Lindsay R M, Sternby J
Optimal Dialysis Research Unit, London Health Sciences Centre, Westminster Campus, 800 Commissioners Road E., London, Ontario N6A 4G5, Canada.
Semin Dial. 2001 Jul-Aug;14(4):300-7. doi: 10.1046/j.1525-139x.2001.00067.x.
The influence of dialysis prescription on outcome is well established, and currently the amount of dialysis prescribed is based on small molecular weight toxin removal as represented by the clearance of urea. The "normalized dose of dialysis" (Kt/V(urea)) concept is well established. Most techniques for dialysis quantification require that blood samples be taken at the beginning and after the completion of dialysis. The postdialysis sample, however, gives cause for concern because of the "rebound phenomenon" due to nonuniform distribution of urea among body compartments. Blood samples give "indirect" measures of dialysis quantification. Thus direct urea concentration measurements in dialysate may be superior in urea kinetic modeling and these may be made "real time" during dialysis. It is with real-time monitoring that future advances in dialysis quantification will take place. These will be of two types. The first will analyze blood water or dialysate samples for urea content multiple times throughout the treatment; the second will assess the on-line clearance of urea using surrogate molecules such as sodium chloride, the clearance being determined by conductivity measurements. On-line urea monitoring is based on the action of urease on urea in a water solution and measurement of the resultant ammonium ions, which are measured directly by a specific electrode or indirectly by conductivity changes. Differences in blood-side versus dialysate-side urea monitors exist which reflect the parameters they can provide, but with both, the standard urea kinetic measurements of Kt/V and nPCR (nPNA) are easily obtainable. A range of additional parameters can be derived from dialysate-side monitoring such as "whole-body Kt/V," "pretreatment urea mass" and "whole-body urea clearance," which are worthy of future studies to determine their roles in adequacy assessment. Conductivity clearance measurements are made by examining the conductivity differences between dialysate inlet and outlet measured at two different dialysate inlet concentrations. This allows for the calculation of the electrolyte (ionic) dialysance, which is equal to the "effective" urea clearance, that is, the clearance that takes into account recirculation effects that reduce hemodialysis efficiency. The continuous reading of effective ionic clearance will allow an average value for K to be obtained for that dialysis, and hence the parameter K x t as an indication of dialysis dose is easily and accurately obtained for every treatment. The conductivity technology is cheap and rugged, and thus expanded use can be expected. Urea monitors have an inherent cost and require maintenance, and perhaps will remain researchers' tools for the present. The methodologies can complement each other; the addition of an accurate and independent value for K to dialysate based urea monitoring is like having simultaneous blood- and dialysate-side monitoring, and allows further increase in measurable parameters.
透析处方对治疗结果的影响已得到充分证实，目前规定的透析量是基于以尿素清除率为代表的小分子毒素清除情况。“标准化透析剂量”（Kt/V(尿素)）的概念已确立。大多数透析定量技术要求在透析开始时和结束后采集血样。然而，透析后的样本令人担忧，因为尿素在身体各腔室中分布不均会导致“反弹现象”。血样只能提供透析定量的“间接”测量。因此，透析液中尿素浓度的直接测量在尿素动力学建模方面可能更具优势，并且可以在透析过程中进行“实时”测量。未来透析定量的进展将基于实时监测。这些进展将分为两类。第一类将在整个治疗过程中多次分析血水样或透析液样本中的尿素含量；第二类将使用诸如氯化钠等替代分子评估尿素的在线清除率，清除率通过电导率测量来确定。在线尿素监测基于脲酶对水溶液中尿素的作用以及对生成的铵离子的测量，铵离子可通过特定电极直接测量或通过电导率变化间接测量。血侧和透析液侧的尿素监测仪存在差异，这反映了它们所能提供的参数，但通过这两种监测仪都能轻松获得Kt/V和nPCR（nPNA）等标准尿素动力学测量值。从透析液侧监测还可以得出一系列额外参数，如“全身Kt/V”、“治疗前尿素质量”和“全身尿素清除率”，值得未来开展研究以确定它们在充分性评估中的作用。电导率清除率测量是通过检查在两种不同透析液入口浓度下测量的透析液入口和出口之间的电导率差异来进行的。这使得能够计算电解质（离子）透析率，其等于“有效”尿素清除率，即考虑到降低血液透析效率的再循环效应后的清除率。持续读取有效离子清除率将使每次透析能够获得K的平均值，因此，作为透析剂量指标的参数K×t能够轻松、准确地为每次治疗获取。电导率技术价格低廉且坚固耐用，因此有望得到更广泛的应用。尿素监测仪有其固有成本且需要维护，或许目前仍将是研究人员的工具。这两种方法可以相互补充；在基于透析液的尿素监测中加入准确且独立的K值，就如同同时进行血侧和透析液侧监测，能够进一步增加可测量的参数。
