Concordance of absolute and relative plasma volume changes and stability of Fcells in routine hemodialysis

Abstract

Central hematocrit (H) measurements are currently used to track the degree of ultrafiltration-induced hemoconcentration with the aim to detect and prevent excessive intravascular fluid depletion during hemodialysis (HD). Failure to maintain hemodynamic stability is commonly attributed to the misinterpretation of H caused by an unaccountable increase in Fcells , the ratio of whole-body hematocrit to H. It was the aim to examine Fcells under everyday conditions in a group of stable HD patients. Absolute plasma volume (Vp ) and H were concomitantly measured during routine HD in the extracorporeal system in hourly intervals by noninvasive and continuous technology (CritLine-Instrument-III) and indocyanine green dye dilution to derive relative plasma volumes from Vp and H (RPVp , RPVH ), respectively, and to calculate Fcells . Thirteen patients were studied during two midweek treatments (n = 26). Both absolute Vp (P < 0.05) and relative plasma volumes RPVH (P < 0.001) decreased during HD. Vp at any time point was positively correlated to RPVH (r = 0.52). Moreover, relative plasma volumes RPVH and RPVp determined by independent techniques were identical and showed negligible bias (-0.2%) but considerable limits of agreement (-15.6% to +15.3%). Fcells was stable and in the range of 0.9 ± 0.05 throughout HD and not different from the value assumed at the beginning of HD. Although Fcells remains constant in patients on routine dialysis and relative plasma volumes (RPVH and RPVp ) determined by independent techniques are therefore comparable, the variability of experimental conditions during dialysis and the limited accuracy of absolute volume measurements using available technology continues to complicate the ultrafiltration control problem.

Keywords: Ultrafiltration; hematocrit; monitoring; plasma volume.

Fig. 1. Plasma volumes

Absolute (V_p, left panel) and relative (RPV_t,, right panel) plasma volumes at four time points (t₁, t₂, t₃, and t₄). Bars represent average values. * p<0.01, different from baseline at t₁ ** p<0.001, different from baseline at t₁

Fig. 2. Comparison of techniques

Identity plot (left panel; linear regression shown as broken line) and Bland-Altman plot (right panel; bias and limits of agreement shown as broken lines) of relative plasma volumes determined from hematocrit changes (RPV_H, Eq. 4) or from plasma volumes (RPV_p, Eq. 6). For parameters of linear regression see Tab. 4.

Fig. 3. Fraction of cells

Fraction of cells (F_cells) at times t₂, t₃, and t₄ assuming an initial F_cell=0.9 at time t₁. Left panel: individual and average data indicated by bars. Right panel: Average data from repeated measurements obtained in each patient.

Fig. 4. Comparison of relative volumes

Identity plots of relative plasma (RPV_H) and blood volumes (RBV_H) calculated from hematocrit changes (Eq. 4) shown for experimental (left panel) and simulated (right panel) data. Simulated data (n=100, open symbols) were derived from evenly distributed random baseline hematrocrit (30<H₀<45%) and an evenly distributed random hematocrit increase (0<(H-H₀)<5%). Linear regressions are shown as broken lines. For parameters of linear regressions see Tab. 4.

Fig. 5. Relative plasma volume and fraction of cells

Simulation of relative plasma volumes (RPV_H) derived from central hematocrit (H_c) according to Eq. 5 assuming a constant F_cells=1 (i.e. single-pool characteristics for hematocrit, full line), a constant F_cells=0.9 (i.e. steady double-pool characteristics for hematocrit, broken line), or a variable F_cells increasing from 0.9 to 1 (i.e. variable double-pool characteristics for hematocrit, dotted line), for baseline hematocrit H₀ and a hematocrit increase representative of this study.