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Application note |
| A2 | Measurements of Water Transport Across the Peritoneal Membrane |
As an example of the measurement of water transport, the DS-1 has been used to study the transport properties of the peritoneal membrane of patients being treated by continuous ambulatory peritoneal dialysis (CAPD).
In CAPD treatment, the peritoneal cavity of the patient is loaded with a solution of the dialysing liquid (dialysate), which causes the flow of water and unwanted toxins from the blood to the dialysate. To study water flow across the membrane using deuterium labelling, deuterated water (D2O) can either be administered orally or the dialysate can be “spiked” with D2O.
The subject drinks a weighted amount of 99.9% pure D2O in 200mL of regular drinking water. The dose of D2O can be 0.3g/kg body weight. This is well within safe limits.Samples of dialysate fluid are taken starting four hours after ingestion.
Samples of about 10 mL of the fluid is placed in 200 mL glass bottles sealed with a septum. Each bottle is then placed in a temperature controlled water bath and the headspace is allowed to develop for about 10 min. A sampling needle connected directly to the input line of the DS-1 then punctures the septum and headspace vapour is drawn into the carrier gas by the existing pressure differential (initially from atmosphere down to the flow tube pressure). Sampling lines are held at about 100 ºC to inhibit the condensation of water vapour and other condensable vapours.
The transport rate of water across the membrane can be accurately determined from the well-defined increase of the deuterium content over time. Conclusions can then be drawn concerning the mechanisms of transport of water through the system of pores that connect the capillary blood vessels with the peritoneal membrane.
In the complementary experiment, in which the dialysate water is labelled with a known quantity of D2O, the breath HDO is monitored with time following the dialysate exchange. Many more sampling points can be obtained using breath analysis, which can be carried out every minute or so. This very well defines the breath/blood increase of deuterium with time and provides even more accurate transport rates. With such data the renal physician is assisted in determining the appropriate dialysis regime for the particular CAPD patient.
Asghar RB, Diskin A, Spanel P, Smith D, Davies SJ. Measuring transport of water across the peritoneal membrane. Kidney Int 2003; 64: 1911-1915
Asghar R, Diskin A, Spanel P, Smith D, Davies S. Influence of convection on the diffusive transport and sieving of water and small solutes across the peritoneal membrane. J Am Soc Nephrol 2005; 16: 437-443
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