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Stapel
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10 Monitoring of Muscle Mass in Critically Ill Patients 117
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Chapter 11
Bioelectrical Impedance Vector Analysis
in Critically Ill Patients
Cristian Deana, Sara Samoni, and Rinaldo Bellomo
Introduction
Critically ill patients with organ failure develop important disruptions of normal
body composition. Acute illness is often associated with increased capillary permeability and inte rstitial fluid accumulation [1].
Robust evidence suggests that a strongly positive fluid balance in intensive care
(ICU) is associated with increased mortality [2, 3].
unit
Finally, critically ill patients are also characterized by wide variations in their
carbohy
catabolism, which leads to muscular dysfunction, and ICU-acquired weakness
(ICU-AW) [4].
drate, lipid, and protein metabolism and critical illness increases protein
Supplementary Information The online version contains supplementary material available at
https://doi.org/10.1007/978-3-031-66541-7_11.
C. Deana (
Department of Anesthesia and Intensive Care, Health Integrated Agency of Friuli Centrale,
Udine, Italy
S. Samoni
Department of Nephrology, Dialysis and Renal Transplantation, Fondazione IRCCS Ca’
Granda Ospedale Maggiore Policlinico, Milan, Italy
R. Bellomo
Australian and New Zealand Intensive Care Research Centre (ANZIC-RC), School of Public
Health and Preventive Medicine, Monash University, Melbourne, VIC, Australia
Department of Critical Care, University of Melbourne, Melbourne, VIC, Australia
Department of Intensive Care, Austin Hospital, Melbourne, VIC, Australia
Department of Intensive Care, Royal Melbourne Hospital, Melbourne, VIC, Australia
e-mail: Rinaldo.BELLOMO@austin.org.au
© The
A. Cotoia et al. (eds.), Nutrition, Metabolism and Kidney Support,
https://doi.org/10.1007/978-3-031-66541-7_11
✉)
Author(s), under exclusive license to Springer Nature Switzerland AG 2024
119
120 C. Deana et al.
As a result, real-time knowledge of body composition may be of value in
optimizing fluid status, nutritional therapy, and drug dosing.
Some techniques are available for this purpose. However, many of them are based
on radiation techniques (Dual-Energy X-Ray absorptiometry (DEXA)), computed
tomography (CT), or need isotopes to determine water content. Others require the
transfer of patients to the radiology suite with increased workload and exposure of
critically ill patients to mobilization-related risks [5].
Given such challenges, bioelectric impedance vector analysis (BIVA) appears
attractive because it is a non-invasive, easy to use, inexpensive bedside tool that has
interesting potential applications in ICU.
Functional Principles
BIVA evaluates some characteristics of tissues, mainly hydration status, in response
to an application of alternate current [
Electrolyte-rich tissues are highly conductive of electrical current, while anhydrous tissues (like fat) resist the flow of current.
In simple words, the opposition to flow of a current is called resistance (R), while
the opposition to a curren t change due to a capacitor is defined as reactance (X
sum of resistance and reactance gives the impedance dimension (I) according to the
following formula: I
2
2
= X
+ R2 [8, 9]. Considering the participant’s height in the
c
regression equations, BIA can estimate lean body mass from impedance values and
body water content [
10].
There are many tools available on the market for this clinical purpose.
Single frequency BIVA (SF-BIVA) machines use a 50 kHz current to estimate
impedance. Low frequency machines usually measure only extracellular water
(ECW); thus, total body water (TBW) is derived from proportional equations [11].
High frequency currents, instead, provide data on intracellular water (ICW) and
ECW directly from measurement [
For this reasons, multifrequency devices have some advantages. Estimation of
TBW, ICW, and ECW is more reliable [13].
In addition, some machines adopt a multifrequency (MF-BIVA or bioimpedance
spectroscopy) current that provides more data.
In the case of huge amount of cell membranes to be crossed by high frequency
current, BIVA will measure a high X
and R are also included in the determination of Phase angle (PA). This
X
c
parameter represents the arctangent (X
The PA derives from a phase shift caused by resistance to flow determined by
capacitors (i.e., healthy cell membranes) that delay the current’s flow [
racti
In p
ce, a high PA correlates with large quantities of intact cell membranes
and body cell mass [15].
PA is
affected by age (lower at higher age) and sex (higher in men due to a greater
muscle mass to water content ratio) [16].
6, 7].
12].
value.
c
/R) * (180/π ).
c
14].
). The
c
11 Bioelectrical Impedance Vector Analysis in
Critically Ill Patients 121
BIVA depicts impedance as the vector of Xc and R in an x–y Cartesian axis. An R-
graph simultaneously describes hydration status and body composition. It has
X
c
been validated in healthy individuals and it is widely used in maintenance hemodialysis and peritoneal dialysis patients [
17].
Finally, bioimpedance spectroscopy (BIS) measures impedance by varying fre-
quencies, from very low to about 1000 KHz. According to Cole’s model, BIS fits the
impedance to a mathematical model that best describes X
and R. Theoretically, BIS
c
does not assume that ECW and ICW are uniformly distributed, so this technique
seems to give more accurate and individualized measure of ECW, ICW, and TBW
when compared to SF-BIVA or MF-BIVA [18].
Nonetheless, equations used to estimate volumes rely on constants that could
introduce some bias during calculation.
Old BIVA machines considered the body as a single cylinder. However, this does
not re flect possible asymmetry between right and left side of the body, or difference
between limbs and trunk, for example.
Technology has overcome this limit with segmental BIVA devices that consider
the body as five separate cylinders, using electrodes on all limbs as shown in
Fig. 11.1.
Fig. 11.1 First BIVA machines considered the body as one cylinder as shown in the left part of the
figure. Whole-body impedance was then used to calculate body water volumes. As devices
improved, with segmental BIVA the body was split into five cylinders with a separate analysis of
impedance in each cylinder. The latter improved accuracy in estimating body composition (right
part of the figure)
122 C. Deana et al.
Hydration Status Evaluations in Critically Ill Patients
Accumulation of fluids in ICU patients is frequently observed [19]. Overzealous
fluid administration in septic patients or those with shock is the typical scenario
where concomitant capillary leak and large amounts of fluids increase the ECW [20].
In contrast, poor hydration status might impair organ perfusion (if concomitant
hypovolemia is present) leading to injury and derangement of physiological
function.
Excessive negative or positive cumulative fluid balance significantly affects
critically ill patients’ outcomes [21, 22].
In fact, excessive fluid accumulation has been related to increased duration of
mechanical ventilation, ICU, and hospital length of stay (LOS), acute kidney injury,
and mortality [23].
However, it is difficult to estimate the patient’s volume status in ICU.
The gold standard for estimating the body water content is the use of tracers such
as deuterium oxide. However, this is not applicable in critically ill patients in daily
clinical practice [24].
In addition, classical hemodynamic parameters such as central venous pressure
(CVP) or arterial pressure (AP) are unreliable markers of TBW or ICW or ECW
[25]. Similarly, estimated cumulative fluid balance is subject to errors (for example,
insensible losses are very difficult to determine in case of fever).
In this regard, BIVA seems to be a reliable and easy to use method for assessing
fluid overhydration in ICU. To allow easier interpretation of BIVA data, an algorithm has been developed to finally convert bioelectrical parameters into a synthetic
measure of lean body mass hydration percentage [26]. According to this numerical
scale, patients can be classified as dehydrated, normohydrated, and hyperhydrated.
Jones and Colleagues described that hydration status changes given by BIVA
were consistent with directional changes in fluid balance. In other words, patients
classified as “dehydrated” at ICU admission had positive cumulative fluid balance at
day 5. In contrast, patients labelled as “hyperhydrated” had a negative cumulative
fluid balance after 5 days of observation in ICU. The authors concluded that BIVA
measurement of hydration status is likely valid and may be useful in critically ill
patients [27].
Samoni and Colleagues, in a similar study carried out in ICU, found that severe
hyperhydration measured with BIVA was the only variable significantly associated
with long term ICU mortality (OR 22.91; p < 0.001) [28].
However, many BIVA limitations have been acknowledged in critically ill
patients.
Acute c
limiting its ability to monitor rapid measurement of body water content [29].
Moreo
given by muscle mass. Considering that muscle mass is rapidly lost during the first
days of critically illness, incorrect estimation of BIVA parameters could increase
biased volume estimation.
hanges i
ver, estimation
n volume status, for example, could not be detected by BIVA,
of hydration status relies on fat-free mass, which is
mainly
11 Bioelectrical Impedance Vector Analysis in Critically Ill Patients 123
Further evidence demonstrated that natriuretic peptides blood levels may not
correlate with BIVA parameters in patients with heart failure [30]. This means that
hydration status alone is not the main actor that guides fluid therapy in ICU [31].
As a result, BIVA could be considered a supportive bedside tool to evaluate
hydration status in critically ill patients that, together wi th clinical evaluation and
other tools (for example ultrasound of the inferior vena cava or lung ultrasound
(LUS) score), may provide clinicians with supportive information on hydration
status and help guide fluid management in critically ill patients [32, 33].
Body Composition and Nutrition in ICU
Patients with long stays in ICU experience deep changes in their body composition.
Muscle wasting and ICU acquired weakness are probably the most clinically
evident events [
modified by critical illness as shown in Fig. 11.2.
It is important to define muscle mass in ICU. Rapid loss of muscle mass predicts
higher ICU mortality [36]. Interestingly, rapid muscle mass depletion affects also
physical functioning and quality of life even months or years after ICU discharge
[37, 38].
Some evidence demonstrated that there is good correlation between muscle mass
determined with BIVA compared to CT-muscle mass investigated at the third
lumbar vertebra [39]. Similarly, low PhA values correlates well with low skeletal
muscle mass and muscular density. In this way, BIVA seems useful to help
34, 35]. However, TBW, fat mass, and bone mineral density are also
HEALTHY INDIVIDUAL
Protein
ICW
ECW
TBW
Mineral mass
Fat mass
BCM
FFM
Body
Weight
ICU PATIENT
BCM
FFM
Body
Weight
Protein
TBW
Mineral mass
Fat mass
ICW
ECW
Fig. 11.2 Body composition changes in critically ill patients. Each body component is altered
during and after ICU stay, although TBW and protein compartment are probably the most affected.
Notwithstanding, also fat and mineral mass do not remain unaltered after critically illness. Legend:
ICW intracellular water, ECW extracellular water, TBW total body water (ICW + ECW), BCM body
cell mass, FFM fatty-free mass
124 C. Deana et al.
clinicians identify sarcopenic patients who are at very high risk of adverse outcomes
in ICU.
Moreover, in a small sample of critically ill patients, Lambell and colleagues
a significant correlation between PA and CT-muscle mass density [
found
9].
Evidence supports the validity of BIVA in estimating muscle mass when com-
pared to DEXA; however, the patients assessed were also from non-ICU setting
[40, 41].
Decrease in body cell mass (BCM) is v ery common in ICU and represents a
potential injury that contributes to ICU-AW.
Despite some unavoidable muscle mass reduction, the identification of patients
with great muscle loss during ICU stay might allow for a closer follow-up after
discharge.
Adequate caloric and protein intake is important during critical illness. If on the
one hand early full nutritional support seems harmful, on the other hand, poor intake
after clinical stabilization confers higher mortality in critically ill patients [42]. Consequently, timing and dosing of energy/protein delivery remains an open question,
also considering some side effects of artificial nutrition [43].
Indirect calorimetry (IC) measures energy expenditure (EE) through the analysis
of inhaled/exhaled gases. IC is a reliable method to measure EE and it has been
validated and compared to direct calorimetry. Recent guidelines suggest targeting
caloric needs with the use of IC [44].
However, IC is not widely used, many formulas are available to estimate the EE
and the caloric needs of ICU patients. Finally, fixed doses based on patient’s weight
are frequently used to simplify nutritional prescription.
Physiologically, the metabolic active part of free fat-mass is represented by body
cell mass (FFM).
Considering this concept, some BIA machines provide basal metabolic rate
estimation using equations that consider FFM. However, these equations are less
accurate than indirect calorimetry [45].
In addition, data from ICU patients on this issue are still lacking.
Another interesting potential use of BIVA is to calculate the protein dosage based
on FFM (sometimes called also lean body mass) derived from bioimpedance.
Protein dosing should consider FFM. However, this prescription becomes diffi-
cult whenever patients are not at their ideal body weight (or FFM). Many ICU
patients are overweight and, frequently even obese. Considering that obese patients
are more prone to become sarcopenic, it is difficult to determine the exact amount of
protein for nutritional planning [46].
In this regard, a recent study on COVID-19 population compared FFM derived
from BIA to four formulas. It demonstrated that none of these had acceptable
agreement after Bland-Altman analysis [47].
Consequent
IA may be a useful bedside tool to optimize protein target in ICU
ly, B
patients.
Moreover,
BIA may be helpful in the post-ICU recovery phase when adequate
calories and protein intake are extremely important for the restoration of
muscle mass.
11 Bioelectrical Impedance Vector Analysis in Critically Ill Patients 125
Limits of BIVA in Critically Ill Patients
Although there are many advantages for BIA in ICU, some limitations in addition to
those already listed above need to be acknowledged.
BIVA derived parameters are determined considering a healthy population, in
whom fixed proportion s between compartments can be assumed. However, critically
ill patients who are subjected to great rapid fluid shifts these assumptions may be
violated. In fact, important hemodynamic derangements in ICU require rapid interventions to restore optimal organ perfusion. Besides vasopressors and inotropes,
fluid challenges are frequently performed in critically unstable patients. During the
early resuscitation phase of septic shock, but also if the patient is suffering from
hemorrhagic shock, large amounts of fluids alter the physiological distribution
across body compartments. BIVA is unable to detect this rapid change, so some
errors could easily be present in the definition of derived parameters.
In this regard, PA is one of the main parameters that is affected by large fluid shift.
Similarly, in case of muscular edema, muscle mass could be overestimated because it
is calculated at a constant FFM hydration.
Another important aspect to consider is the presence of ascites and pleural
effusions, given that they represent an accumulation of extravascular fluids that,
when whole body BIVA is adopted, can influence measurements.
Hyper- or hyponatremia could alter the ECW estimation because of lower or
higher extracellular resistance respectively.
Another problem with BIVA is that many ICU patients are connected to machines
(ventilators or dialysis machines or chest drains) which lead to electrical flux
dispersion to environment in a way that cannot be reliably quanti fied.
Finally, one of the main caveats is the absence of standard ICU reference and
cut-off values.
Conclusions
In conclusion, while BIVA shows promise as a bedside tool for assessing hydration
status and body composition in critically ill patients, its limitations should be
carefully considered. It is recommended as a supportive tool alongside clinical
evaluation and other diagnostic techniques. The review encourages further research
to establish standard reference values for BIVA in the ICU setting.
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