The Impact of A Reduction of Dialysate Calcium on Diastolic Function in Patients on Peritoneal Dialysis: A Prospective Study

Abstract

Peritoneal dialysis (PD) allows continuous therapy that allows the maintenance of uid, toxins and electrolyte balance, leading to the preservation of residual kidney function [5]. However, several PDassociated factors lead to an increased cardiovascular risk, mainly due to glucose overload, causing changes in the lipid pro le, hyperinsulinemia and formation of advanced glycation end products [6]. The complex interaction between these risk factors causes a high prevalence of abnormalities of cardiac structure and function in patients with CKD [7].
Reliable and accessible tools that can early detect changes in cardiac function may facilitate the understanding and management of CVD in patients with CKD. In daily clinical practice, left ventricular ejection fraction (LVEF) is calculated with two-dimensional echocardiography, by measuring left ventricular end-diastolic and end-systolic volumes according to the biplane Simpson's method [8]. Despite a high prevalence of cardiovascular insults and progressive symptoms of heart failure, less than 15% of patients in dialysis have detectable systolic dysfunction [9,10]. Hence, the majority of CKD patients will develop heart failure with preserved left ventricular function and different degrees of diastolic function [11][12][13][14][15].
Calcium (Ca) is a key messenger in the contraction of muscle, including the myocardium. At the myocyte level, changes in Ca homeostasis cause an increased diastolic cytosolic Ca, which leads to abnormalities in both active relaxation and passive stiffness. Ca is the central element of excitation-contraction coupling, so that hypercalcemia impairs the relaxation [16]. It has been described an association between serum Ca and ventricular end-diastolic diameter [17], and other parameters of myocardial relaxation [18].
In patients on hemodialysis, high dialysate calcium concentration (d [Ca]) causes an impairment in the diastolic function [19] and leads to a high incidence of myocardial infarction [20]

Study Population and design of the study
This was a single-center study in which patients were evaluated at baseline, using a standard 1.75 mmol/L d [Ca] and after switch to the 1.25 mmol/L d[Ca] for at least 3 months but no longer than 6 months. All prevalent patients on PD in the Institution were invited to participate. The inclusion criteria were patients >18 years old who were on PD for at least 6 months. Exclusion criteria were active episodes of decompensated heart failure or acute coronary syndrome, atrial brillation or another arrhythmia, and poor echocardiographic image quality. A total of 26 patients were selected after screening. After exclusions due to arrhythmia (N=2), cardiac failure (N=2), and refusal to participate (N=3), the nal sample included 19 patients.
The Local Institution Review Board at the Hospital da Clinicas da Universidade de São Paulo (Cappesq# 30284714.0.0000.0068) has approved the study protocol, which was conducted in accordance with the Declaration of Helsinki. All participants provided written informed consent to participate in the study.

Bioimpedance Analysis (Bia)
The multi-frequency segmental BIA was performed using the InBody 720® equipment (Biospace Co, Ltd, Seoul, Korea). The measurement was performed at baseline and 3 months after the intervention.
This device evaluates the amount of intracellular and extracellular body water in the different segments of the body, through the passage of an alternating electric current at different frequencies detected by electrodes that are placed on the ankles and hands. This technique was previously validated (accuracy of 0.5% and repeatability 0.3%) [22] for measurements of body uids and has a close correlation with the gold standard of volume assessment in dialysis patients, as methods of diluting markers (Deuterium and sodium bromide) or magnetic resonance imaging [23].
The procedure was performed with the patient in the supine position. Electrodes were placed on the ankles and hands ( rst and third ngers) according to the manufacturer's manual. The patient was instructed to remove metal objects and to remain in a comfortable position and as immobile as possible.

Echocardiographic Measurements
An experienced cardiologist blinded to the study group performed and analyzed all exams. Twodimensional echocardiography was performed in the left lateral decubitus position in both experimental situations (d[Ca] 1.75 and 1.25 mmol/L). The cardiologist obtained the following echocardiography images: the standard apical 2-, 3-and 4-chamber views. The images were obtained (1.5-3.6 MHz 3 S probe, Vivid I; GE Medical Systems, Sonigen, Germany).
Images and parameters were evaluated according to the American Society of Echocardiography. The LVEF was calculated using Simpson's biplane method. Left ventricular mass index was determined as the ratio of left ventricular mass to body surface area. Left ventricular hypertrophy (LVH) was present when index mass was > 116 g/m 2 for men and > 96 g/m 2 for women [24].
Presence of diastolic dysfunction was determined after evaluating the following parameters: peak E-wave velocity (cm/sec), peak A-wave velocity (cm/sec), wave deceleration time (wave E), and duration of wave A. Diastolic function was them classi ed into 1 of 4 categories: normal, alteration of left ventricular relaxation, pseudo normal pattern, and restrictive pattern (reversible and irreversible) according to previous guidelines established by the American Society of Echocardiography and the European Association of Cardiovascular Imaging [24,25]. DD refers to impaired left ventricular lling capacity due to abnormalities in relaxation or stiffness of the myocardium. The reference ranges for the structural and functional echocardiography variables used to assess DD have been derived from studies employing transthoracic echocardiography parameters.

Statistical analysis
The results are presented as the mean ± SD or median and (25-75) quartiles depending on the normality of the data. Comparisons between continuous variables while using D[Ca] 1.75 and D[Ca] 1.25 mmol/L] were done using paired t-test or Wilcoxon test according to the Gaussian distribution. We used the McNemar test to compare categorical variables before and after the intervention. Correlations between independent variables were tested by Spearman coe cients.

Results
A total of 19 were included in the study and 16 (84,2%) were in automated PD. They aged 55 ± 17 years, (11 women, 57.9%), and were on PD for a median time of 10.4 (3.7, 17.0) months. Primary kidney disease was chronic glomerulonephritis in 12 patients (63.1%), hypertension in 2 patients (10.5%), diabetes in 4 patients (21.1%), and adult polycystic kidney disease in 1 patient (5.3%). Previous cardiovascular events were identi ed in 3 patients (15,8%) and most of the patients were treating systemic hypertension (68.4%). At the study entry systolic and diastolic blood pressure were 128.8 ± 21.3 and 76.0 ± 12.5 mmHg, respectively. Anti-hypertensive medication in use included β-blocker (42.1%), calcium antagonist (3,8 %), and angiotensin-converting enzyme inhibitors or block aldosterone receptor blocker (ACE/ARB) (57.9%). Residual diuresis was 1,307 ml (from 400 to 2,160 ml). One patient was anuric. During the follow-up, the doses of antihypertensive medication did not change, and blood pressure levels remain stable for both systolic (p=0.468) and diastolic (p=0.363) levels. patients, which led to a decreased in the percentage and in the grade of DD, as shown in Table 2 and  Sidney Ringer discovered in the 1880s that Ca is essential for the heart's contraction, and many roles of Ca in the cell continue to astonish us. Maintaining serum Ca at physiological levels is especially important for several biological processes, including vascular tone, muscle contractility, and stimulus conduction activity in the nervous system. The myocardium uses Ca in a positive feedback loop to trigger contraction. Indeed, Ca is a crucial element in cardiac systolic and diastolic function [6] and hypercalcemia impairs especially myocardium relaxation [16]. Therefore, there might be a link between high Ca concentration, worse ventricular relaxation, DD, and the development of clinical manifestation of heart failure with preserved LVEF.
The prevalence of DD is becoming more frequent than systolic dysfunction [26]. According to a recent systematic review, DD affects approximately 36% of the population older than 60 years [27]. Importantly, not all patients with DD develop clinical heart failure. Diastolic heart failure occurs more often in heart failure with an LV ejection fraction of more than 50% [28]. DD is associated with a 3.53-fold higher risk of combined events of MACE and death and a 3.13-fold increased risk of death [26]. Patients with CKD have a high burden of cardiovascular risk factors closely related to accelerated atherosclerosis, left ventricular dilatation with hypertrophy, systolic dysfunction, and high left ventricular lling pressure [29]. In line with these ndings, most of our patients were hypertensive and had LVH and, although EF was normal in 63.15%, DD was found in 78.9% of cases.
Pathophysiological changes that lead to DD have a deleterious impact on cardiac function. Hypertension, obesity, hypercholesterolemia, and diabetes are associated with systemic in ammation, myocardial oxidative stress, and coronary microvascular dysfunction, common risk factors for cardiovascular disease, contributing to myocardial stiffening and left ventricular DD [30]. As explained by Ogawa and Nitta [31], the central mechanism of left ventricular DD is LVH with myocardial interstitial brosis, which induces myocardial stiffness and impairs heart function during diastole. In patients with CKD, congestive heart failure is caused by LVH due to arterial hypertension and chronic anemia. Left ventricular DD causes an increase in left ventricular lling pressure, which may lead to pulmonary congestion. The severity of CKD is the most independent predictor of elevated left ventricular lling pressure and might be responsible for systolic and diastolic dysfunction in patients with CKD not on dialysis [32]. The prevalence and severity of LVH increase as the CKD progresses, according to Levin et al. [33]. Structural changes combined with anemia and hyperparathyroidism promote maladaptive LVH, leading to systolic and DD [34].
Echocardiography, a non-invasive technique, allows the assessment of multiple indices of diastolic function, with good concordance with invasive hemodynamic monitoring [29,35]. However, no single echocardiographic parameter is considered su ciently accurate and reproducible to establish the diagnosis of DD [36]. DD is di cult to characterize, and refers to abnormal mechanical properties of the myocardium, including abnormal LV diastolic distensibility, impaired lling, and slow or delayed relaxation, regardless of whether the EF is normal or depressed and regardless the presence of symptoms [37]. Since DD implies that the myo brils do not rapidly or entirely return to their resting length, the ventricle cannot accept blood at low pressures [34]. In this situation, the ventricular lling is slow or inadequate unless there is an increase in the atrial pressure [38]. Consequently, there is an increased dependence on lling through the atrial contraction and higher atrial pressures to maintain lling or cardiac output [38]. These parameters were improved in our patients after a reduction in the d[Ca], suggesting that lower d [Ca] improved diastolic function in patients on PD. During diastole, the left ventricle, the left atrium, and the pulmonary veins form a common chamber continuous with the pulmonary capillary bed [39]. In late diastole, the ventricle is most compliant and easily distensible, offering minimal resistance to ventricular lling over a normal volume range [39]. The active phase is myocyte dependent and relies on the rapid decline in [Ca2+] at the beginning of diastole, leading to dissociation of the thick and thin laments. In the subsequent passive phases of diastole, the pressure gradient distends the ventricle [40]. Our results need to be interpreted considering some limitations: it was a single-center study that included a small number of patients, with a short follow-up. Although the precise prevalence rate of diastolic heart failure in PD patients is unknown, a higher prevalence of left ventricular diastolic heart failure in patients with CKD than in the general population is expected in light of the occurrence of in ammation, uid overload, hypertension, renin-angiotensin-aldosterone system activation, and LVH [43]. Strengths of our study rely upon having included patients on automatic PD, the most applied technique nowadays, echocardiography was performed by the same expert observer, and the same researcher followed all patients during the study.

Conclusions
In conclusion, we demonstrated that low d Consent to participate statement: All participants provided written informed consent to participate in the study.

Consent for publication
Not applicable in the declarations Availability of data and materials All data supporting the ndings of this study are available within the article. Raw data are available at https://www.scidb.cn/en/datalist.

Competing interests
The authors have no con icts of interest to declare.
Funding RMAM and RME are supported by CNPQ, Conselho Nacional de Desenvolvimento Cientí co e Tecnológico (grant numbers 303545/2020-8 and 304901/2021-0, respectively). This nancial support had no role in study design; collection, analysis, and interpretation of data; writing the report; and the decision to submit the report for publication.

Authors' contributions
The authors contributions were as follows: 1. Conception or design, or analysis and interpretation of data, Cross tabulation of lling pressure and diastolic function at baseline and post-intervention.