Rippe B, Carlsson O
Department of Nephrology, University Hospital of Lund, Sweden.
Perit Dial Int. 1996;16 Suppl 1:S97-103.
In this article the difference between osmotic fluid flow (ultrafiltration) as driven by osmotic pressure and diffusion through thin leaky membranes is discussed. It is pointed out that water transport induced by osmosis is fundamentally different from the process of water diffusion. Applying modern hydrodynamic pore theory, the molar solute concentration and the solute concentration in grams per 100 mL, exerting the same initial transmembrane osmotic pressure as a 1% glucose solution, was investigated as a function of solute molecular weight (MW). It was then assumed, base on experimental data, that the major pathway responsible for the peritoneal osmotic barrier characteristics is represented by pores of radius approximately 47 A. With increasing solute radius, the osmotic reflection coefficient (sigma) and, hence, the osmotic efficiency per mole of solute will increase. However, simultaneously, the molar concentration per unit solute weight will decrease. The balance point between these two events apparently occurs at a solute MW of approximately 1 kDa. An additional advantage of using solutes of high MW as osmotic agents during peritoneal dialysis (PD), rather than increased osmotic efficiency per se, lies in the fact that large solutes, due to their low peritoneal diffusion capacity, will maintain a sustained rate of ultrafiltration (osmosis) over a prolonged period. To illustrate this, we have performed computer simulations of peritoneal fluid transport according to the three-pore model of peritoneal permselectivity. According to these simulations, 4% of an 800 Da polymer solution (+50 mmol/L above isotonicity) will produce the same cumulative amount of intraperitoneal fluid volume ultrafiltered (UF) during 360-400 minutes as 4% of a 2 kDa polymer solution (+20 mmol/L) or 6.5% of a 10 kDa polymer solution (+6.5 mmol/L) having the same electrolyte concentration as dialysis solutions conventionally used for PD. Similar cumulative UF volumes (during 400 minutes) can be obtained by a 2.5% glycerol (+272 mmol/L) or a 3.2% glucose-containing dialysis solution (+177 mmol/L) with conventional electrolyte composition.
本文讨论了由渗透压驱动的渗透流体流动（超滤）与通过薄的渗漏膜的扩散之间的差异。文中指出，渗透作用引起的水运输与水扩散过程有着根本的不同。应用现代流体动力学孔理论，研究了与1%葡萄糖溶液具有相同初始跨膜渗透压的每升摩尔溶质浓度和每100毫升克数表示的溶质浓度随溶质分子量（MW）的变化关系。然后根据实验数据假设，腹膜渗透屏障特性的主要途径由半径约为47埃的孔所代表。随着溶质半径的增加，渗透反射系数（σ）以及每摩尔溶质的渗透效率都会增加。然而，与此同时，单位溶质重量的摩尔浓度会降低。这两个因素之间的平衡点显然出现在溶质分子量约为1 kDa时。在腹膜透析（PD）期间使用高分子量溶质作为渗透剂的另一个优势，并非在于其本身的渗透效率提高，而是在于大分子溶质由于其低腹膜扩散能力，将在较长时间内维持持续的超滤（渗透）速率。为了说明这一点，我们根据腹膜通透性选择的三孔模型对腹膜液运输进行了计算机模拟。根据这些模拟，800 Da聚合物溶液（高于等渗浓度50 mmol/L）的4%在360 - 400分钟内产生的腹腔内液体超滤（UF）累积量，与具有相同电解质浓度的2 kDa聚合物溶液（+20 mmol/L）的4%或10 kDa聚合物溶液（+6.5 mmol/L）的6.5%相同，这些电解质浓度与传统用于PD的透析液相同。通过具有传统电解质组成的2.5%甘油（+272 mmol/L）或3.2%含葡萄糖的透析液（+177 mmol/L）也可获得类似的（400分钟内）累积UF体积。
