Volume Overload in Dialysis: The Elephant in the Room, No One Can See

Abstract

interests of the authors. However, considering the totality of the evidence, I am less convinced that this marker is superior to other volume markers. First, there are few head-to-head studies comparing BIA to other established markers of volume. Second, there is no range that defines ‘normal’. Unlike blood pressure (BP) or cholesterol, there is no target that has been established. The normal values are obtained from healthy subjects whose body composition differs substantially from those on dialysis. Extrapolating from healthy subjects to those with ESRD on hemodialysis does not appear scientifically valid. Third, there is no study to my knowledge that demonstrates convincingly that compared to probing dry weight without BIA, BIA-guided dry weight reduction results in improved outcomes. Most data provided to support the use of BIA are associative and observational. A large amount of observational data does not prove causation. We therefore have to be cautious in interpreting these data as ‘strongly indicative of fluid overload’. Nearly all markers of volemia suffer from the same shortcomings. These shortcomings include lack of normative data, and data being largely observational and associative in nature. As an example, several observational studies suggest that B-type natriuretic peptide (BNP), NT-proBNP or other natriuretic peptides are associated with volume state among people with ESRD on dialysis [2–10] . However, only one study has been reported to date that relates the change in BNP to an intervention of probing dry weight [11] . In this study, a direct relationship existed at baseline between BNP and interdialytic ambulatory BP. Since interdialytic ambulatory BP is reVolume overload is the invisible threat to the health of dialysis patients. Currently, Medicare incentivizes the measurement of the dose of dialysis (urea reduction ratio) but has no measure of volume. Perhaps this is because the few who can recognize the epidemic nature of volume excess cannot agree on how to define it. Whereas, some have proposed interdialytic weight gain (IDWG) as a marker of volume, the in-depth review of Hecking et al. [1] quite clearly and importantly makes the distinction that IDWG should not be confused with hypervolemia. One has to wonder how IDWG got confused with dry weight in the first place. It appears that epidemiologists decided to use IDWG as a proxy of dry weight because of its ready availability in large databases. IDWG is a marker that is convenient; it can be measured easily and it can be related to outcomes such as all-cause mortality. This analysis was performed and a relationship emerged between IDWG and all-cause mortality. But establishing a relationship between IDWG and mortality does not establish causality. It could be numerous other factors, measured and unmeasured, associated with IDWG which may be causally related to mortality. Factors such as missing or cutting short dialysis, not taking medications as prescribed, and consuming a diet rich in phosphorus and potassium are some confounding factors that may also be associated with IDWG but may be more plausibly related to mortality. The authors discuss several markers of volume excess. Although several exist, one marker, bioimpedance analysis (BIA), is discussed in great detail. This is understandable given both the academic and intellectual property Published online: July 6, 2013 Nephrology American Journal of