Study population
A total of fifty young professional athletes dedicated to wrestling recruited from the Athletics Center were enrolled in the athlete group. According to their different heart rates, they were split up into the Low HR group (HR of 45 ~ 60 bpm, average 52.38 ± 4.72 bpm, n1 = 25) and the High HR group (HR of 60 ~ 80 bpm, average 66.68 ± 6.45 bpm, n2 = 25). The inclusion criteria were as following: (i) years of training ≥ 5, time of training per week ≥ 30 hours; (ii) never stopping intensive strength exercise; (iii) without records of stimulant use; (iv) sinus rhythm. The exclusion criteria included: (i) without good image quality for offline analysis; (ii) coronary heart disease, myocardial infarction, or arrhythmia; (iii) valve disease such stenosis or regurgitation; (iv) hypertension, diabetes, kidney disease and other systemic disease. Meanwhile, thirty sedentary individuals who underwent physical examination at the same period in the First Affiliated Hospital of Zhengzhou University were collected as the control group (HR of 60 ~ 96 bpm, average 71.65 ± 10.17 bpm, n3 = 30). The ones have no history of continuous training were included and the exclusion conditions were performed as described above. The study protocol has obtained the review and approval by the ethics committee and informed written consent was provided by all participants.
Echocardiography
Transthoracic echocardiography image acquisition was compiled by using a Vivid E95 color Doppler ultrasound diagnostic apparatus (GE Vingmed Ultrasound, Horten, Norway), equipped with a M5S transducer (frequency of 2.0 ~ 4.0mHz). Brachial artery pressure including systolic and diastolic blood pressure detected by electronic sphygmomanometer in a quiet state before examination and then recorded. The subjects were instructed to place in the left lateral decubitus position and breath calmly, with simultaneous electrocardiogram displaying. The standard LV long-axis, apical two-chamber, and four-chamber views of the gray-scale dynamic images at frame rates ≥ 60 frames/sec for three consecutive cardiac cycles were collected and restored in the offline analysis workstation of Echo PAC software (ver. 203, GE Vingmed Ultrasound, Norway).
LV diameter in diastolic (LVD), posterior wall thickness (PWT) and diastolic interventricular septum (IVST) on the LV long-axis view were measured using 2DE, and the relative ventricular wall thickness (RWT) was calculated by the equation: (IVSTd + PWTd)/LVDd. Measuring LV ejection fraction (EF), end-systolic volume (ESV), end-diastolic volume (EDV) and stroke volume (SV) utilized Simpon’s biplane method.
Myocardial strain and work analysis
Import the images into the Echo PAC workstation and determine the three points of the mitral valve annulus and the apex on the long-axis, apical two-chamber, and four-chamber views respectively. Then, the system automatically traced the LV entire myocardial movement trajectory after identifying the endocardial borders, and the region of interest could be manually adjusted if necessary. Next, the brachial artery systolic and diastolic pressure value were entered, and the aortic valve closure time was automatically defined by the software on the long-axis view to obtain the LV-PSL and LVMW 17-segment bull’s eye diagram (Fig. 1).
The 17-segment of LV was included three levels, the basal, middle and apical level, and several wall compartments, anterior, anterolateral, inferolateral, inferior, postseptal and anteroseptal, among which, the basal and middle level with six parts and the apical level with five parts. The regional myocardial work index (RMWI) and regional myocardial work efficiency (RMWE) were automatically calculated form three levels of LV and get the average for three times. According to our previous research, GWE was considered as the best predictor of LV contractility, so we analysis the linear relationship between GWE with general parameters to better understand the work efficiency of athletes. Global and regional MW parameters were compared between three groups and the 17-segment RMWI and RMWE were showed (Fig. 2).
Non-invasive PSL combined 2D-STE and arterial pressure to acquire the dynamic changes of LV pressure and strain during the mitral valve closing to opening process, which had been proven to have good consistency with invasive cardiac catheterization measurements[12]. Among them, a non-invasive LV pressure curve was constructed by the system using the brachial artery pressure that based on the period of LV isovolumetric and ejection obtained by echocardiography. Global myocardial work (GWI) presented the area of PSL, that was the total work calculated by the LV from mitral valve closure to mitral valve opening. Global myocardial constructive work (GCW) represented the work that conductive to LV ejection, including myocardial contracting in systole and elongating in isovolumic relaxation. Global myocardial wasted work (GWW) was constructed by lengthening myocytes in systole adding shortening myocytes in isovolumic relaxation, which was not conductive to LV ejection. Global myocardial work efficiency (GWE) was the ratio of constructive work divided by the sum of constructive and wasted work.
Statistical analysis
Statistical analysis was carried out with the aid of SPSS (ver. 24.0, IBM, Chicago, IL). All measurement data conforming to a normal distribution were presented as mean ± standard deviation (SD). Comparison among the three groups were conducted by one-way ANOVA which was followed by Tukey-Kramer test when variances were homogeneous or Games-Howell test when not. Multiple linear regression analysis was applied for relations of GWE. Intra- and inter-observer reliability of MW parameters measurement was interpreted using intraclass correlation coefficient (ICC) with 10 randomly selected athletes. P-values ༜ 0.05 were identified statistically significant.