Supernormal functional reserve of apical segments in elite soccer players: an ultrasound speckle tracking handgrip stress study

Regional myocardial function is currently evaluated by with 2D echocardiography in the four-chamber view[1] at rest and after pharmacological or physical stress. The evaluation of heart dysfunction, that is often due to ischemia, usually derives from myocardial kinetics modifications[2].

In athletes, where ischemia is an uncommon event, the main information on myocardial performance is related to global contractility, normally expressed by left ventricular Ejection Fraction (EF)[3, 4].

Traditionally, a 2D echocardiographic exam is used in the four-chamber view for analysing the motion of the segments in the LV near the apex [5]; however this type of assessment can often be problematic due to the poor resolution of the lateral wall [6]. Currently the availability of new non-invasive methods for deformation parameters will make a significant contribution to a more accurate analysis of whole and segmental myocardial performance [7].

The segmental contributions of myocardial systolic function can be evaluated using non-invasive quantification of myocardial strain [8]. Strain is a physically-based measure of myocardial deformation, and it may be a tool for detecting systolic function in physiological hypertrophy [7, 9, 10]. Of the components of strain (longitudinal, circumferential and radial), longitudinal strain is conventionally calculated from the gradient of tissue velocity evaluated with Tissue Doppler Imaging (TDI) [9]. Estimation from TDI is limited to the segments that are well-aligned with the scan line, and it is therefore best applied to basal and medial regions [10, 11]. Strain can be also estimated with "speckle tracking", a novel non-Doppler-based method that yields tissue velocity values from grey scale imaging, with high reproducibility of results [12, 13] and no angle dependence. While the results of the two methods are consistent when applied to the basal segments of myocardial walls [14], only the latter allows accurate measurement of strain in the left ventricle para-apical-segment [13, 15, 16]. For these reasons, the regional contribution of left ventricular wall segments to overall heart performance has not yet been evaluated, especially when the medium-apical segments of the left ventricle are investigated. Experience led us to hypothesise that the efficiency of an athlete's heart during sporting activity might differ in the different LV segments, and that study of this aspect might allow us to better distinguish the various contributions of the single segments to overall heart performance. The present study thus aims to quantify the Longitudinal Peak Systolic Strain (LPSS) and Longitudinal Velocity (LV) values in a group of athletes at rest and during stress.

For reasons of homogeneity, all the subjects studied were men. Twenty-five elite soccer players (ACF Fiorentina, Premier League) aged 26 ± 3.5, regularly trained for two hours at approximately 80% of heart rate five days a week for 10 months a year for 5 years or more, and played one game a week during the regular season. All were matched for age, weight and body surface with 25 controls who perform occasional sporting activity (Table 1).

All subjects gave their written consent to the study protocol and all procedures were approved by our Local Ethics Committee.

Subjects in both groups were submitted to anamnesis, including information about drugs and smoking; also to a general examination and cardiological check-up, in order to exclude family history of sudden death, hypertension, diabetes, congenital disease. All the subjects enrolled were evaluated with a standard echocardiograhic exam including systolic and diastolic parameters, as well as LPSS and LV values starting from the four-chamber view.

All the subjects were healthy, with no statistically significant differences among the general parameters (Table 1). In both groups, Systolic Pressure (SP), Diastolic Pressure (DP) and HR values increased during HG, demonstrating that the effort was efficient (Table 2). In athletes, the left ventricular dimensions were higher than in controls. However only left ventricle CMI showed a significant difference (101.54 g/m^{2 athletes} vs 92.100 g/m^{2 controls} with p < 0.05 Table 3). Left ventricle systolic and diastolic parameters including EF did not show any differences in either group at rest or after HG (Table 3). In athletes, the LPSS values of the left ventricle medium apical segments increased significantly during HG, while the control group did not show any significant increase in the same values (Table 4, 5; Fig 3, 4). Statistical power was 0.90. The LV profile in both groups maintained the physiological decrease from the basal to medium-apical segments, but only in athletes did the peak value present a statistical difference with HG (Table 4, 5). No relationship was observed between basal and apical segment LPSS values after stress and CMI in athletes (LPSS_Basal -20.99, CMI 101.54, R = 0.09, LPSS _{Medium-Apical} -24.47, CMI 101.54, R = 0,13).

Interest has recently grown [11, 13] in evaluating the deformation parameters of heart walls in addition to the EF that normally estimates overall myocardial performance The development of the new speckle tracking software for the calculation of strain, whose application is possible in all myocardial wall sections due to its angle independence [13, 15], has enhanced this interest. Commonly, the standard 2D echo exam, has several limitations, like the pitfalls in the evaluation of myocardial performance when it is focussed on the segments close to the heart apex [5, 6]. On the other hand, 2D-strain evaluation offers high reproducibility of the results with mainly high-quality images.

In athletes, where a major curvature of the walls is present [3, 4] as a consequence of physiological hypertrophy, the importance of estimating strain in the left ventricle medium-apical wall segment is linked to the possibility of verifying the involvement of these particular sections in overall myocardial function.

This investigation, aimed to evaluate, using LPSS, the contribution of the regional contractility of left ventricle medium-apical segments to overall heart performance in the athlete with physiological myocardial hypertrophy.

The results demonstrate the applicability of the new speckle tracking method to the calculation of strain in all sections of the heart, also in the medium -apical segments, with results which come within the normal and validated range [19] A particular effective increase in the LPSS of the left ventricle medium-apical as compared to the basal segments after an HG test is evident in athletes. HG normally produces a pressure overload in the left chamber, and the application of this study protocol enhanced the differences in LPSS values in the athletes' group, showing a gradient between the basal and medium- apical segments which was not recognizable in controls. No significant modification of EF is evident in these parameters in either groups studied after HG.

These results point to the importance of considering myocardial deformation parameters in LV segments adjacent to the apex, particularly in athletes where regular training produces increased myocardial contractility. We must remember that the actual performance of the apical segments is not correctly assessable with standard 2D; also the current application of the method of strain estimation by TDI is normally limited to basal and medial wall regions due to its angle dependence.

In this study, the measurements of myocardial fibre velocity values in the same segments where strain was determined were made with the speckle tracking method, thus overcoming the limitations of aligning the beam with the scan line.

Our results show a physiological gradient with a decrease in values from basal to apical segments in the two groups, confirming the literature data

In conclusion, the significant increase in LV longitudinal strain in athletes suggests the importance of evaluating deformation parameters in order to more thoroughly assess those morphological and functional modifications in the heart which cannot be discovered by traditional echo measurements.

In agreement with the normal values recently reported in the literature [19], these results, also suggest the possibility of a routine application of these parameters in order to evaluate the athlete's heart and also to define myocardial performance in several other pathologies (involving athletes and patients alike) where heart contraction is of differing degrees according to the different wall segments.

In athletes, this evaluation might be extended in future to the investigation of the strain of the two ventricles contracting in real time, in order to determine a the different behaviour of value of these two chambers that normally work at different load pressures.