Two-dimensional tracking and TDI are consistent methods for evaluating myocardial longitudinal peak strain in left and right ventricle basal segments in athletes

30 soccer-players, 25 ± 3 years, regularly trained three times a week for 10 months a year for almost five years, were studied (Table 1). The athletes were submitted to a complete clinical evaluation which included anamnesis and an objective examination to exclude important lung, heart, or metabolic diseases. A complete cardiological check-up including basal ECG was performed. The study protocol was approved by the ethics committee of the University of Florence and subjects gave their written informed consent

Following the study protocol, each subject remained at rest for 10 minutes, then was submitted to a trans-thoracic echographic test at rest and after a hand-grip stress (HG). The latter was performed with an adequately-calibrated handle dynamometer, placed in the athlete's dominant hand in line with his forearm and hanging straight down his side. The athletes were required to keep the handle pressed at 30% of the maximal effort previously established for a period of 3 minutes Maximum grip strength without swinging of the arm was then determined. Systolic and diastolic blood pressure were measured at rest and at the end of maximal stress. This sequence was performed twice for each athlete to record the 2D images, which were processed by both TDI and B-mode tracking methods to calculate strain values. The Ejection Fraction was calculated at rest and after HG from the apical 4-chamber view using Simpson's algorithm. The study protocol was repeated twice 7 days later by a different, expert operator.

Echocardiographic imaging was performed with subjects lying in left lateral position. Echo GE Vivid 7 and Philips SONOS 5500 echocardiographs were utilized to record the images of the two ventricles. In accordance with the recommendations of the American Society of Echocardiography [11], the usual parameters were measured in M-mode, two-dimensional mode, and continuous and pulsed-wave Doppler echocardiography. LV (Left Ventricular) 4-chamber-view images were obtained with conventional 2D grey scale echocardiography. Two-dimensional grey scale images with real-time left and right ventricular endocardial detection were stored digitally for subsequent offline analysis. In the TDI method, ellipsoid ROI was defined so that the left and right endocardial layer could be encapsulated for the whole cardiac cycle. The feasibility of feature-tracking analysis was ensured by images free of reverberation and with a good lateral wall resolution.

With the Vivid 7 echocardiograph, it was possible to determine LPS (longitudinal peak strain) in real time with the Tissue Doppler method. With the Philips SONOS 5500, the myocardial images were collected for the next feature tracking analysis. Tracking and subsequent strain calculations were performed in MatLab (The MathWorks, Natick MA, U.S.A.) with custom software based on a previously-validated algorithm [5, 7].

Longitudinal Peak Strain was calculated in the lateral wall (LW) and interventricular septum (IVS) basal segments of the left and right ventricles of the athlete's heart (Figures 1, 2, 3, 4, 5). In order to reduce regional artifacts, only the high-quality images without reverberations were considered and processed. A good echocardiographic myocardial view was always obtained in the athletes enrolled in this study, allowing identification of the endocardial border and left and right ventricle wall thickening.

Following the indications in the literature [12, 13], and in order to reduce random noise, each sample was obtained by averaging more than one consecutive heart cycle (usually three), setting the frame rate between 75–80 FPS (Frames Per Second). These settings are recommended to combine temporal resolution with an acceptable lateral definition, to enhance the feasibility of the tracking technique, and to decrease the possibility of angle problems in the Doppler-derived method. With the TDI technique, for each segment a stationary region of interest (ROI) was manually set in early systole in the middle wall portion of each segment. The ROI size was 5 mm laterally and it was adjusted longitudinally to cover the length (mean 20 mm) of the segment. This procedure was performed in double-blind by two expert operators a week later and following the same protocol. All the echocardiographic analyses were performed without knowledge of the results of the reference methods. To assess inter-observer variability, 6 echocardiographic tests were randomly selected and then independently analyzed by two different observers with the two methods.

Values were expressed as [average ± SD]. Comparisons among data were performed with Student's t test, p < 0.05. For intra-observer and inter-observer variability 95% limits of agreement were used.

In all the athletes enrolled, the myocardial wall basal segments were analyzed using the two methods, and only the good-quality echocardiographic images were processed. One reason for exclusion was inability to obtain the ROI (region of interest) closely enough to the shape of the left and right ventricular endocardium, which would result in underestimation of data and their contamination by signal components outside the left ventricular cavity. The 4-chamber view was the one which allowed the best delineation of the ventricular cavity and the best detection of the endocardial border.

All the athletes maintained a normal EF % at rest and after stress, without any significant differences. The studied group showed a higher cardiac mass index than the upper limits of the normal range (132 ± 5 g/m²), confirming that the athletes were regularly trained. Heart rate and systolic blood pressure increased significantly after stress. The results of the echocardiographic parameters are reported in Table 2

The results of the strain analysis performed with the two different methods are shown in Table 3. The values refer to systolic peak strain on basal segments of the heart. Comparison of the values shows the consistency between the results of the Doppler-derived technique and those obtained with the feature tracking method. The values are also in agreement with the data reported in the literature [12‐15]. The 95% limits of agreement of the two methods were: lower 0.51-upper 0.54 for TDI, and lower 0.44- upper 0.47 for tracking.

In addition, we wish to note that the longitudinal strain in the athlete's heart appears to be higher in the right than in the left ventricle, although the statistical differences are not sufficient to draw any conclusion.

The accurate identification of regional myocardial function in athletes by strain analysis is important for follow-up and management of the training itself. In this particular population, the physiological hypertrophy of the heart supports the hypothesis that longitudinal strain may present changes in basal segments that are nearly rectilinear. A mild increase in longitudinal strain values appears in the right more often than in the left ventricle, although a wider and heterogeneous group of athletes should be studied to investigate this particular trend. It should be remembered that the two methods present several limitations to a correct interpretation of the results that preclude routine use of the strain concept. The Doppler-derived method is limited by angle dependence, which makes it unable to evaluate LV deformation with uniform accuracy along the different ventricular walls and in the different echocardiographic projections. Feature tracking is performed on grey scale (B-mode) echocardiographic images where the local grey-scale pattern (including speckles) remains reasonably stable; unlike TDI, the velocity evaluated with this method is a vector, and does not therefore present any angle dependence. It can be applied with the same accuracy to the different myocardial regions; on the other hand it is a novel technique that requires further practical experience validation studies. Both methods are influenced by image quality, and in particular clinical conditions they can present limitations due to the physiological growth of the myocardial chambers, which prevents the perfect framing of the image in the echographic window.

In conclusion, strain values computed with the TDI-derived method and the tissue tracking technique are equivalent in the basal segment. The former has more literature to support it, but is limited to basal and low-median segments. Tracking appears to open up possibilities of extending strain analysis to the whole of the myocardial wall. It might allow complete analysis of the development of myocardial contractile adaptation in athletes in training and during their career.

The potential clinical applications of the two methods used to estimate myocardial contractility are not further explored here since this is not within the scope of the study and would necessarily imply the use of subjects with different pathologies.