Relationship between effective ionic dialysance and in vivo urea clearance during hemodialysis.

Effective ionic dialysance (EID) can be measured from dialyzer inlet and outlet conductivity changes following two steps of dialysate conductivity. Relationships between EID and in vivo urea clearances were studied four times per hemodialysis treatment in eight patients, each undergoing six hemodialysis treatments (192 data sets). Dialyzer blood flow was varied from 190 to 500 mL/min. Dialysate flow was constant (751 to 771 mL/min), and a standard dialyzer (700 HG; Cobe, Lakewood, CO) was used. Double samples were drawn for arterial, venous, and dialysate urea measurements. Two laboratory values were missing. Twelve unreliable laboratory values indicated by divergent results were excluded. Urea clearances were calculated by formulae converting whole-blood to blood-water urea clearances. EID was measured using Diascan (Gambro-Dasco, Medolla, Italy). Mass balance was checked by comparison of dialysate and blood-water urea clearances. Divergent results between dialysate and blood-water urea clearance values led to the exclusion of an additional three laboratory values. A small error (4.2%) in urea mass balance was found (dialysate greater than blood-water urea clearances). A total of 175 data sets were compared. EID showed excellent correlation with blood-water urea clearances (r = 0.92) over the line of identity, with a mean difference of -3.5 mL/min (-1%), and similarly with dialysate urea clearances (r = 0.92; mean difference, -13.4 mL/min; -5%). For both blood- and dialysate-side comparisons, differences increased with greater clearances. Because EID is an effective clearance and urea clearance is a measure of dialyzer clearance, the curves were corrected for cardiopulmonary recirculation; access recirculation was zero (Transonic monitor; Transonic Systems Inc, Ithaca, NY). For cardiopulmonary recirculation correction, cardiac output and access flows were assumed to be 6.4 L and 1.46 L/min. Corrected data show EID correlates with blood-side urea clearance (r = 0.92), with a mean difference of +7.3 mL/min (3.3%), and is constant over the range of clearances. EID correlated with dialysate urea clearance (r = 0.92) with virtually no difference. The difference on the blood side is consistent with the urea mass balance error found. These data indicate that EID using Diascan can provide an accurate indication of effective urea clearances obtained during hemodialysis and is of value in monitoring dialysis adequacy.

Significance of distribution volume in dialysis quantification.

The urea distribution volume is in several ways an important parameter in the treatment of end-stage renal disease (ESRD) patients. It has a major impact on the relative dose of dialysis treatment as measured by Kt/V. It is also important in the assessment of fluid status, which has a direct influence on blood pressure. Nevertheless, urea distribution volume is usually not measured on a regular basis, probably because it has been perceived as a relatively complicated measurement. With the arrival of on-line monitors for dialysate urea this situation has improved radically. In this article a number of volume determination methods are discussed, including some new methods based on on-line dialysate urea monitoring, which are shown to perform well in comparison to reference methods.

Future directions in dialysis quantification.

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.

Hydrodynamic properties of a new percutaneous intra-aortic axial flow pump.

Cardiac intervention, myocardial infarction, or postoperative heart failure will sometimes create a need for circulatory support. For this purpose, a new, minimally invasive intra-aortic cardiac support system with a foldable propeller has been developed. In animals, the pump has been shown to have a positive hemodynamic influence, and the present study evaluates the hydraulic properties of the pump in a bench test. The axial flow pump is a catheter system with a distal motor driven foldable propeller (0-15,000 revolutions per minute). To protect the aortic wall, filaments forming a cage surround the propeller. In the present study, tests were done with two different pumps, one with and one without the cage. Two different models were used, one for testing pressure generation and one for obtaining flow-pressure characteristics. Propellers and tubes with different diameters were studied, and pressure and flow characteristics were measured. The mathematical relationships between pressure and rotational speed, pressure, and diameter of propeller and tube were determined. There was a positive relationship between the revolutions per minute and the generated pressure, a positive relationship between the diameter of the propeller and pressure, and a negative relationship between the diameter of the tube and the generated pressure. Within the physiologic range of cardiac output, there was a small drop in pressure with increasing flow in the tubes with a small diameter. With an increasing diameter of the tube, a smaller pressure drop was seen with increasing flow. The present cardiac support system has hydraulic properties, which may be of clinical relevance for patients with left ventricular heart failure.

Urea sensors--a world of possibilities.

The arrival on the market of several monitors for the on-line measurement of urea has opened up a new range of possibilities. Already established dose measurements can be performed more easily, and a number of new parameters can be calculated. In this paper, different technologies for urea measurements are first discussed. Devices available on the market are then described according to the technology that is used and the parameters that are calculated. Finally, to show the versatility of this new possibility, a number of additional parameters are discussed that can be determined from dialysate side measurements of urea.

Whole body Kt/V from dialysate urea measurements during hemodialysis.

A new method for the calculation of dialysis dose from continuous measurements of dialysate urea concentrations has been developed. It is based on urea mass in the patient instead of plasma concentrations, and results in a measure of dialysis dose that has been named whole body Kt/V. The measured urea mass removal rate and the slope of the dialysate urea concentration curve are the key parameters needed for the calculations. No assumptions have to be made about urea distribution in the body (single or double pool, etc.). Blood sampling is not needed. This simplifies the logistics and eliminates the problems with rebound and timing in taking samples. The total urea mass present in the body before treatment is also obtained. It can be used directly, or in relation to body weight or water volume, as a measure of the level of urea in the body. This may serve as an alternative to pretreatment plasma concentration. If a pretreatment plasma urea concentration is available, the urea distribution volume can be calculated, which may be of separate clinical interest.

Whole body urea kinetics.

Improved methods are needed for dose quantification in dialysis because the kinetic modelling based on blood side urea concentrations, as currently used, is subject to several theoretical and practical problems. Since the treatment induces a disequilibrium, it is necessary to make assumptions about the distribution and exchange of urea within the body. A new method is presented which avoids these problems. The amount of urea removed is determined from continuous measurements in the spent dialysate. It is shown that the relative dialysis efficiency K/V can be calculated from the mass removal rate in relation to the mass remaining in the body, which is also determined from the measurements. As a by-product the total mass of urea in the body before the treatment is obtained. This can be used together with the blood concentration to calculate the urea distribution volume, which might be used as an additional clinical parameter. The new method was tested in three in vitro treatments of a container with a known amount of urea added to a known volume of dialysate. The calculated dose KT/V and initial urea mass differed from the true values by less than 3%.

Beneficial effect of cold dialysate for the prevention of hemodialysis-induced hypoxia.

Hypoxia occurs frequently during routine hemodialysis (HD). In this study the effect of dialysate temperature on arterial blood gas parameters was investigated. Ten stable HD patients (2 smokers) were dialyzed for 240 min with each of three different dialysate temperatures: 36.5 degrees C (normal temperature HD; NHD), 38.5 degrees C (warm HD; WHD) and 34.5 degrees C (cold HD; CHD). A cuprophane plate dialyzer was used. The ultrafiltration volume was identical in each patient. Arterial blood gas samples were frequently (approximately 10 times/treatment) taken during the dialysis and immediately analyzed. The dialysate temperature significantly affected PaO2 (p < 0.001) but not PaCO2. We also compared the effect of NHD with that of WHD and CHD, respectively, as regards PaO2. NHD and WHD differed significantly p < 0.01), whereas NHD and CHD were not significantly different. However, the relative PaO2 value (% of the baseline value) at the end of CHD (105 +/- 5%) was significantly higher than after both NHD (96 +/- 4%, p < 0.01) and WHD (91 +/- 3%, p < 0.01). In the case of NHD and WHD the fraction of time during which the patients had a PaO2 value below 80 mm Hg was 62 and 64%, respectively. The corresponding figure for CHD was 44%. Arterial oxygen saturation (SaO2) increased during CHD from 95.2 +/- 0.6 to 96.7 +/- 0.6% (p < 0.05), while SaO2 was unchanged during NHD and WHD. The positive effect of CHD was evident in 7 patients. In 1 patient PaO2 was not affected by the dialysate temperature, while in the remaining 2 patients (smokers) a decrease in PaO2 was induced by WHD as well as CHD. A separate statistical analysis with the 2 smokers excluded was performed, which showed that the dialysate temperature significantly affected PaO2 (p < 0.001). A comparison between NHD and CHD showed a significant difference (p < 0.001), whereas NHD and WHD did not differ significantly. When the 2 smokers were excluded from the analysis the fraction of time with a PaO2 value below 80 mm Hg was 60% during NHD and 56% during WHD, but it was reduced to 31% during CHD. In conclusion, despite the existence of interindividual variations most patients seemed to benefit from cold dialysate for the prevention of dialysis-induced hypoxia.