It is readily accepted that the majority of patients attending for outpatient haemodialysis are volume overloaded and require ultrafiltration during a haemodialysis session. In our study of asymptomatic stable peritoneal dialysis patients attending for routine assessments of peritoneal dialysis adequacy and transport status (who were not thought to be clinically volume overloaded), volume status assessed by MF-BIA was similar to that of patients prior to haemodialysis, supporting earlier smaller studies [16
]. Thus, despite greater urine output and urinary sodium excretion peritoneal dialysis patients are similarly volume overloaded as haemodialysis patients.
The results of MF-BIA depend upon the resistance and reactance to the passage of an electrical current, and our MF-BIA device has been validated in both healthy controls [17
] and haemodialysis [14
] and peritoneal dialysis patients [19
]. MF-BIA is affected by body composition which changes with age, sex and race [20
], and as such it has been suggested that volumes should be corrected for height [21
] or body surface area [22
]. Volume overload in incident peritoneal dialysis patients is known to adversely determine outcome; however, it is not generally recognised that peritoneal dialysis patients are chronically volume overloaded [23
]. However, we did not find any differences in ECW, or when corrected for height or body surface area with the haemodialysis group, either before or after haemodialysis. Other reports have reported that ECW is greater in peritoneal dialysis patients compared to after dialysis [24
]. This was a smaller study of predominantly Caucasoid patients with fewer diabetics. However, an increased ECW/TBW ratio can also be caused by a reduction in TBW due to a loss of intracellular water (ICW). Nearly all our peritoneal dialysis patients used icodextrin, which can increase plasma osmolality [25
], due to the accumulation of metabolites [26
], and could therefore potentially lead to water removal from and reduction in ICW. Another possible cause of a reduction is ICW protein energy malnutrition [27
]. However, in our study both NT-proBNP values, a marker of extracellular volume expansion [15
], and ECHO cardiography dimensions were not different between the peritoneal and haemodialysis groups, in keeping with the changes in ECW/TBW which were due to ECW expansion, and not ICW loss. This is supported by body composition which was similar between the groups in terms of skeletal muscle and fat mass. As such, the ECW/TBW in the peritoneal dialysis patients cannot be explained by a loss of ICW and supports ECW expansion.
The lower serum albumin and sodium in the peritoneal dialysis group could be due to volume overload and dilution, and one previous study showed that serum albumin concentrations increased following deliberate ultrafiltration [28
]. The lower sodium, however, could also be an artefact of laboratory error [29
] due to the usage of icodextrin [26
As expected, blood pressure fell after haemodialysis [31
], but pre-dialysis blood pressure was not different compared to the peritoneal dialysis cohort. However, both post-dialysis mean arterial pressure and diastolic blood pressure were lower than those in the peritoneal dialysis group. Both groups had similar antihypertensive medication prescription, although more peritoneal dialysis patients were prescribed loop diuretics, reflecting greater urine volumes. In keeping with blood pressure, transthoracic echocardiography assessments of cardiac size were similar between the groups, as were NT-proBNP levels. It could be speculated that as NT-proBNP is degraded by the kidney, then levels should have been lower in the peritoneal dialysis group due to better residual renal function.
After peritonitis [32
], ultrafiltration failure and clinically apparent volume overload are the next most common causes of peritoneal dialysis technique failure [33
]. Our study shows that stable asymptomatic peritoneal dialysis patients are equally volume overloaded when compared to haemodialysis patients. As such, this chronic volume overload may account for the prevalence of hypertension and increased cardiovascular death rate in this group of patients [33