[Factors which influence phosphorus removal in hemodialysis].

BACKGROUND The sustained elevation of phosphorous among patients with end-stage renal failure is associated with elevated mortality rates. Phosphate binding agents are usually necessary to control serum phosphate levels. Phosphate removal during dialysis is limited largely due to the intracellular location of most inorganic phosphorous. The membrane surface, the frequency and the duration of therapy have proved to be very important factors in the serum phosphate control. THE AIM of our work is to investigate the influence on phosphate removal of factors that normally participate in the haemodialysis session: Plasma phosphate level (Php), treatment duration, membrane surface, high or low-flux membranes, the vascular access, dialysate flux , the volume of blood passing through the dialyzer (L) in each dialysis session and the blood flow during the first hour of dialysis. On 16 patients, we also had the possibility of comparing phosphate removal with 1.8 m(2) high-flux haemodialysis, 1.8 m(2) on-line hemodiafiltration and the on-line technique with the new Helixone dialyzer Fresenius Fx100. METHODS 108 haemodialysis patients, 62% men, 38% women aged 21-82 years (61+/-14;mean+/-sem),) were selected for the study. Mean treatment time 4.14+/-0.41 hours (range 3.5-5 hours). The vascular access was an arterio-venous fistula in eighty five (78%) and a double lumen tunnelled catheter 23 (22%). Patients were studied under their normal every day conditions. High-flux membrane was used by 31 (30%) patients and low-flux membrane by 77 (70%). Membrane surface was: 1.7 m2:17 (16%); 1.8 m2:77 (71%); 2,1 m2:14 (13%). Dialysate flux was: 500 ml/min. 55 patients; 700 ml/min: 53 patients. In 16 out of 108 patients we had the possibility of using on-line hemodiafiltration with ultrapure bicarbonate-buffered dialysate. Phosphate mass removal (MPO4) was calculated using the formula:MPO4=0.1 t-17+50 Cds 60+11Cb 60 (1), where t is treatment time in minutes, Cds60 and Cb60 are phosphate concentrations in dialysate and plasma measured at 60 min from the beginning of hemodialysis in mg/dl, and MPO4 is the estimated phosphate removed in mg/treatment. RESULTS We found a good correlation between phosphate removal and serum phosphate levels (p=0.01), but not with the membrane surface or treatment duration. Phosphate removal was 640+/-180 mg/session with low-flux membrane and 700+/-170 mg/session with high-flux membrane (p=0.280). The MPO4 was 720+/-190 mg/treatment in patients with a AV fistula and 620+/-180 in patients with a tunnelled catheter (p=0.023). We found a good correlation between phosphate removal and the volume of blood (L) that passed the dialyzer in each session (r=0.001) but we did not find a correlation between phosphate removal and KT/Vurea, the dialysate flux or the ultra filtration. On-line technique did not increased the MPO4(733+/-280 mg, p=0.383). The on-line technique with the new dialyzer (Fresenius Fx100), increased the phosphate removal to 759+/-199 mg/session (p=0.057). CONCLUSION Phosphate removal during dialysis is influenced by Plasma phosphate levels, the volume of blood that passed the dialyzer and the vascular access. Uniformity on time and membrane surface could explain the abs cense of influence in our case. The ultra filtration, dialysate flux, membrane permeability or on-line hemodiafiltration does not influence the phosphate removal. The new membrane helixone with 2,1 m2 (Fresenius Fx100) increases phosphate removal probably because the membrane surface is higher.