Influence of hydration state on plasma volume changes during ultrafiltration

To assess the influence of hydration state on plasma volume (PV) changes during ultrafiltration, 11 clinically normhydrated patients on maintenance hemodialysis were studied intraindividually during 2 hydration states differing by 2-15% of lean body mass (LBM). Plasma volume was measured continually during a 15-min ultrafiltration test including an ultrafiltration of 1% of target weight and during a 45-min follow-up period of blood recirculation through the dialyzer without ultrafiltration. In all patients a maximal decrease in relative PV was more pronounced when there was less hydration and when the grade of hydration correlated inversely with the maximal response of PV decrease (r = -0.68, p < 0.001). The same was observed between the hydration state and the slope of the PV decrease (r = -0.69, p < 0.001). Plasma refilling was estimated in the period following ultrafiltration. The slope of the plasma volume increase correlated with the grade of hydration (r = 0.31, p < 0.05) as did the asymtote of PV (r = 0.4, p < 0.01). It is concluded that the less hydrated a dialysis patient is, the more pronounced will be the fall of the ultrafiltration-induced plasma volume and the less distinct will be the recovery of the plasma volume after the discontinuation of ultrafiltration.