Study population
A total of 26 uremic patients who were treated in the nephrology department of our hospital from November 2020 to June 2021 were consecutively identified. All uremic patients were treated with regular haemodialysis three times a week and with LVEF ≥ 50% according to the 2016 European Society of Cardiology Guidelines [21]. Patients were excluded if there were valvular disease, LV outflow tract obstruction, arrhythmia, essential hypertension, diabetes, previous myocardial infarction, medium and large pericardial effusion, LV systolic dysfunction with LVEF < 50%, and patients with a poor acoustic window of transthoracic echocardiography, which was mainly manifested as unclear display of endocardium or endocardium located outside the sampling frame.
Twenty-seven healthy volunteers matched for sex, age and body surface area (BSA) were selected as the normal group. All volunteers had no history of heart disease, hypertension, diabetes or medication. Their physical examination, electrocardiogram, chest radiograph, renal function and echocardiography results were normal.
All subjects provided informed consent prior to participating in this study, and the study protocols were approved by the Hospital Ethics Committee. The participant selection process is illustrated in the flowchart in Fig. 3.
Echocardiography
GE Vivid E95 (GE Vingmed Ultrasound AS, Horten, Norway) colour Doppler ultrasound system equipped with M5Sc-D 1.4–4.6 MHz probe was used for echocardiographic image acquisition.
The subjects were instructed to assume the left-lateral position and an electrocardiogram was attached. Transthoracic echocardiography was used to measure the LVEDd, LVPWTd and IVSTd of the participants. The modified biplane Simpson method was used to calculate LVEF from the apical four- and two-chamber views. The LVMI was calculated as LV mass divided by BSA. We obtained the dynamic two-dimensional images from the LV apical four-, two- and three-chamber views at frame rate of 58—69 frames/s (average of 67.4 ± 6 frames/sec) for at least three cardiac cycles in the resting state. The images were copied, saved and exported to the workstation in Digital Imaging and Communication in Medicine (DICOM) format on a mobile hard disk for offline analysis.
All two-dimensional images and measurements were performed according to American Society of Echocardiography guideline [22], and all parameters were averaged over three consecutive cardiac cycles. The SBP and DBP of the participants were measured using a brachial artery cuff.
Two-dimensional speckle tracking image analysis
The stored images were analysed offline by an Echo PAC BT203 (GE Vingmed Ultrasound, Horten, Norway) workstation. The durations of the LV isovolumic systole and ejection period were first determined according to the opening and closing of the aortic and mitral valves, and then the automatic functional imaging mode was selected. The endocardium of the LV long axis, four-chamber and two-chamber view were depicted from the level of the mitral valve annulus to the LV apex in sequence. The software automatically tracked the endocardium and epicardium of the left ventricle. The intracardiac and epicardial wrap were manually adjusted to ensure satisfactory tracking. Finally, Approve was clicked, and the blood pressure measured by the bronchial artery cuff was entered into the MW menu to obtain the LV pressure–strain loops (PSL), GLS and NIMWI. The NIMWI parameters included GWI, GCW, GWW and GWE, as shown in Fig. 4.
Statistical analysis
All statistical analyses were performed using SPSS version 25.0 (SPSS Inc., IBM, Chicago, USA). Continuous variables were expressed as mean ± standard deviation if normally distributed. Independent-sample t-test was used to compare continuous variables between the uremic group and normal group. The Chi-squared test was used to compare count data. Correlation between continuous variables was carried out using Pearson’s or Spearman’s correlation coefficient as appropriate.
ROC curves were performed to determine the optimal sensitivity and specificity of MW parameter. The AUC was calculated to assess the performance of MW parameter for the detection impaired myocardium in uremic patients. According to the Youden index, the best threshold of each tested MW parameter was estimated. Ten subjects were randomly selected, and two observers used infra-class correlation coefficients to carry out repeatability tests for the myocardial work parameters GWI, GCW, GWW and GWE. A P-value < 0.05 was considered statistically significant.