Measuring transport of water across the peritoneal membrane.

INTRODUCTION Mechanisms of water flow across the peritoneal membrane include diffusion, convection, and reabsorption. OBJECTIVES To understand these processes more clearly we have developed a method to measure transport of water across the peritoneal membrane. METHODS An artificial gradient of deuterated water (HDO) between blood and dialysate compartments was created in five subjects who took 0.3g per kg of body weight of D2O, which was allowed to equilibrate with total body water. During a test dwell (2 L, bicarbonate:lactate buffer, 1.36% glucose to minimize convection), frequent dialysate samples were drawn to determine the abundance of deuterium and other solutes and to calculate their time constants. Dialysate deuterium abundance was measured using flowing afterglow mass spectrometry (FA-MS). The method was combined with 125iodine-labeled albumin (RISA) to enable simultaneous estimates of intraperitoneal volume and thus calculation of the mass transfer area coefficient (MTAC) for small solutes using the Garred equation. RESULTS The appearance of HDO in dialysate in four subjects is described by a single exponential fit with residuals of <1%, similar to method precision. In a fifth subject, the resolution of this method demonstrated that the best fit was a double exponential. When compared to other solutes, the time constant for water was as predicted by its molecular weight, with a MTAC of 38.7 +/- 4.4 mL/min. Total body water could also be estimated from the equilibrated dialysate deuterium abundance, with repeat estimates within 0.5%. CONCLUSION Transport of water across the peritoneum can be measured with remarkable accuracy and when combined with an intraperitoneal volume estimation can be used to determine mass transfer. In conditions of low convection, the relative rate of deuterium appearance and mass transfer compared to other solutes suggests that water diffuses predominantly through the intercellular small pores.