ASN95 Hot Topic: New techniques for dry weight assessment

Fluid balance is an integral component of hemodialysis (HD) treatments to prevent under or over hydration both of which have been demonstrated to have significant effects on intradialytic morbidity and long term cardiovascular complications. The consequences of large interdialytic weight gains (IDWG) have not been well described. Two presentations showed different results: Ifudu et al (820/536) found a significant correlation between IDWG and pre dialysis BP and need for antihypertensive medications and LVH. The correlation between IDWG and LVH was stronger in white patients. In contrast, Luik et al (817/607) found that interdialytic fluid loading did not result in elevation of blood pressure except in 3 of 10 patients. Interestingly cardiac index was higher following fluid loading and was associated with decrease vascular resistance. Although the two reports appear contradictory a possible explanation lies in the definition of dry weight.

Fluid removal is usually achieved by ultrafiltration to achieve a clinically derived value for 'dry weight'. Unfortunately there is no standard measure of dry weight and as a consequence it is difficult to ascertain adequacy of fluid removal for an individual patient. The majority of hemodialysis (HD) treatments incorporate a prescription for fluid removal targeted to a patients 'dry weight'. In most centers dry weight is clinically determined and usually reflects the lowest weight a patient can tolerate without intradialytic symptoms and hypotension in the absence of overt fluid overload. At best this is an imprecise measure of dry weight as it is determined by trial and error and does not account for changes in nutritional status and lean body mass. As a consequence it is difficult to determine if an individual patient is over or under hydrated. Additionally, there is a lack of information on the effect of ultrafiltration on fluid shifts in the extracellular (ECF) and intracellular (ICF) fluid spaces. Several new methods are now being developed to assess changes in different compartments during hemodialysis.

Three different techniques have emerged for measuring volume changes during HD, (a) Single or multifrequency Bioimpedance Spectroscopy (BIS) which allows computation of extracellular fluid and total body water (b) a continuous hematocrit monitor (Critline) which allows evaluation of blood volume (BV) changes and (c) Ultrasonic measurement of the inferior vena cava diameter (IVCD) to determine the state of hydration. These methods have previously been used separately however in combination provide more information.

Single frequency bioimpedance was utilized to assess hydration status during dialysis runs by three groups. Spiegel et al (810/599) found that fluid retention was predominantly in the ECF as there appeared to be no change in ICF and TBW in pre dialysis assessments in comparison to controls. Other investigators (1018/603) described variations in the distribution of fluid in different compartments. Of 20 patients with overhydration 25% of the patients had ECF fluid gains alone, 40% with ICF alone and 35% with ICF and ECF combined. 11 patients who were not felt to be fluid overloaded also demonstrated expanded ECF in 55% cases. The decrease in TBW in these cases was attributed to a decrease in ICF secondary to malnutrition. In both sets of patients the clinical dry weight was inaccurate and both over and underhydration were common as estimated by BIA. In a third study (2219/574) a pattern of diminished body cell mass and total body water with supranormal body mass index suggests an underestimated prevalence of protein energy malnutrition in these patients.

All these studies suggest that current estimates of dry weight are not valid and need to be revised. Although BIA techniques appear to be promising methods, sequential multifrequency BIA measurements combined with simultaneous hematocrit measurements to assess blood volume (1013/618) showed variations in ECF greater than expected for the amount of UF. As BIA techniques are potentially influenced by changes in conductivity and temperature these factors may account for the different results seen.

Measurement of IVCD has been described as a method to assess hydration status (Nephrol Dial Transplant, 1989,4:563). Katzarski et al (812/604) compared changes in IVCD diameter and BV by Critline in short vs long dialysis and found that the UF rate influences the pattern of change in BV and IVCD with the post dialysis values reflecting different periods of requilibration. Other investigators (815/611) on the other hand demonstrated a significant correlation between IVCD and TBW with UF. Whereas IVCD appears to track changes in fluid volume as shown by Luik et al (817/607) as discussed above, there do not appear to be uniform criteria for using these methods for dry weight assessment. Additionally the complexity of the techniques may be daunting.

Monitoring BV changes in dialysis may be helpful in identifying patients more likely to have hypotensive episodes. It is well recognized that intradialytic complications are influenced by the balance between ultrafiltration rates and plasma refilling. UF rates in excess of plasma refilling capacity predispose to dialysis induced hypotension. Raja et al (1010/611) and Knoflach et al (1016/605) have both demonstrated the effect of sodium variation on maintaining higher plasma refilling rates. Step Na variation combined with step ultrafiltration is superior to linear UF. Our own group (819/537) has shown that using a non-linear UF protocol reduces the frequency of hypotensive episodes however it is important to define the endpoints, as the BV changes predicting a hypotensive event are strongly influenced by the definition. These findings possibly explain the difficulty in ascertaining the equivalent of a "crash crit" which could predict adverse events and thus could be avoided (1021/614). One of the benefits of monitoring BV changes is that it allows identification of patients who are relatively overhydrated as their BV curves remain flat and do not demonstrate a decline (1359/505). Reduction in target weight in these individuals results in improved fluid removal.

All of the above techniques are still in their infancy with respect to application in dialysis patients, however it is likely that combinations of these methods will provide a better estimate of dry weight.

(Ravindra Mehta)

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