Discussion
Cardiovascular complications have a serious impact on the survival and quality of life of uremic patients, and left ventricular diastolic dysfunction is its most prominent feature. Echocardiography needs to find subclinical damage of left ventricular function early. 2D-STI is a new echocardiography technique, which can provide objective quantification of cardiac function on the basis of analyzing myocardial deformation.
Previous studies have confirmed [1, 13] that left ventricular global longitudinal strain (LVGLS) can accurately evaluate left ventricular systolic function, while GLS is affected by volume load[14]. Recently, experts recommended[15-17] to add left atrial strain to the evaluation of left ventricular diastolic function. Therefore, speckle tracking imaging was used to evaluate left atrial strain function before and after dialysis in ESRD patients, and to explore the influence of volume load on left atrial function.
During the cardiac cycle, LA has three main functions: the reservoir function when blood fills LA in systole, constrit function corresponding to passive LV filling in early diastole, and the active contractile function in late diastole.
Reservoir function is the ability of left atrium to receive blood flow from pulmonary vein during ventricular systole and mitral valve closure. It is determined by atrial stiffness and compliance, and is also affected by left ventricular contractile force. After hemodialysis, the strain value of LASr decreased, which proved that the blood entering the left atrium from pulmonary vein corresponding to the decreased volume.
Conduit function is the difference between reservoir function and contractile function. In the early stage of ventricular diastole, blood flows directly into left ventricle through left atrium, which is mainly determined by left atrial compliance, and is also closely related to left ventricular diastolic tension and stiffness. After hemodialysis, the strain value of LAScd also decreased, indicating that the blood flowing through left atrium and entering left ventricle decreased in early diastole.
The contractile function is the plateau strain value in the late diastolic period of the ventricle, which reflects the active pump function of the left atrium. In the late diastole, the left atrium plays an autonomous booster pump function, and the atrial muscle actively contracts, pumping out the remaining blood and increasing the left ventricular filling [18]. It plays a vital role in maintaining the best cardiac circulation. Once the left ventricular myocardium is damaged and the compliance is decreased, in order to maintain sufficient cardiac output, the left atrium will do compensatory work to supplement the left ventricular filling[15]. In this study, after hemodialysis, while blood volume decreased, the cardiac output was still in the normal range, the passive function of atrium (LASr, LAScd) decreased, but the pump function of atrium itself (LASct) did not change significantly. Furthermore, uremic patients are in a state of volume overload for a long time, and the cardiac output after hemodialysis is more in line with their physiological needs, so there is no need for extra work of the left atrium.
Consistent with Frank-Starling law: in a certain range, when the volume load increases, the myocardial fiber adjusts its initial length, which improves the myocardial deformation ability, that is, the myocardial contractility increases. In this study, the volume was overloaded before hemodialysis, and the LVGLS parameters (subendocardium, midmyocardium and subepicardium) were close to those of the normal control group. After the excess fluid was removed, the heart cavity became smaller, the myocardium returned to physiological length, and the LVGLS decreased. Consistent with Breetveld N-M et al. [19], under the condition of normal myocardial contractility, left ventricular myocardial fibers reshape with the change of volume load, and the absolute value of myocardial strain decreases with the decrease of volume load after hemodialysis. However, Sun MM et al.[20] put forward different viewpoints. The incidence of myocardial damage in patients with renal insufficiency is higher, and the compensatory ability of myocardium is weaker than that of normal people. A large amount of fluid is removed, and myocardial blood perfusion is reduced, which can cause myocardial cell injury and stunning, resulting in a decrease in left ventricular strain.
It may be possible to detect subclinical cardiac changes in patients early by strain of left atrium. The thickness of atrial muscle is thinner than that of ventricular, and atrial myocardium is more vulnerable to damage due to long-term deposition of uremic toxins and metabolites and retention of water and sodium. In this study, the atrial strain parameters (LASr, LAScd, LASct) before hemodialysis were lower than those in the control group. It can be found that ESRD patients are overloaded with volume, and the damaged left ventricular myocardial fibers are passively stretched. At this time, the left ventricular strain value has not decreased, and the atrial strain parameters have already decreased. After hemodialysis, the pump function of atrium itself has no obvious change, which further indicates that the passive function of atrium (LASr, LAScd) is greatly affected by volume load, while the active deformation (LASct) is less affected.
Consistent with previous studies, traditional Doppler parameters of diastolic function and left ventricular globe strain parameters depend on volume load [21]. When these parameters are still normal, LA strain parameters evaluated by STE can find subclinical LA dysfunction in ERSD patients early, provide more quantitative indicators for cardiac function evaluation and prognosis evaluation of such patients, and find subclinical myocardial damage in time, so as to obtain more timely treatment and improve clinical results.
Limitations : 1. She is a study of left atrial function with single center; 2. The patients with end-stage renal disease in this study had left ventricular wall hypertrophy and left atrial strain had decreased. It is necessary to to expand the size to study the changes of left atrial function in the early stage of renal insufficiency.
Author contributions
Z B-Y and L Y-P contributed the study conception and design. Y Y contacted with the patients for the echocardiography. M Y-X and W L wrote the manuscript. M Y-X and W L performed the statistical analysis. M Y-X and W L were responsible for image off-line analysis. Z B-Y and L Y-P reviewed and edited the manuscript before submission. All authors read and approved the final manuscript.