Atrial natriuretic peptide: a regulator of transvascular fluid transport in dialysis?

Moreover, a wide variation in the course of relative blood volume during dialysis was previously described by Krepel et al. [5] . In this paper, no statistical relationship could be found between changes in relative blood volume and either inferior caval vein diameter (ICVD) or blood pressure, indicating that the initiation of hypotension may result from defective cardiovascular defense mechanisms. Natriuretic peptides are released by the heart in response to derangements in blood pressure and extracellular volume. Wallin et al. [6] described the high correlation between plasma ANP levels and cardiac output in patients on dialysis. However, they also observed a discrepancy between the reduction in cardiac output and central blood volume during dialysis. In this journal, Yashiro et al. [7] describe the relationship between atrial natriuretic peptide (ANP), ICVD and Lp. They demonstrate a highly signifi cant relationship between ANP levels and Lp values. The observed relationship between plasma ANP levels and Lp raises the question whether the increased ANP levels may have been instrumental in the increased hydraulic permeability of the capillaries. Their approach corrects for a changing plasma protein concentration, but the transcapillary oncotic gradient may decrease due to increased transcapillary escape of protein [8] . Moreover, changes in the transcapillary hydrostatic pressure gradient cannot be excluded as ICVD also correlated with Lp. The determination of dry weight in dialysis has always been elusive, and to this day remains the Holy Grail in dialysis treatment. Various techniques have been propagated to give an estimation of dry weight, including bioimpedance, blood volume measurement, measurement of inferior caval vein diameter and the concentration of various natriuretic peptides. In the event of circulatory underfi lling, due to excessive ultrafi ltration, symptomatic hypotension may ensue. The maintenance of plasma volume is determined by both ultrafi ltration rates and the rate of vascular refi lling from the interstitium. Schneditz et al. [1] have attempted to determine the capillary fi ltration coeffi cient (Lp), that describes net water fl ow (J v ) in Starling’s equation. Several other authors have also used these computations, based on an open two-compartment model, to study the contribution of (changes in) Lp on blood volume preservation during dialysis [2, 3] . Changes in blood volume, estimated by changes in the concentration of circulating blood components such a hematocrit, may be even greater than apparent from relative blood volume measurements. As hematocrit is not homogeneously distributed throughout the body, Mitra et al. [4] compared relative blood volume measurements to the gold standard measurement of plasma volume using indocyanine green dilution techniques. They demonstrated that the difference between systemic and whole-body hematocrit (F-cell ratio) increases progressively during dialysis, indicating a redistribution of the red cell mass. Published online: September 9, 2005