventricular mass index >134 g/m² for men and >110 g/m² for women,21 secondary to either arterial hypertension (12 patients) or aortic valve stenosis (nine patients), were also enrolled as a reference group. Patients with HCM or arterial hypertension/aortic stenosis having mitral valve regurgitation characterised by a vena contracta width ⩾0.7 cm were excluded. The study was approved by the institutional ethics committee and informed written consent was obtained from all patients. Additionally, to eliminate the possible interference of medication on echocardiographic measurements, all drugs had been discontinued for at least 3 days before the echocardiographic evaluation.

Echocardiography

All echocardiographic studies were performed using a Vivid 7 ultrasound system (GE Medical systems, Horten, Norway). Three cardiac cycles were stored in cineloop format for offline analysis. All data were analysed by two independent expert observers who were unaware of the patients’ clinical status. Measurements and tracings were carried out according to the leading edge principle and in accordance with the recommendations of the American Society of Echocardiography and other published reports related to HCM.22 23 The following parameters were calculated: left atrial diameter and volume index in end-systole, left ventricular end-systolic and end-diastolic diameter and hence fractional shortening, and intraventricular and posterior wall thickness in end-diastole. Left ventricular ejection fraction was measured using the Simpson’s method. Left atrial volume was measured offline using the biplane area-length method, as previously described and indexed to body surface area.24 25 Obstruction of the left ventricular outflow tract was considered to be present when the peak instantaneous outflow gradient was estimated to be at least 30 mm Hg under resting conditions.26 27

Strain measurements

Colour Doppler myocardial imaging data were stored in digital format and analysed offline by proper software (Echopac, GE Medical systems, Horten, Norway); regional myocardial velocities and strain values were measured on three consecutive cardiac cycles. All tissue Doppler and 2D measurements were performed on the same three cardiac cycles. The recorded wall was positioned in the centre of the sector to minimise artefactual data and re-aligned so that the direction of motion interrogated was as near as possible parallel to the direction of the insonating beam. The colour Doppler myocardial imaging range setting was adapted in order to avoid aliasing within the image.28 Sectors were adjusted to achieve frame rates ⩾100 frames/s (100–167 frames/s) for TDI and ⩾40 frames/s (40–82 frames/s) for 2D strain imaging.

Left ventricular longitudinal strain was estimated using both tissue Doppler and 2D strain imaging. Tissue Doppler left ventricular longitudinal strain was interrogated in the basal, mid and apical myocardial segments in each left ventricular wall (anterior, inferior, septal and lateral). Two-dimensional left ventricular longitudinal strain was measured using the 17-segment model, as previously described.14 After having defined by anatomical M-mode mitral and aortic valve opening and closure as well as atrial contraction, left atrial longitudinal strain, obtained by both tissue Doppler and 2D strain, was measured in all three atrial phases (reservoir, conduit and atrial contraction) as follows: reservoir period was defined as the interval between mitral valve closure and mitral valve opening, conduit period as the interval between mitral valve opening and the onset of atrial contraction, as assessed by anatomical M-mode, and contractile period as the interval between the onset of atrial contraction, as assessed by anatomical M-mode, and mitral valve closure (fig 1). Subsequently, the atrial deformation of each period was calculated by measuring the respective difference of strain (values at the end minus values at the beginning). Additionally, the sum of maximum positive and maximum negative atrial strain observed during a cardiac cycle (peak reservoir strain minus peak contractile strain) was calculated as an index of the overall atrial longitudinal function (total atrial deformation, TAD).

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Figure 1 Left atrial strain measurement in a normal subject. (A) Curved anatomical M-mode. (B) Mean two-dimensional strain (dotted curve). (C) Tissue Doppler strain at the lateral (lower curve) and septal (upper curve) wall. AC, atrial contraction; ACb, atrial contraction beginning; AVC, aortic valve closure; AVO, aortic valve opening; EDp, early filling period; MVC, mitral valve closure; MVO, mitral valve opening.

For tissue Doppler atrial longitudinal strain, analysis was performed from the apical four-chamber view. The sample was positioned between the middle and the inferior edge of the intra-atrial septum and of the left atrial lateral wall (fig 1) and the average value was calculated. A two-segment instead of a six-segment analysis, as in the case of 2D strain, was used as the latter would have been quite time consuming. For longitudinal measurements, a region of interest of 9×2 mm with an elliptical shape was chosen.29 To derive strain profiles from the studied segment, the examined region was continuously positioned within the segment being studied.

Two-dimensional strain is a novel non-Doppler-based method to evaluate systolic strain from standard bi-dimensional acquisitions. By tracing the endocardial contour from the apical four-chamber view, the software automatically tracks the contour on subsequent frames. Adequate tracking can be verified in real time and corrected by adjusting the region of interest or manually correcting the contour to ensure optimal tracking. Following this, regional 2D longitudinal strain of six segments of the atrial wall was automatically obtained and hence mean 2D longitudinal strain was calculated.

Statistical analysis

Statistical analysis was performed using the SPSS 10.0 statistical software package. Continuous variables were tested for normal distribution by the Kolmogorov-Smirnov test. Comparison of continuous variables among study groups was performed by one way analysis of variance, with Bonferroni post-hoc analysis for the multiple comparisons between groups. Bivariate correlation was used to investigate for potential relations between variables. Univariate and multivariate logistic regression analyses were performed to identify predictors of HCM among conventional echocardiographic indices and tissue Doppler and 2D strain parameters; variables univariately related to HCM were entered into multivariate models. To identify optimal cut-off values for the discriminators between HCM and non-HCM left ventricular hypertrophy, a receiver-operating characteristics (ROC) analysis was performed; specificities and sensitivities for each cut-off value were calculated and expressed as percentages. The intra-observer and inter-observer variability for atrial tissue Doppler and 2D strain measurements were assessed by the coefficient of variation in the first 10 patients and the first 10 controls recruited in the study. Inter-observer variability was assessed by two independent observers, while intra-observer variability by one observer twice within four weeks. All statistical tests were two-sided and a p<0.05 was considered statistically significant. Continuous variables are expressed as mean (plus or minus 1 SD).

RESULTS

Patients’ age did not differ among the three groups. Conventional echocardiographic parameters are shown in table 1. In the non-HCM left ventricular hypertrophy group, the left ventricular mass index did not differ between patients with aortic stenosis (nine patients, 137 (5) g/m²) and those with arterial hypertension (12 patients, 141 (15) g/m², p = 0.399). Patients with HCM had a significantly higher interventricular septal wall thickness and septal-posterior wall thickness ratio (p<0.001 for both) and a higher left ventricular mass index (p<0.001), along with higher left atrial diameter and volume index and smaller left ventricular diameter (p<0.001). Left ventricular ejection fraction as well as transmitral E wave velocity, E wave deceleration time and early diastolic wave (e′) velocity at lateral mitral annulus were similar in the three groups. However, the E/A ratio was significantly different among the three groups (p<0.001) suggesting the presence of different diastolic left ventricular function. Moreover, the E/e′ ratio was significantly higher in HCM patients. Mitral regurgitation severity, as estimated by the vena contracta width, was similar in HCM patients and those with non-HCM left ventricular hypertrophy (0.35 (0.11) versus 0.32 (0.12) cm, p = 0.359).

Left atrial and left ventricular strain variables are reported in table 2. Left atrial strain measurement was feasible by both tissue Doppler and 2D strain imaging in all patients. The same applied to left ventricular strain, and no subject was excluded because of the inability to measure tissue Doppler or 2D strain in two or more left ventricular segments. Both tissue Doppler and 2D strain values were significantly reduced in patients with HCM in all three atrial time phases (fig 2), compared to normal subjects (fig 1) and patients with non-HCM left ventricular hypertrophy (overall p values <0.001 in most of the cases). Similarly, TAD, as estimated both by tissue Doppler and 2D measurements, was significantly decreased in HCM patients (table 2, overall p values <0.001). In contrast, atrial strain variables, including TAD, did not differ significantly between patients with non-HCM left ventricular hypertrophy and normal controls, with the exception of tissue Doppler conduit strain (p = 0.018), which was higher in patients with non-HCM left ventricular hypertrophy. Similarly, left ventricular systolic strain, both by tissue Doppler and 2D imaging, was significantly lower in patients with HCM compared to normal subjects and patients with non-HCM left ventricular hypertrophy (overall p values <0.001), while it did not differ between the latter two groups.

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Figure 2 Left atrial strain measurement in a patient with hypertrophic cardiomyopathy. (A) Curved anatomical M-mode. (B) Mean two-dimensional strain (dotted curve). (C) Tissue Doppler strain at the lateral (lower curve) and septal (upper curve) wall. AC, atrial contraction; ACb, atrial contraction beginning; AVC, aortic valve closure; AVO, aortic valve opening; EDp, early filling period; MVC, mitral valve closure; MVO, mitral valve opening.

Left ventricular hypertrophy: HCM versus non-HCM

For the discrimination between HCM and non-HCM left ventricular hypertrophy, different multivariate logistic regression models were constructed incorporating variables that were univariately related to HCM. In a model of conventional echocardiographic measurements, including interventricular septal thickness, left atrial diameter, left atrial volume index and E/A and E/e′ ratios, none of those variables were independently significantly correlated with HCM. In a second model including TDI strain measurements (left ventricular systolic strain, left atrial reservoir, contractile and conduit strain and total atrial strain), only left ventricular systolic strain was significantly correlated with HCM (b = −0.559, p = 0.038). In a third model with 2D strain variables (left ventricular systolic strain, left atrial reservoir, contractile and conduit strain and total atrial strain), only left atrial contractile strain was significantly related to HCM (b = −2.407, p = 0.046) (fig 3). Adding tissue Doppler left ventricular systolic strain to the conventional echo model did not enhance its prognostic significance (p of the change, 0.998). In contrast, when 2D atrial contractile strain was added to the conventional echo model, which also included left atrial volume index, it seemed to have an additive prognostic value (p of the change, <0.001).

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Figure 3 Boxplot diagrams of two-dimensional (2D) contractile left atrial (LA) strain in patients with hypertrophic cardiomyopathy (HCM), hypertension or aortic stenosis (HT/AS) and normal controls. The broken reference line represents the optimal cut-off point for differentiating between HCM and non-HCM left ventricular hypertrophy (HT/AS).

In ROC analysis, 2D atrial contractile strain discriminated HCM from non-HCM left ventricular hypertrophy with an area under the curve of 0.885 (standard error, 0.049; 95% confidence intervals, 0.789–0.981; p<0.001) and a cut-off of −10.82% discriminated HCM from non-HCM LVH with a sensitivity of 82% and a specificity of 81.

Obstructive versus non-obstructive HCM

Twenty-four out of 43 HCM patients (56%) had obstructive HCM (mean resting peak left ventricular outflow track pressure gradient of 68 (34) mm Hg) and 19 non-obstructive. Left atrial diameter did not differ significantly between those two subgroups (46 (4) mm in obstructive versus 43 (6) mm in non-obstructive, p = 0.076), but left atrial volume index was significantly higher in obstructive HCM (40 (7) cm²/m² in obstructive versus 35 (6) cm²/m² in non-obstructive, p = 0.019). A comparison of atrial strain parameters among patients with obstructive and non-obstructive HCM and non-HCM left ventricular hypertrophy is provided in table 3. Tissue Doppler atrial strain variables, with the exception of contractile strain, were significantly lower in obstructive HCM patients compared to non-obstructive HCM ones. In contrast, none of the 2D atrial strain measurements differed between obstructive and non-obstructive HCM. However, 2D strain values in both HCM subgroups were significantly reduced compared to non-HCM left ventricular hypertrophy patients.

Variability

Intra-observer and inter-observer variability for atrial strain variables in HCM patients was better for 2D (8% and 9.5%, respectively) than for tissue Doppler measurements (16% and 17.5%, respectively). In normal subjects, intra-observer and inter-observer variabilities for tissue Doppler atrial strain were 10.5% and 11.5%, respectively.

Finally, the mean processing time per case in HCM patients was 12.5 (2.6) minutes for tissue Doppler strain versus 3.8 (1.2) minutes for 2D strain (p<0.001).

DISCUSSION

This study showed that left atrial longitudinal strain was reduced in HCM patients compared to patients with non-HCM left ventricular hypertrophy or healthy subjects. This finding was evident in all three atrial phases and in the overall longitudinal atrial function and was observed both by tissue Doppler and by 2D atrial strain imaging. In addition, 2D strain seemed to have an additive prognostic value in differentiating HCM from non-HCM left ventricular hypertrophy, when combined with conventional echocardiographic indices. Finally, 2D strain was more reproducible and less time consuming than tissue Doppler strain.

Comparison between tissue Doppler and two-dimensional strain

Clinical use of tissue Doppler strain has been limited by artefacts resulting from myocardial translational motion, by the requirement for optimal alignment, by variable reproducibility and by time-consuming offline analysis. In the present study, a greater beat-to-beat variation for tissue Doppler longitudinal strain than for 2D strain was observed, along with a longer post-processing time.30 31 Although in diseased heart there are no data regarding inter-observer or intra-observer variability for atrial strain measurements, slightly lower values have been published for left ventricular tissue Doppler strain. The slightly higher inter-observer and intra-observer variability observed in the present study probably reflects a higher variation as a result of left atrial myocardial displacement along with the regional heterogeneity32 and/or because of the thinner left atrial wall. On the other hand, measures of left atrial 2D strain, as observed in this study, have a high reproducibility, comparable with that reported for the left ventricle.33 34 Additionally, the intra-observer and inter-observer variability of left atrial tissue Doppler strain in normal subjects observed in the present study is comparable with that reported by previous studies,30 in which values between 11% and 10%, respectively, have been encountered. Thus, it could be suggested that in patients with HCM, left atrial 2D strain is feasible, less time consuming and more reproducible than regional tissue Doppler imaging. It should be stressed that although the left ventricular strain values derived by the two techniques were similar, a greater variation was encountered in the case of left atrial strain. This may be attributed to the fact that left atrial walls are much thinner than the left ventricular ones and although strain measurements are feasible, they may be characterised by a higher variability between the different imaging techniques.35 Moreover, the fact that atrial strain analysis by TDI was based on a two-segment model instead of a six-segment model, as in the case of 2D strain, may also be another potential explanation.

Tissue Doppler and two-dimensional strain in hypertrophic cardiomyopathy

Previous studies have applied TDI to evaluate left ventricular hypertrophy,11 36 suggesting that left ventricular myocardial velocity gradients are sensitive indices for differentiating between physiological and pathological left ventricular hypertrophy caused by pressure overload. However, more recent studies suggested that left ventricular 2D strain is superior to tissue Doppler measurements and can identify early abnormalities in patients with HCM and normal systolic function.30 To our knowledge, this is the first study that evaluates left atrial function, by using both tissue Doppler and 2D strain, in patients with different causes of left ventricular pressure overload. The results of the present study showed that although the E/E′ ratio (an index of left ventricular filling pressure) was significantly higher in the HCM group this could not be considered as a marker of increased left atrial afterload since the reported values are within the grey zone.8 On the other hand using 2D strain we found that all three atrial periods and the total atrial function are reduced in patients with HCM, indicating a decreased contractile and relaxation deformation,8 37 of both left ventricle and left atrium, thus suggesting myocardial dysfunction with normal ejection fraction.8 The reduced left atrial deformation, therefore, might indicate an increased atrial afterload, or an atrial myopathy that might precede and provide the stimulus for the development of left ventricular dysfunction.38

Left ventricular hypertrophy in patients with HCM is presented with or without left ventricular outflow obstruction at rest. The results of the present study revealed that the average tissue Doppler atrial strain variables were significantly lower in obstructive HCM (mainly due to the lateral TDI values, since atrial septum strain was not significantly different), suggesting a more severe form of the disease.39 On the other hand, none of the 2D strain measurements differed significantly between these two subgroups, probably because 2D strain is the average of six segments, including three segments of atrial septum, whereas tissue Doppler represents the values obtained from one or the average of two segments. However, 2D strain measurements were clearly impaired in the HCM group, regardless of the presence of left ventricular outflow obstruction.

Left atrial function

It is known that left ventricular filling pressures are elevated in HCM patients.40 Elevated filling pressures are reliably estimated by the E/e′ ratio41 and have been associated with increased left atrial size in HCM.42 Left atrial size is an indicator of pathophysiological processes in HCM, including increased left ventricular wall thickness, mitral regurgitation and diastolic dysfunction.43 In the present study, patients with mitral regurgitation characterised by a vena contracta width ⩾0.7 cm were excluded, while although both left atrial size and E/e′ ratio were significantly higher in HCM compared to other forms of left ventricular hypertrophy, none of them was an independent predictor of HCM. This may imply the interference of an additional pathogenetic mechanism, specific for HCM, such as a particular atrial myopathy, which is reflected by the contractile atrial deformation and not by atrial size or left ventricular diastolic dysfunction per se. Indeed, the results of the present study indicate that left atrial contractile function, as assessed by 2D strain imaging, had an additive prognostic value in differentiating HCM from other forms of pressure overload, when added to conventional echocardiographic measurements. This is in keeping with previous knowledge indicating the pivotal role of left atrial function in HCM,44 showing a decreased left atrial contractile function,45 which has lately been shown to follow the Frank-Starling law.46

Limitations

In the present study, neither histopathological nor sarcomeric protein gene analyses were performed. It should be stressed though that the myocyte disarray, the most characteristic histopathological finding in HCM, is difficult to identify in vivo47 and is not specific for autosomal dominant HCM.48 Moreover, mutations of sarcomeric protein genes account for only 60% of HCM cases.49 50 Additionally, we used a myocardial wall thickness cut-off of 15 mm47 rather the conventional value of 13 mm,51 which is less accurate than the 2D left ventricular hypertrophy score. Radial, transverse or circumferential left atrial strain were not measured because left atrial wall is too thin.30 Atrial deformation was not investigated in the two-chamber view, because of its low reproducibility and because the corresponding data post-processing is both complex and time consuming. Finally, mitral valve regurgitation severity was quantified using the vena contracta width; this is the preferred quantitative method for the assessment of eccentric regurgitant jets that are frequently encountered in HCM. However, the intermediate values of vena contracta (ranging between 0.3 and 0.7 cm) may sometimes underestimate mitral regurgitation severity.