A total of 371 healthy volunteers (60.6% women), mean (SD) age was 43 (17), were recruited from the physical examination center of the Second Affiliated Hospital of Harbin Medical University from August 2018 to March 2019. Afterward, they were separated into four groups according to their age quartiles. Including criteria were normal at clinical presentation, no history of any comorbidity, and ECG of sinus rhythm.All subjects provided informed consent for study participation and anthracycline therapy administration. This study was approved by the ethics committee ofthe Second Affiliated Hospital of Harbin Medical University.Conflictsofinterest：The authors have no conflicts of interest to declare.
The study was performed on Hitachi Aloka LISENDO 880 ultrasound system (Hitachi-Aloka Medical, Ltd, Tokyo, Japan), with the phased array single crystal probe(probe frequency 1.0-5.0 MHz, frame rate 66-78 Hz). All subjects were connected to a 12-lead ECG and examined in the left lateral decubitus position. Conventional and tissue Doppler transthoracic echocardiography (TTE) was performed by experienced sonographers and reviewed by senior physicians. In two-dimensional (2D) echocardiographic assessment, left ventricular end-diastolic diameter (LVEDd), left ventricular end-systolic diameter (LVESd), end-diastolic septal thickness (IVST), and left ventricular posterior wall thickness (LVPWT) were measured. In PW/TDI evaluation, the early diastolic peak velocity (E) and late diastolic peak velocity (A) of the mitral valve were measured in the apical four-chamber view, with the E/A ratio calculated. The early diastolic peak velocity of mitral annulus (lateral e' and septal e') was also measured for further calculation of lateral E/e' and septal E/e'. The left ventricular ejection fraction (LVEF) was generated by the biplane Simpson method. In terms of VFM mode, the 2D gain was adjusted to optimized visualization of the endocardium, mitral valve, and aortic valve. The size of sampling frame was also adjusted to completely envelop the LV, while the Nyquist limit for CDFI was set high enough so that the flow filled the left ventricle without aliasing and blood flow spilling. The 2D and CDFI images of the apical 4-chamber (Apic 4C), 3-chamber (Apic 3C), and 2-chamber (Apic 2C) views were recorded (Fig. 1).
The acquired images were then imported into the DAS-RS1 workstation for offline analysis. Firstly, the endocardial border was traced to the clearest frame manually, and then the software automatically traced to the remaining frames. The user was allowed to check and edit the image frame by frame. The diastolic period was defined as the first frame after aortic valve closure to the first frame after mitral valve closure. And the diastolic period was then divided into four phases based on the ECG, time-flow curve, and two-dimensional cardiac valvular opening and closing, including isovolumic diastolic period (D1), rapid filling period (D2), slow filling period (D3), and atrial contraction period (D4).
The WSS images, together with the raw data of the Apic 4C, 3C, and2C, were processed on the offline VFM workstation, and the raw data were subsequently imported into the WSS segmentation template [see Additional file 1], in which LV was divided into six walls: anterior, anterolateral, inferolateral, inferior, inferoseptal and anteroseptal (Fig. 2). Since the VFM technique is a combination of CDFI and two-dimensional speckle tracking, it could acquire both the radial and axial flow velocity, as well as the boundary conditions of bilateral walls, allowing the continuity equation to calculate the intraventricular flow quantitatively (Figure 3). The Newton inner friction equation of WSS was as follows:
WSS＝μ(Vmax-0) /dy ＝μVmax/dy
μ: blood viscosity coefficient 4.0×10-3 ( N・s・m-2 )
In the equation, Vmax means the near-wall maximum flow velocity in the cardiac chamber, and the dy means the distance between the point of Vmax and the adjacent ventricular wall. As such, the (Vmax-0)/dy represents the gradient of near-wall flow velocity. And WSS, which is a vector with size and direction, is the product of blood viscosity (μ) and (Vmax-0)/dy[18, 19].
The SPSS statistic software (IBM SPSS, version 22.0, Chicago, IL) was applied.Continuous data were expressed as mean± standard deviation (SD), and those with skewed distributions are presented as medians
(25th and 75th percentiles).Comparisons among different age groups were analyzed using the Kolmogorov-Smirnov test。Comparisons between men and women were carried out by the Mann-Whitney tests. Moreover, Log-transformed WSS value in the single LV wall among different phases were compared by one-way repeated measures analysis of variance (ANOVA).The independent correlations between log-transformed WSS and conventional parameters in evaluating diastolic function were explored using stepwise multiple regressions. P <0.05 was considered statistically significant.
The study was approved by the Harbin Medical University and was conducted in accordance with the principles of the Declaration of Helsinki