HD delivery methods
The most effective and physiologic HD delivery methods currently available are frequent or long dialysis sessions. By maintaining a strict control of patient fluid volume through eight-hour dialysis sessions and strict limitation of salt and fluid intake, Charra et al. have reported an impressive 10-year patient survival of 75% associated with excellent BP control without BP medication.17,18 Every study of both short daily 19-23 and long nocturnal24 treatments has found a decrease in systolic and mean arterial pressure, often with reduction of all BP medications, reduction of extracellular fluid (ECF), or regression of LVH.
For reasons of economics, quality of life, and patient compliance, thrice-weekly HD remains the norm in the United States. Therefore, dynamic, objective measures of fluid compartments with more controlled changes in plasma volume may play an important role in improving HD outcomes when frequent or longer treatments cannot be implemented.
Recent efforts have evaluated the use of non-invasive technologies to monitor intra- and extravascular fluid volumes independently. Methods for evaluating intravascular fluid include ultrasound assessment of inferior vena cava diameter 25-30 and several biochemical parameters, such as the natriuretic peptides.31-35
These methods have been proposed to guide fluid management and have been shown in certain studies to limit intradialytic morbidity. Their use is limited by interpatient variability, test variability, intra-operator ability, the presence of right-sided heart failure, and cost.
Radiotracer dilution methods provide quantitative measurement of the intravascular compartment. These methods are not suitable for intra-dialysis testing but may lend themselves to better analyze the intravascular volume at steady state. Their application may enable the clinician to better control blood volumes over long time intervals and assist in the prevention of chronic hypo- and hypervolemia.
A semi-automated, FDA-approved, blood volume analyzer (BVA) has made it possible to obtain rapid direct measurement of intravascular fluid, eliminating many of the previous time-consuming steps of other dilution techniques. This method may help further validate tools such as the continuous hematocrit measurements by providing quantitative intravascular compartments measurements at pre- and post-dialysis steady states.
BIA is a noninvasive method for assessing electrical tissue properties, predominantly of the limbs. Hundreds of BIA methods derived from models and regression equations have been used successfully to estimate soft-tissue hydration and have been validated by isotope dilution.
Using different single frequency, multi-frequency and bioimpedance spectroscopy (BIS) methods, compartments of ECF, TBW and ICF have been measured in healthy individuals. The concept is based on the assumption that TBW can be measured with single frequency (50 KHz) and formulas using the resistance. Low frequencies (1-5 kHz) pass through ECF whereas high frequencies (50-500 kHz) pass through both ECF and ICF, hence the TBW. Unfortunately, these methods fall short in HD patients who are malnourished, have active inflammation, or have abnormal tissue hydration.
In addition, an unknown and variable amount of low frequency current passes through cells due to anisotropy, particularly through muscle fibers in parallel to the current. Prediction errors are comprised of the sum of measurement error, variability among subjects, and the large bias introduced by the many assumptions that are necessary when using the equations. In contrast, vector-BIA uses direct measurements of a patient’s impedance (Z). This method has been shown to have a precision error of only 2%, with subject variability the only potential source of additional error.
High UF rates are required to achieve desired fluid removal during thrice-weekly HD treatments. This increases the likelihood of an imbalance between UF and vascular refilling, frequently resulting in intradialytic hypovolemia. To address this important issue, hematocrit-guided blood volume monitoring (Hct-BVM) has been advocated and used to guide intradialytic volume removal, both by assessing changes in relative blood volume or hematocrit, vascular refilling, and central oxygen saturation. The promising individual methodologies are reviewed below.
The RXc graph 36 classifies an individual patient’s fluid and nutritional status (by means of vector with length and direction according to the distance of the vector from the mean value of a reference population). Vector-BIA is a method for measuring the voltage drop as an 800 mAmps alternating current at 50 KHz is sent through the body by way of peripheral electrodes placed on the skin of a patient’s ipsilateral hand and foot.
The impedance is made up of the sum of height-standardized R and Xc measurements, where R is the opposition to the flow of the current as it flows through an electrolyte solution—as the percentage of water in the body rises, current flows more freely, and the impedance falls. Xc is the shift in the phase angle of the current as it passes through cell membranes and tissue interfaces, which act as capacitors—the larger and healthier the cell membranes and proteoglycan meshwork, the greater the Xc. The phase angle is the arctangent of the Xc over the R, and it is visualized as the angle made by the vector and the X axis. At a fixed resistance, the phase angle increases with an increase in cell mass, and decreases with a decrease in cell mass. Simply, vector-BIA is a method for converting the body’s electrical properties into clinically useful information, analogous to an electrocardiogram (ECG).
In 1998, vector-BIA was assessed in 1,367 chronic HD patients from more than 40 dialysis centers across Italy, and compared to results from 726 healthy subjects. Asymptomatic HD patients tended to stay within the 75% tolerance (ellipse) interval of the normal population, whereas symp-tomatic patients had smaller phase angles, outside of the 75% reference tolerance intervals for normal healthy population.37
These results were consistent with the study by Chertow et al., which showed that low phase angles were associated with an increased relative risk of death in chronic HD patients.38 Pillon et al. found that shorter vector lengths, indicating greater soft tissue hydration, were associated with an increased one-year relative risk of death, even after adjustment for case mix and several nutritional and inflam-matory indicators.39
Vector-BIA provides a promising direct approach for interpreting soft tissue fluid status in dialysis patients. Longitudinal studies evaluating the clinical effectiveness of using vector-BIA to guide UF has yet to be performed. One of the perceived drawbacks of vector-BIA is that it does not directly indicate an easily understood physical property. This problem is largely one of familiarity. With continued use of vector-BIA and the development of clear protocols for incorporating results into adjustment of UF by the nursing staff, the method will be more globally understood and more easily integrated into treatment.