P1401THE REMOVAL EFFICIENCY OF PHOSPHATE FROM INTRACELLULAR FLUID DURING HEMODIALYSIS

Abstract

Abstract Background and Aims In hemodialysis (HD) patients without residual renal function, almost all of phosphate (P) absorbed through intestine is eliminated with HD. To avoid hyperphosphatemia, which is major risk for mortality in HD patients, reduced P absorption and /or improved P removal efficiency should be required. The P elimination during HD from intracellular fluid (ICF) remarkably differs from that from extracellular fluid (ECF). Because the total P removal is too complicate to analyze, few studies about P removal efficiency have been performed. In this study, we tried to separately estimate the amount of P removal from ICF and ECF. Method Fifty-eight patients undergoing 4-hour HD with BMI 22±3 were enrolled this study. ECF and ICF volumes were considered respectively as 20% and 40% of body weight (BW). The amount of urea nitrogen (UN) removal (Run) was calculated using the values of serum UN concentration (UN0, UN4) and total body fluid (60% of BW) at pre and post HD as 0.6(UN0 x pre BW – UN4 x post BW). The amount of intradialytic total P removal (Rp) was calculated using the formula previously reported. At starting phase of HD, P is considered to be removed only from ECF, and from ECF and ICF at later stage. In initial hours, when P is removed only from ECF, serum P concentration change exponentially (P = KptP0) as serum UN concentration (UN = KuntUN0). (Where, Kun and Kp are exponential coefficient of UN and P respectively, t is time (min), P0 is serum P concentration before HD). If P outflow from ICF is disregarded, the exponential change in P persists, and serum P concentration at the end of 4-hour HD is Kp240P0. Consequently, the amount of P removal from ECF (Rp(ex)) was calculated as 0.2(P0 x preBW - Kp240P0 x postBW). The exponential coefficient in P change was reported to be 0.997788 times of that in UN. The amount of P removal from ICF (Rp(in)) was calculated as difference between Rp and Rp(ex). Each removal efficiency was calculate as Run/UN0, Rp(ex)/P0 or Rp(in)/P0. Intradialytic removal of P from ECF and ICF were compared with that of UN. Regression analysis was performed on 24 factors which might affect the efficiency. The relationship between drug administration and the removal efficiency was investigated as for 22 drugs. Results UN removal and P removal from ECF were closely related. Run and Rp(ex) had positive correlation (0.564, p<0.001). And Run/UN0 also correlated positively to Rp(ex)/P0 (R=0.970, p<0.001). Rp(ex)/P0 and P0 had a positive correlation (R=0.334, p<0.01) as well as Run/UN0 and UN0 (R=0.382, p<0.01). P removal from ICF showed different pattern. In comparison between P removal from ECF and ICF, removal amount showed positive correlation (R=0.634, p<0.001), but removal efficiency showed no correlation(R=0.006, ns). Notably, Rp(in)/P0 and P0 had negative correlation (R=0.315, p<0.02). Rp(in) accounted for 44.6±6.2% of Rp. On regression analysis concerning the 24 factors, only P0 and its confounding factors showed correlation with Rp(ex), Rp(in), Rp(ex)/P0 or Rp(in)/P0. Rp(ex)/P0 or Rp(in)/P0 were not affected with administration of 22 investigated drugs. To exclude the influence of P0 on Rp(in)/P0, adjusted Rp(in)/P0 (removal efficiency of P from ICF not affected by P0) was calculated. Investigation on iron containing P binders and ion exchange resins revealed each drug groups ameliorated adjusted Rp(in)/P0. Conclusion This is the first report to analyze separately P removal from ECF and ICF during HD. Increased UN removal efficiency results in increased the removal efficiency of P from ECF, but did not improved that from ICF. Rp(in) accounts for about half of Rp. For improving total P removal efficiency, removal efficiency of P from ICF should be increased. Some drugs were suggested to increase removal efficiency of P from ICF.