title: The contribution of free water transport and small pore transport to the total fluid removal in peritoneal dialysis.
abstract: BACKGROUND
  Water transport in peritoneal dialysis (PD) patients is across the small pores and water channels, the latter allowing free water transport. The objective of the study was to investigate the contribution of each transport route on transcapillary ultrafiltration (TCUF).
  
  
  METHODS
  Standard peritoneal permeability analyses of 80 stable PD patients were analyzed. Twenty-nine patients were followed longitudinally. Fluid transport through small pores (SPT) was assessed by the amount of transported sodium. Free water transport (FWT) was calculated by subtracting SPT from TCUF. The contribution of FWT and SPT to the TCUF and water transport rates at any time point was computed.
  
  
  RESULTS
  The ultrafiltered volume due to SPT increased gradually during the dwell, while FWT reached its maximum around 3 hours. The rate of FWT decreased continuously during the dwell. SPT decreased during the initial 2 hours and remained stable thereafter. At 60 minutes, the SPT (P < 0.05) and its contribution (P < 0.05) were positively related to the MTAC(creat). The contribution of FWT after 1 hour, but not the absolute amount, showed an inverse relationship. Peritoneal solute transport parameters (P < 0.01) and the contribution of SPT (P= 0.08), but none of the other fluid parameters showed a U-shape with the lowest values in the second year of PD (P < 0.01).
  
  
  CONCLUSIONS
  The dwell courses of water transport suggest that the activity of water channels is dependent and limited by the crystalloid osmotic pressure gradient, while other determinants are important in SPT. The time-course of SPT paralleled that of peritoneal solute transport parameters.
authors.affiliation: Division of Nephrology, Department of Medicine, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. Alenaparikova@hotmail.com
authors.forename: Alena
authors.initials: A
authors.lastname: Parikova
authors.forename: Watske
authors.initials: W
authors.lastname: Smit
authors.forename: Dirk G
authors.initials: DG
authors.lastname: Struijk
authors.forename: Machteld M
authors.initials: MM
authors.lastname: Zweers
authors.forename: Raymond T
authors.initials: RT
authors.lastname: Krediet
mesh_terms: D000328:Adult; D000368:Aged; D020346:Aquaporins / Q000378:metabolism; D001826:Body Fluids / Q000378:metabolism; D015314:Dialysis Solutions / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000378:metabolism* / Q000628:therapy*; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009997:Osmotic Pressure; D010530:Peritoneal Dialysis; D010537:Peritoneum / Q000378:metabolism*; D014462:Ultrafiltration; D014867:Water / Q000378:metabolism*; D014882:Water-Electrolyte Balance / Q000502:physiology
chemical_list: D020346:Aquaporins; D015314:Dialysis Solutions; D014867:Water