Oxidative stress, endothelial function, carotid artery intimal thickness and their correlates among chronic peritoneal dialysis patients

Introduction

Cardiovascular disease (CVD) is leading cause of mortality amongst patients of chronic kidney disease (CKD) including those undergoing dialysis.[12] However, cause of this increase in CVD in these patients is not explained by traditional risk factors alone[3] and various nontraditional factors including oxidative stress (OS) and endothelial dysfunction have been implicated.

Endothelial dysfunction is an early marker of atherosclerosis.[4] Flow-mediated dilation (FMD), is reliable noninvasive technique to assess endothelial function, which correlates very well with coronary endothelial function.[5]

OS is defined as tissue damage resulting from an imbalance between oxidant compounds and antioxidant defense. OS, by causing NO breakdown,[6] can be a major mechanism of endothelial dysfunction.

Various circulating uremic toxins can be responsible for these abnormalities of endothelial function and OS. Correction of uremia by dialysis should therefore improve OS and endothelial function. However, loss of antioxidants and soluble factors in dialysate, use of bioincompatible fluids and increased advanced glycation end products could increase these abnormalities.

Carotid intimal medial thickness (CIMT) is a reliable, reproducible, noninvasive method of detecting and monitoring progression of atherosclerosis and reflects to a high degree the atherosclerotic situation of the coronary artery.[7]

The present understanding of these important nontraditional cardiovascular risk factors in peritoneal dialysis (PD) patients and their inter-relationship is limited due to small patient numbers and limited number of studies which have simultaneously assessed all these parameters.The present study was designed to assess endothelial function, OS and CIMT in stable patients on PD and to examine their correlates and the relationship if any between them.

Materials and Methods

This cross-sectional study was conducted at a tertiary care hospital in north India. Informed consent was obtained from all participants and the Institute Ethics Committee approved the study. All patients who were on PD for >6 months and were aged 18-65 years were screened for the study. Fifteen healthy and age-matched subjects who were being worked up as renal donors were recruited as controls.

Subjects who smoked, had an episode of peritonitis or received intravenous iron within 6 weeks were excluded. Patients who had evidence of malignancy, autoimmune disease or chronic liver disease, active infection, history of cardiovascular or peripheral vascular disease or were taking drugs that cause release of nitric oxide were also excluded. Forty-five patients were screened and 38 patients who consented and satisfied all criteria were recruited.

Seven days prior to evaluation angiotensin converting enzyme inhibitors (ACE-inhibitors), lipid lowering drugs, angiotensin receptor blocks and folic acid were withheld and appropriate antihypertensive agents were substituted. All subjects were advised to refrain from caffeine containing drinks for at least 12 hours before evaluation.

Detailed demographic data, duration of dialysis (DOD), urine output (UO), total weekly Kt/V urea and biochemical and lipid profile were noted in all patients.

Results

Thirty-eight patients on PD and 15 age and sex-matched controls were studied. Mean age of study population was 49.3 ± 11.6 years, with 52.5% (20 patients) being male. Cause of renal failure was diabetes in 13 patients (34.1%), chronic interstitial nephritis and chronic glomerulonephritis in seven patients each, ischemic nephritis in four, miscellaneous diseases in three and in four patients cause of renal failure was unknown. Thirty patients were on CAPD, while remaining eight were on CCPD with automated PD at night and day dwell with Icodextrin. Majority of CAPD patients were performing daily 3, 2-liter exchanges of standard lactate-based glucose solutions (Ultra Bag Baxter, Manesar, India), while four patients were performing daily 3, 2.5-liter exchanges of same solution.

Mean duration of PD was 19.4 ± 11.8 months (median 15.5, range 6-44 months), mean UO was 508.2 ± 422.9 ml/day (median 425, range 0-1400 ml/day) and mean weekly Kt/V urea was 1.73 ± 0.24 (median 1.7, range 1.3-2.2).

Table 1 shows the important demographic and biochemical parameters in PD patients and their age and sex-matched controls. As expected, patients on PD had higher systolic and diastolic blood pressure, serum creatinine, phosphorus, LDL cholesterol, LDL/HDL cholesterol ratio and triglycerides and lower hemoglobin, serum albumin, calcium and HDL cholesterol. Patients on PD had impaired endothelium-mediated dilatation as indicated by lower absolute and percent change in diameter from the baseline after occlusion but preserved endothelium independent dilatation [Table 2]. Levels of pro-oxidant TBARS were significantly higher, while levels of antioxidant capacity FRAP and vitamin C was significantly lower in PD patients as compared to controls [Table 3]. CIMT was also significantly higher among PD patients as compared to controls. There parameters were not significantly different between diabetics and nondiabetics [Table 4].

Table 1 Important biochemical parameters and special investigations in the control and PD population

Table 2 Parameters of flow-mediated dilatation of brachial artery in the control and PD population

Table 3 Pro-oxidant, antioxidant parameters and the carotid intimal media thickness in the control and PD population

Table 4 Endothelial-dependent dilatation, pro-oxidant, antioxidant parameters and the carotid intimal media thickness in the nondiabetic and diabetic chronic peritoneal dialysis patients

On Pearson's correlation CIMT had significant negative correlation with Kt/V urea (r = –0.371, P = 0.05), hemoglobin (r = –0.356, P = 0.05), EDD (r = –0.474, P = 0.01) and vitamin C (r = –0.456, P = 0.01). EDD of the brachial artery had significant positive correlation with residual UO, Kt/V urea, hemoglobin and vitamin C (r = 0.393, 0.405, 0.385 and 0.408, respectively, P < 0.05 for all). TBARS correlated positively with DOD (r = 0.418, P = 0.012) and negatively with hemoglobin and FRAP level (r = –0.462 and –0.45, respectively, P = 0.05). Vitamin C correlated positively with serum albumin (r = 0.352, P = 0.003) besides CIMT and EDD. While FRAP correlated positively with only serum albumin (r = 0.694, P = 0.05). When stepwise multiple regression model was applied with all variables having significance of <0.25 for a particular study parameter [Table 5], CIMT was predicted only by hemoglobin; EDD by hemoglobin and residual UO; TBARS by hemoglobin and DOD; vitamin C by EDD and FRAP was predicted by serum albumin.

Table 5 Stepwise multiple regression model of study parmeters with all variables having significance of <0.25

Discussion

Accelerated atherosclerosis is important cause of increased morbidity and mortality in patients on dialysis. OS and endothelial dysfunction are key factors in the development and progression of atherosclerosis. However, only a few studies have looked at these factors, their correlates and their relationship with atherosclerosis simultaneously among patients on PD. We in this study have looked at OS, endothelial function and their correlation with structural atherosclerosis as measured by CIMT in 38 stable chronic PD patients. Our patient population is unique as being from an emerging economy where around two-third of our patients pay for the therapy themselves, the cost of therapy often takes precedence over the quality of dialysis. This is reflected by lower serum hemoglobin and weekly Kt/V urea in our patients as compared to those from the developed world.

We found that patients on PD had higher OS as documented by higher serum concentration of TBARS and lower serum concentrations of antioxidants FRAP and vitamin C as compared to normal controls. We also found no significant difference in these parameters among diabetic and nondiabetic patients and modality of dialysis (CAPD and CCPD). Many authors have similarly reported increased OS among PD patients.[11–16] However, the correlates of increased OS among PD patients are not clearly defined. We found that TBARS correlated positively with DOD and negatively with hemoglobin level, while FRAP correlated positively with serum albumin levels. Similar to our study Ignace et al.,[14] and Sundl et al.,[15] also found a negative correlation of dialysis vintage with lipid peroxidation and AGEs levels in 23 and 37 PD patients, respectively. Kim et al.,[13] also found a direct correlation between total antioxidant capacity and serum albumin, which, however, is not consistent with majority of other studies. Serum albumin in our study was lower than other studies, with 23.4% patients having serum albumin of <3.5 gm/dl and is comparable to 33% of such patients in study by Kim et al. This higher percentage patients having low albumin might have unmasked the association between albumin and total antioxidant capacity. Low serum albumin has been shown to reflect underlying inflammation besides malnourishment and has been shown to be a predictor of long-term mortality in dialysis patients.[17] Although we have not formally assessed the inflammatory status in our patients but higher inflammation in these malnourished patients could be responsible for lower antioxidant capacity in these patients.

We also found that PD patients have markedly impaired endothelial function as documented by impaired FMD of brachial artery (EDD). The nitrate-mediated dilatation (EID) was preserved in these patients indicating that the disease process does not significantly affect the vessel wall. This is supported by majority of studies in literature.[18–21] In addition we found that EDD correlated directly with hemoglobin and residual UO and was unaffected by traditional risk factors like diabetes and lipid abnormalities. Association between EDD and hemoglobin in adult PD patients has not been highlighted previously. It is postulated that anaemia related lower whole blood viscosity reduces physiological laminar shear stress hence decreasing the EDD.[22] Verbeke et al.,[22] found that EDD in HD patients correlated with hemoglobin and partial correction of hemoglobin to 11 gm/dl significantly improved EDD. Dursun et al.,[23] found higher endothelial microparticles, a marker of poor endothelial function in 25 paediatric PD patients, which negatively correlated with hemoglobin. We also found that patients with better residual UO had better EDD. Not many studies have looked at this association before. Wang et al.,[24] reported that every 1 ml/min/1.73 m² increase in residual GFR is associated with a near 50% reduction in all cause mortality in PD patients. This survival advantage seen in patients has been attributed to lower degree of inflammation, malnutrition and calcification and better middle molecule clearances seen in these patients besides better volume status.[25] Our findings suggest another novel way in which preserved residual renal function by improving endothelial function can decrease CV mortality. Recently Cheng et al.,[21] have reported a negative association between index of volume status and FMD. Although we have not formally assessed volume status in our patients we believe patients with higher residual UO would have better volume status.

We also found an increase evidence of structural atherosclerosis in our PD patients as documented by higher CIMT. This is in concordance with some recently reported studies.[26–30] CIMT has even been shown to be raised in children on PD.[31] We found that CIMT had a significant negative correlation with hemoglobin. Although association of hemoglobin with CIMT in PD patients has not been observed by other studies, a negative correlation between the two has been reported in a large study among non-CKD population. Dijk et al.,[32] in 2514 patients who had manifest arterial disease found that for each mmol/L increase in hemoglobin CIMT was found to be lower by 0.03 mm. It has been postulated that a low hemoglobin by inducing an adaptive response in the vessel wall due to increase in blood flow and shear stress would increase vascular remodelling and hence CIMT.

There are some limitations of our study. Interpretation of our study is limited by small sample size and cross-sectional design of study, due to which establishment of cause and effect relationship between these parameters is not possible. Also we have not studied the effect of these parameters on long term patient outcomes. However, our sample size is still larger than many previous studies. We have also studied uniquely the correlation of two important nontraditional risk factors endothelial dysfunction and OS with each other and with structural atherosclerosis. The initial association of the two with CIMT and lack of its association with established risk factors like diabetes, blood pressure and lipid abnormalities should encourage further prospective multicentric trials enroling larger patient population looking at the relationship and effect of these parameters on long-term patient outcomes and possible role of corrective intervention by use of antioxidants.

To conclude, we demonstrate high OS, poor endothelial function and increased structural atherosclerosis among stable chronic PD patients. These parameters are interrelated and are negatively influenced by increased DOD, poor residual renal function, low hemoglobin, serum albumin and small solute clearance. This highlights the need for achieving the target levels of hemoglobin, albumin and weekly Kt/V urea besides preserving residual renal function among PD patients.