NATURE AND RATE OF VASCULAR REFILLING DURING HEMODIALYSIS AND ULTRAFILTRATION

The change of blood volume, of blood and plasma density (rho(b), rho(p)) following a short ultrafiltration pulse (duration: 20 min; mean rate -35 ml/min) within the first hour of hemodialysis was analyzed in 13 hemodynaniically stable patients (30 single measurements). Protein concentration of refilling volume (7 g/liter) was calculated from its density (1009.25 +/- 3.7 kg/m3, at 20-degrees-C) and from the linear relationship between plasma density and protein concentration (c(p)) of uremic plasma samples (rho(p) = 1007.46 + 0.2422 x c(p)). The filtration coefficient (L(p,calc)) determined from a relation derived from Starling's hypothesis was 5.6 +/- 1.4 ml/(min . mm Hg . 50 kg lean body mass); N = 13, mean +/- SD, minimum 3.2, maximum 8.0. A model describing the dynamics of blood and plasma volume was developed. It was fit to on-line measurements of relative blood volume changes by variation of the filtration coefficient and of initial blood volume (L(p,fit), V(b,fit)). The linear regression between V(b,fit), and blood volume determined from anthropometry (V(b,calc)) was highly significant (r = 0.79, N = 30, P < 0.001). Compared to V(b,calc), V(b,fit) was typically increased by 21 +/- 11 %, reflecting a fluid overload at the beginning of the treatment. L(p,fit) was not different from L(p,calc). L(p,fit) significantly increased with blood volume excess. Due to the small but definite protein content of refilling volume, the model accounts for increased blood volume recovery and occasional overshoot of blood and plasma volumes following ultrafiltration.