EFFECT OF AN INCREASED INTRAPERITONEAL PRESSURE ON FLUID AND SOLUTE TRANSPORT DURING CAPD

The effect of an increased intraperitoneal pressure on fluid and solut e transport was studied in eight stable CAPD patients. Two permeabilit y tests of two hours each with continuous registration of the intraper itoneal pressure were performed while patients were in supine position . The intra-abdominal pressure was raised by means of a tightening gir dle with inflatable cuffs in one of the experiments. Intraperitoneally administered dextran 70 was used as a volume marker in order to deter mine the peritoneal fluid kinetics. The increment in the intra-abdomin al pressure of 10.0 +/- 1.0 mm Hg caused a decline in the net ultrafil tration. This was mainly determined by an increase in the lymphatic ab sorption: 1.07 +/- 0.18 ml/min (without compression) versus 1.86 +/- 0 .25 ml/min (with compression; P < 0.01), whereas the transcapillary ul trafiltration rate tended to decrease: 2.02 +/- 0.23 versus 1.73 +/- 0 .27 ml/min (P = 0.08). External compression also diminished solute tra nsport from the circulation to the peritoneal cavity. The decline in t he mass transfer area coefficient of urea, creatinine, urate and beta2 -microglobulin was 13%, indicating a smaller effective peritoneal surf ace area caused by external compression probably due to a decrease in the number of the perfused peritoneal capillaries. The fall in the per itoneal protein clearances was more pronounced the higher the molecula r weight of the protein, consistent with a decline in the intrinsic pe rmeability of the peritoneum. Kinetic modeling using computer simulati ons was used to analyze these effects in terms of the pore theory, usi ng a convection model (large pore radius 184 +/- 14 angstrom) and a di ffusion model (large pore radius 1028 +/- 218 angstrom) for the transp ort of macromolecules. External compression led to a decrease in the u nrestricted area over diffusion distance from 183 +/- 16.6 m to 151 +/ - 15.3 m and a large pore radius of 174 +/- 9 angstrom. The diffusion model gave a decrease of the unrestricted area over diffusion distance from 140 +/- 12.8 m to 120.5 +/- 12.5 m and a large pore radius of 83 0 +/- 122 angstrom. The diffusion model fitted the measured clearances slightly better than the convection model (P < 0.001). As the main di fference between the two models is the large pore size, the contributi on of diffusion or convection to the transport of macromolecules can o nly be elucidated when the morphological counterpart of the large pore system has been identified. It can be concluded that the effects of e xternal compression on the transport of fluid and solutes include an i ncreased lymphatic absorption rate, a decreased transcapillary ultrafi ltration, a reduced effective surface area, that is, the number of por es and a lower intrinsic permeability.