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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