Effect of an increased intraperitoneal pressure on fluid and solute transport during CAPD

Effect of an increased intraperitoneal pressure on fluid and solute transport during CAPD. The effect of an increased intraperitoneal pressure on fluid and solute transport was studied in eight stable CAPD patients. Two permeability tests of two hours each with continuous registration of the intraperitoneal pressure were performed while patients were in supine position. The intra-abdominal pressure was raised by means of a tightening girdle with inflatable cuffs in one of the experiments. Intraperitoneally administered dextran 70 was used as a volume marker in order to determine the peritoneal fluid kinetics. The increment in the intra-abdominal pressure of 10.0 ± 1.0 mm Hg caused a decline in the net ultrafiltration. This was mainly determined by an increase in the lymphatic absorption: 1.07 ± 0.18 ml/min (without compression) versus 1.86 ± 0.25 ml/min (with compression; P < 0.01), whereas the transcapillary ultrafiltration rate tended to decrease: 2.02 ± 0.23 versus 1.73 ± 0.27 ml/min (P = 0.08). External compression also diminished solute transport from the circulation to the peritoneal cavity. The decline in the mass transfer area coefficient of urea, creatinine, urate and β2-microglobulin was 13%, indicating a smaller effective peritoneal surface area caused by external compression probably due to a decrease in the number of the perfused peritoneal capillaries. The fall in the peritoneal protein clearances was more pronounced the higher the molecular weight of the protein, consistent with a decline in the intrinsic permeability of the peritoneum. Kinetic modeling using computer simulations was used to analyze these effects in terms of the pore theory, using a convection model (large pore radius 184 ± 14 Å) and a diffusion model (large pore radius 1028 ± 218 Å) for the transport of macromolecules. External compression led to a decrease in the unrestricted area over diffusion distance from 183 ± 16.6 m to 151 ± 15.3 m and a large pore radius of 174 ± 9 Å. 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 830 ± 122 Å. The diffusion model fitted the measured clearances slightly better than the convection model (P < 0.001). As the main difference between the two models is the large pore size, the contribution of diffusion or convection to the transport of macromolecules can only be elucidated when the morphological counterpart of the large pore system has been identified. It can be concluded that the effects of external compression on the transport of fluid and solutes include an increased lymphatic absorption rate, a decreased transcapillary ultrafiltration, a reduced effective surface area, that is, the number of pores and a lower intrinsic permeability