Hct-BVM (Crit Line III, Hemametrics) is an optical density monitor that can detect and visually
display the continuous recording of changes in hematocrit, relative blood volume and oxygen saturation in real time during HD. It uses a transmissive photometric technique to measure the Hct on the basis of both the absorptive properties of hemoglobin and the scattering properties of red blood cells passing through the blood chamber.
During UF, intravascular blood volume is inversely and linearly correlated to hematocrit changes: as fluid is removed, the Hct rises. If the UF rate exceeds the vascular refill rate, hematocrit rises sharply. Intradialytic symptoms are often preceded by a rapid rise in hematocrit (fall in relative blood volume)40 or fall in central oxygen saturation.41 Timely intervention by the nurses has been shown to reduce symptoms and prevent morbidity.
Several small, observational studies have supported this hypothesis. They showed improved outcomes when Hct-BVM is used to guide dialysis.40,42 In contrast to these studies, however, the multi-center, randomized, controlled CLIMB study did not demonstrate improved patient outcome when using Hct-BVM to guide dialysis.43 This study, however, should be interpreted with caution for several reasons.
First, the use of Hct-guided BVM was encouraged but not mandated or monitored. The study only evaluated the availability of Hct-BVM, not its implementation. Second, the annualized mortality rate in the patients treated without Hct-guided BVM was extremely low (6.4%), substantially lower than for average patients within the United States (23.7%).
This suggests that, as often occurs with randomized trials, the healthier patients tend to be enrolled and results cannot be generalized to the average American population. Third, all-cause mortality was reported instead of volume-related death. This study cannot be used to discount the potential benefits of using Hct-BVM, although it does emphasize the need for developing clear and effective protocols for the use of this methodology. Baseline BVA coupled with Hct-BVM changes has potential to provide quantitative intravascular volume measurements.
Radioisotope BVA (BVA-100, Daxor Co.) uses a single tracer dilution technique to measure plasma volume. Red cell mass is calculated utilizing the hematocrit and the plasma volume in the intravascular space. Total blood volume then, can be calculated. BVA has been used to assess volume requirements in heart failure44,45 with promising results. If used in conjunction with vector-BIA and Hct-BVM, the three combined tools may provide more objective measures of fluid status in all body compartments.
Vector-BIA, Hct-BVM, and BVA provide complementary volume meas- urements. At the New York University School of Medicine’s VA New York Harbor Health Care System dialysis unit, we have been developing strategies using a combination of vector-BIA (TBW), BVA (intravascular blood volume), and Hct-BVM (dynamic, intradialytic response to volume removal) to define dynamic fluid compartment volumes in adults and the elderly.
By using more objective, accurate methods to dynamically evaluate fluid status in the different compartments, we ultimately hope to develop an improved method to guide ultrafiltration during thrice weekly hemodialysis.
Dr. Pillon is assistant professor of medicine (Nephrology) at the New York University School of Medicine, Director of Dialysis at the VA New York Harbor Health Care System. Dr. Feldschuh is President and CEO of Daxor Corporation.
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