Transperitoneal transport of creatinine. A comparison of kinetic models

Paradoxes in Peritoneal Transport of Small Solutes

Analysis of kinetic studies of peritoneal solute transport involves the need for discrimination between three transport components: diffusion, convective transport, and peritoneal absorption. The description of convective transport in standard clinical conditions of continuous ambulatory peritoneal dialysis (CAPO), as well as in isochratic measurements, has met some problems related to the paradoxical and often anomalous values of sieving coefficient, a parameter that characterizes solute drag with the flow of ultrafiltrate. A possible explanation of some of these results is the time dependence of the transport parameters, which is in contrast to their assumed steadiness. These anomalies as well as the time dependence of the transport parameters are confined more to the standard glucose-based dialysis fluid than to some alternative dialysis fluids. Furthermore, the most striking anomalies have been found for small electrolytes as well as for osmotic agents, which are applied in high, unphysiological concentrations. These solutes may be involved in the transport between intracellular and extracellular compartments within the peritoneal membrane, which phenomena are not included in the current modeling.

Transperitoneal transport in diabetic and non-diabetic patients on peritoneal dialysis

To investigate differences in the transport characteristics of the peritoneal membrane between diabetic and non-diabetic patients on chronic peritoneal dialysis, a study was conducted in 21 non-diabetic and 18 diabetic patients. Transperitoneal transport of small solutes was evaluated in terms of the mass transfer area coefficients (urea, creatinine and glucose), ultra-filtration sieving coefficients (urea and creatinine) and by peritoneal equilibration test results. The capacity of the peritoneal membrane to transport macromolecules was evaluated by albumin mass transfer rates and clearances of albumin. Transperitoneal water transport was evaluated by the ultra-filtration properties and the lymphatic flow rates. Finally, the whole-body capillary permeability was estimated by measuring the unidirectional flux of albumin across the capillary wall, i.e. the transcapillary escape rate of 125I-labelled human albumin. Despite a significantly increased transcapillary escape rate of albumin in the diabetic patients, no differences in peritoneal membrane characteristics could be demonstrated between diabetic and non-diabetic patients on peritoneal dialysis.

A prospective study of transperitoneal transport in patients undergoing peritoneal dialysis

A prospective one-year follow-up study of the functional characteristics of the peritoneal membrane was conducted in 20 peritoneal dialysis patients. Ten patients had at least one episode of peritonitis during the follow-up period. Changes in the transperitoneal transport of small solutes were evaluated by the mass transfer area coefficients (urea, creatinine, and glucose), ultrafiltration sieving coefficients (urea and creatinine), and peritoneal equilibration test results. Changes in the capacity of the peritoneal membrane to transport macromolecules were evaluated by means of albumin mass transfer rates. Finally, changes in transperitoneal water transport were evaluated by means of the ultrafiltration properties and the lymphatic flow rates. After one year of follow-up, transport of water and the investigated solutes had not changed significantly. Even episodes of peritonitis had no permanent influence on the transport function of the peritoneal membrane. The intraperitoneal residual volumes before instillation were significantly larger in patients who developed peritonitis during the follow-up period compared to patients who did not. Over a one-year period, no changes in peritoneal membrane characteristics could be demonstrated. A large residual volume of dialysate may be a marker of increased risk of getting peritonitis.

Aspects of osseous, peritoneal and renal handling of bisphosphonate during peritoneal dialysis: a methodological study

The accumulation of methylene bisphosphonate (MBP) in increased bone turnover is an index of the total skeletal turnover. Accordingly, a model has been described where uptake of the osseous tracer can be estimated, regardless of renal function. In the present study, the model was adapted to continuous ambulatory peritoneal dialysis (CAPD). The aims were: to assess the kinetics of 99m-technetium MBP (99mTc-MBP) in CAPD, and to evaluate the correctness of the assumption that the peritoneal and renal clearances of 99mTc-MBP equal the total plasma clearance of 51-chromium ethylenediamine tetra-acetic acid (51Cr-EDTA). Eight patients on CAPD were studied cross-sectionally. The mean plasma clearances of 99mTc-MBP and 51Cr-EDTA in the steady state (4h) were 38.2 and 12.2 ml min-1 (p < 0.01), the peritoneal clearances (0-4 h) were 5.2 and 7.2 ml min-1 (p < 0.01), and the renal clearances (0-4 h) were 2.5 and 2.8 ml min-1 (not significant), respectively. The bone bisphosphonate clearance (BBC) at steady state was 26.0 ml min-1, a value which was significantly higher than that at infinity (16.5 ml min-1, p < 0.01). We conclude that the assumption that the sum of peritoneal and renal clearances of 99mTc-MBP equal the clearances of 51Cr-EDTA is correct from a clinical point of view. We found that the uptake of the tracers in soft tissue and the renal handling of 51Cr-EDTA and 99mTc-MBP are also similar. The differences between the clearance values for 51Cr-EDTA and 99mTc-MBP may be explained by their different molecular weight. The differences in the BBCs at infinity and at steady state are most probably due to late recirculation of MBP from the bone compartment. Hence, the BBC technique can be applied in the CAPD setting.

The transport of phosphate between the plasma and dialysate compartments in peritoneal dialysis is influenced by an electric potential difference

Six kinetic models of transperitoneal phosphate transport were formulated and validated on the basis of experimental results obtained from 22 non-diabetic patients undergoing peritoneal dialysis. The models were designed to elucidate the presence or absence of diffusive, non-lymphatic convective, and lymphatic convective phosphate transport. Calculations allowed for a 20% protein binding of phosphate. The validation procedure demonstrated that only diffusive and non-lymphatic convective phosphate transport mechanisms were identifiable. A lymphatic convective phosphate transport mechanism was not identifiable. Furthermore, it was demonstrated that the electrochemical gradient between plasma water and dialysate favours the diffusive phosphate transport, and both electric and chemical potentials must be taken into account in calculations of the transperitoneal phosphate transport.

Transperitoneal transport of sodium during hypertonic peritoneal dialysis

The mechanisms of transperitoneal sodium transport during hypertonic peritoneal dialysis were evaluated by kinetic modelling. A total of six nested mathematical models were designed to elucidate the presence or absence of diffusive, non-lymphatic convective and lymphatic convective solute transport. Experimental results were obtained from 26 non-diabetic patients undergoing peritoneal dialysis. The model validation procedure demonstrated that only diffusive and non-lymphatic convective transport mechanisms were identifiable in the transperitoneal transport of sodium. Non-lymphatic convective sodium transport was the most important quantitative transport mechanism during the first 90 min of the dwell. Significant sodium sieving was demonstrated and explains the observation of hypernatremia in dialysis with hypertonic dialysis fluid.

Parameter estimation in six numerical models of transperitoneal transport of potassium in patients undergoing peritoneal dialysis

The mechanisms of transperitoneal potassium transport during peritoneal dialysis were evaluated by validation of different mathematical models. The models were designed to elucidate the presence or absence of diffusive, non-lymphatic convective and lymphatic convective solute transport. Experimental results were obtained from 26 non-diabetic patients undergoing peritoneal dialysis. The validation procedure demonstrated that models including both diffusive and non-lymphatic convective solute transport were superior to the other models. Lymphatic convective solute transport was not identifiable. Furthermore, it was demonstrated experimentally that the equilibrium distribution of potassium between plasma water and dialysate did not differ from a Donnan equilibrium, although the precondition of the Donnan equilibrium was not fulfilled, i.e. the volumes on each side of the membrane were not constant and dialysate was not an ultrafiltrate of plasma.