Clinical Nutrition 31 (2012) 854e861

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Clinical Nutrition
journal homepage: http://www.elsevier.com/locate/clnu

Review

Bioelectrical phase angle and impedance vector analysis e Clinical relevance and
applicability of impedance parameters
Kristina Norman a, *, Nicole Stobäus a, Matthias Pirlich b, Anja Bosy-Westphal c, d
a
Medizinische Klinik für Gastroenterologie, Infektiologie und Rheumatologie (inkl. Arbeitsbereich Ernährungsmedizin) Charité, Universitätsmedizin Berlin, Berlin, Germany
b
Abteilung für Innere Medizin, Evangelische Elisabeth Klinik, Berlin, Germany
c
Institut für Humanernährung und Lebensmittelkunde, Christian-Albrechts-Universität Kiel, Kiel, Germany
d
Institut für Ernährungsmedizin, Universität Hohenheim, Stuttgart, Germany

a r t i c l e i n f o s u m m a r y

Article history: Background & aims: The use of phase angle (PhA) and raw parameters of bioelectrical impedance analysis
Received 8 September 2011 (BIA) has gained attention as alternative to conventional error-prone calculation of body composition in
Accepted 11 May 2012 disease. This review investigates the clinical relevance and applicability of PhA and Bioelectrical
Impedance Vector Analysis (BIVA) which uses the plot of resistance and reactance normalized per height.
Keywords: Methods: A comprehensive literature search was conducted using Medline identifying studies relevant to
Bioelectrical impedance analysis
this review until March 2011. We included studies on the use of PhA or BIVA derived from tetrapolar BIA
Phase angle
in out- and in-patient settings or institutionalized elderly.
BIVA
Nutritional status
Results: Numerous studies have proven the prognostic impact of PhA regarding mortality or post-
Malnutrition operative complications in different clinical settings. BIVA has been shown to provide information about
Mortality hydration and body cell mass and therefore allows assessment of patients in whom calculation of body
Clinical outcome composition fails due to altered hydration. Reference values exist for PhA and BIVA facilitating inter-
Hydration status pretation of data.
Conclusion: PhA, a superior prognostic marker, should be considered as a screening tool for the identi-
fication of risk patients with impaired nutritional and functional status, BIVA is recommended for further
nutritional assessment and monitoring, in particular when calculation of body composition is not
feasible.
Ó 2012 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.

1. Introduction appropriate regression equations. Body composition reference

methods, such as isotope dilution or dual X-ray absorptiometry,
For over 25 years, bioelectrical impedance analysis (BIA) has have been used to generate the dependent variable for regression
been in use for the estimation of body composition. BIA is models. Various empirical equations for the calculation of body
a portable, easy-to-use, inexpensive and non-invasive method, composition from BIA measurements of tissue impedance and
which can be repeated frequently and is independent of patient reactance have thus been developed with considerable variation in
cooperation. It measures whole-body impedance, the opposition of the estimated body compartments.3,4 Moreover, homogenous
the body to alternating current consisting of two components: composition, fixed cross-sectional area and consistent distribution
resistance (R) and reactance (Xc). Resistance is the decrease in of current density are necessary assumptions for the correct esti-
voltage reflecting conductivity through ionic solutions. Reactance is mation of body composition. In healthy subjects who have no fluid
the delay in the flow of current measured as a phase-shift, imbalance, no body shape abnormalities and who are within
reflecting dielectric properties, i.e., capacitance, of cell a certain BMI range (16e34 kg/m2), BIA offers reliable information
membranes and tissue interfaces.1,2 BIA is therefore not a direct on body composition provided that suitable (i.e. age-, sex- and
method for assessment of body composition and its accuracy as an population-specific) equations for the calculation of body
indicator of body composition relies largely on the use of compartments are applied.5
However, these conditions are frequently violated in sick and
hospitalized patients since disturbed hydration or altered distri-
* Corresponding author. Tel.: þ49 30 450 514139; fax: þ49 30 450 514936. bution of extra- and intra-cellular water are often present, e.g., in
E-mail address: kristina.norman@charite.de (K. Norman). liver cirrhosis, renal failure, cardiac insufficiency and obesity.6e8

0261-5614/$ e see front matter Ó 2012 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved.
doi:10.1016/j.clnu.2012.05.008
 K. Norman et al. / Clinical Nutrition 31 (2012) 854e861 855

Therefore, the use of raw impedance parameter has gained extracellular to intracellular water ratio of adipose tissue.20 More-
attention. They provide information on hydration status and body over, physical activity appears to play a role, as shown in
cell mass and cell integrity without algorithm-inherent errors or athletes12,21 as well as in 60e90 year old healthy adults13,18 where
requiring assumptions such as constant tissue hydration. They have subjects in the highest quartiles of physical activity had higher
moreover proven to be of prognostic value in various diseases. phase angles. Since these studies did not investigate both muscle
This review outlines the clinical applicability of the most mass and physical activity it is tempting to speculate that the
commonly used impedance parameter, the phase angle, and the physical activity influences phase angle via a higher amount of
combined use of reactance and resistance normalized by height in muscle mass.
the R/Xc Graph using Bioelectrical Impedance Vector Analysis. Although the reliability of BIA is high18 one limitation regards
measurement differences between BIA devices from different
2. Phase angle manufacturers.22 Since there is no international manufacturing-
standard, values from different devices differ which hampers
The most clinically established impedance parameter is the direct comparison of results from different studies as well as the
phase angle. It has gained popularity over the past years since it has application of generally accepted reference values. Therefore,
shown to be highly predictive of impaired clinical outcome and harmonization of technology as well as cross-calibration of the
mortality in a variety of diseases. It expresses both the amount and electrical resistors should be a mandatory future goal for imped-
quality of soft tissue and can be calculated directly as its arc ance companies. Also, only BIA devices that can detect phase
tangent: (Xc/R)  180 /p. Phase angle has been suggested to be an sensitive impedance variation indicate Xc and can be used for
indicator of cellular health9,10 where higher values reflect higher assessment of phase angle.
cellularity, cell membrane integrity and better cell function. In
healthy subjects phase angle usually ranges between 5 and 7 11 but 2.2. Phase angle in disease
values above 9.5 can be reached in athletes.12
Phase angle indeed correlates with various indices of functional Phase angle is frequently lower than normal in disease since
(e.g. r ¼ 0.53 with grip strength in liver cirrhosis and r ¼ 0.4 with influences such as infection, inflammation or disease-specific
knee extension strength and r ¼ 0.35 with Barthel Index in elderly parameters may impair phase angle. Co-infection with tubercu-
institutionalized subjects) and nutritional status (e.g. r ¼ 0.6 with losis and human immunodeficiency virus (HIV) was, for example,
albumin, r ¼ 0.53 with the Subjective Global Assessment)13e17 and associated with significantly lower phase angles than HIV infection
has early been suggested to be an index for muscularity.15 alone in one study. In these patients with pulmonary tuberculosis,
there were no significant differences in BMI, or anthropometrically
assessed fat and fat-free mass between HIV-positive and HIV-
2.1. Determinants of phase angle negative adults.23 Among HIV-positive subjects, phase angles
were significantly lower among those with CD4þ lymphocytes
In healthy adults, age, sex and BMI are the major determinants <200 cells/ml compared with those who had >200 cells/ml.23
of phase angle,11,18 as shown in Table 1. Phase angle decreases with Lower phase angle values were found with volume overload or
increasing age, due to a reduction in reactance which parallels the anaemia in heart failure patients24 and volume overload and anuria
loss of muscle mass and an increase in resistance due to the in patients on peritoneal dialysis.25 Repeated measurements during
declining proportion of body water at the expense of increasing fat one year in patients on haemodialysis revealed decreasing phase
mass in higher age. Men have higher phase angles than women due angles whereas no significant changes in body weight, fat mass,
to the higher amount of body muscle mass. Moreover, phase angle lean body mass, or laboratory variables were observed.26 In obese
increases with increasing BMI due to the increased number of patients, women in the lowest phase angle tertile group revealed
muscle and fat cells. Interestingly, this association is only observed a significantly severe cardiovascular risk profile because their fat
in BMI values <30 kg/m2, in severely obese subjects with BMI mass, glucose, interleukin 6, leptin and insulin resistance were
>40 kg/m2, an inverse correlation is found.11 This has been attrib- significantly higher.27 C-reactive protein, atherosclerosis parame-
uted to higher tissue hydration due to fluid overload19 or increased ters, albumin and creatinine correlated significantly with phase
angle in peritoneal dialysis patients.25
Table 1
Studies have reported various disease-specific determinants of
Main determinants of phase angle. phase angle. In patients on haemodialysis, creatinine and log
soluble leptin receptor (sOB)/leptin ratio were significant inde-
Authors
pendent predictors of phase angle next to age,28 and higher inter-
In healthy populations:
leukin 1 concentrations were associated with lower phase angles
Age Bosy-Westphal et al.11
Dittmar et al.18 and with greater decrease of phase angle over time.26 Maggiore and
Barbosa-Silva et al.33 colleagues found serum albumin, age, mid arm muscle circumfer-
Sex Bosy-Westphal et al.11 ence, nutritional status according to Subjective Global Assessment
Dittmar et al.18 (SGA) and normalized protein catabolic rate to be significant
Barbosa-Silva et al.33
BMI Bosy-Westphal et al.11
predictors for phase angle in haemodialysis patients.29 In HIV
Dittmar et al.18 infected men, insulin-like growth factor 1 was the only significant
Disease-specific parameters: predictor in a multivariate analysis adjusted for number of drugs,
Malnutrition disease severity age and nutritional intake.30 We investigated
Subjective Global Assessment Maggiore et al.29
determinants of phase angle in a large observational cross-
Stobäus et al.31
Prealbumin Avram et al.92 sectional sample of hospitalized patients and found that weight
Inflammation loss and inflammation were independent predictors of phase angle
C-reactive protein Gunn et al.13 next to age and sex. When studying the standardized phase angle
Demirci et al.25 (Z-score for patients’ individual deviation of sex-, age-, and BMI-
Stobäus et al.31
Interleukin-6 Johansen et al.26
stratified mean reference values) only malnutrition and inflam-
mation were identified as predictors of phase angle.31
856 K. Norman et al. / Clinical Nutrition 31 (2012) 854e861

2.3. Phase angle as indicator of nutritional status malnutrition (SGA C) although SGA was identified as one of the
predictors of phase angle.29
When interpreting phase angle in sick populations, R and Xc This implies that valid cut-off values need to be identified in
must always be considered. Only if R is comparable between order to use the phase angle as clinical indicator for disease-related
groups, the decrease of phase angle is due to the reduction of soft malnutrition in various disease settings. One approach is offered by
tissue and the phase angle might be seen as indicator of nutritional percentiles of reference databases,33,42 such as the reference values
status. generated in a large cohort of healthy subjects that offers sex-, age-
Since phase angle is a marker of amount and quality of soft and BMI-stratified percentiles.11 Standardizing phase angle
tissue mass as well as hydration status it has been considered according to the reference values provides an immediate measure
a useful marker of nutritional status by many authors. In disease- for the patients’ individual deviation from population norms.
related malnutrition, the characteristic early shift from intracel-
lular to extracellular water and increased extracellular to body cell 2.4. Phase angle as indicator of functional status
mass ratio32 is reflected by the phase angle.15 Not surprisingly,
disease-related malnutrition has in fact been shown to be associ- As phase angle correlates with total body protein and muscle
ated with altered electric properties of the tissue which are mass as well as hand grip strength,43 it has been suggested to be
detectable with BIA.17 a useful muscle index, thus also offering a qualitative, dynamic
Several studies have corroborated this notion showing a close aspect of functional status.15 Dittmar et al. demonstrated that
correlation between nutritional status and phase angle. One study elderly non-institutionalized men and women (60e90 years) with
in patients with benign gastrointestinal disease clearly showed higher physical activity levels in household, sport, and leisure-time
a gradual decrease of the phase angle with progressing malnutri- activities also exhibited significantly higher mean phase angle
tion determined with SGA.17 Similarly, in the preoperative setting, values.18 Similarly, in elderly nursing home residents, we observed
phase angle was also closely correlated with the nutritional status a significant relationship between phase angle and simple muscle
determined with SGA33 and in haemodialysis patients, phase angle function parameters such as hand grip strength and knee extension
exhibited a negative correlation with the SGA-1 score modified for strength as well as Barthel Index of the activities of daily living.16 In
renal disease.14 In elderly nursing home residents16 as well as in a large cohort of ambulatory rehabilitation patients, phase angle
free living elderly,34 impaired nutritional status defined by the Mini was also indicative of functional status, as patients with highest
Nutritional Assessment was reflected by lower phase angle values. values in various functional measurements such as Timed Up and
Phase angle also correlates well with biochemical markers such as Go test, Functional Independence Measure and higher quadriceps
albumin, total serum protein, creatinine, and blood urea nitrogen in strength also revealed significantly higher phase angle values,
patients35 with renal disease and with measures of nutritional independent of sex.13 In patients on haemodialysis, the level of
status such as total body protein and muscle mass in liver and renal spontaneous physical activity measured by number of daily steps
disease.14,15 taken correlated significantly with phase angle.44
Underweight patients such as Anorexia Nervosa patients clearly In colorectal cancer patients, increase in phase angle was asso-
have lower phase angles than well nourished subjects.36 Moreover, ciated with an increase in physical and role function scales and
phase angle appears to discriminate between different forms of likewise a decrease in fatigue of the European Organization for the
underweight as shown by Marra et al.21 When compared to normal Research and Treatment of Cancer questionnaire, indicating
weight controls, Anorexia Nervosa patients revealed significantly improved functional aspects of quality of life.45
lower values (5.09  0.52 ), whereas ballet dancers (6.40  0.51 )
had significantly higher values reflecting their higher muscle mass 2.5. Phase angle as prognostic indicator in disease
and habitually lean subjects (5.94  0.93 ) did not differ from
normal weight controls. Similarly, obese and overweight haemo- Given the close correlation between phase angle and nutritional
dialysis patients revealed lower phase angles than normal weight as well as functional status it is not surprising that a high predictive
patients and BMI-matched controls which were associated with potential of the phase angle e in particular with regard to mortality e
anthropometric measures of lean body mass and a higher protein has been reported by many studies.
catabolic rate.37 Compared to healthy subjects, a low phase angle frequently
Improvement of nutritional status is also accompanied by an occurs in sick patients correlating with disease sever-
increase of phase angle. Studies in patients with Anorexia Nervosa ity.13,15,23e25,44,46e55 It has consequently been shown to be predic-
reported a mean increase of 0.6 after 15 weeks of successful tive of impaired prognosis (mortality, disease progression,
nutritional therapy,38 and 1.1 in the stable refed state.36 In incidence of postoperative complications, length of hospital stay) in
malnourished patients with benign gastrointestinal disease, phase pancreatic,56 colorectal,57 breast and lung cancer,58e60 as well as in
angle improved by 0.34  0.91 after three month intervention HIV/AIDS,61 liver cirrhosis,15 renal insufficiency on peritoneal- or
with oral nutritional supplements.39 Similarly, nutritional therapy haemo-dialysis,29,62 amyotrophic lateral sclerosis,63 systemic scle-
improved phase angle in malnourished children (2.6  2.6 years) as rosis,64 bacteraemia/sepsis65 and surgical patients.66 In HIV
shown in small observational study. Interestingly, although the patients, phase angle was even the best single predictor of survival,
increase in body weight paralleled the increase in phase angle in superior even to CD4þ cell count.67 Table 2 gives an overview on
these children, phase angle did not change in case of weight gain studies demonstrating the prognostic impact of low phase angle in
due to oedema.40 various disease settings.
Despite the close correlation between nutritional status and However, most authors generated phase angle cut-offs within
phase angle, however, not all studies found the phase angle to be their study population by using primarily the median or the lowest
a reliable indicator of disease-related malnutrition. Gupta et al.41 quartile or created cut-offs in comparison with a healthy control
observed only modest sensitivities and specificities for different group. A major drawback of this method is that these cut-offs are
cut-offs of the phase angle when comparing it with the SGA in not necessarily transferable to other populations and might thus
patients with advanced colorectal cancer. One study in haemo- not be applicable in the general clinical setting. Also, since these
dialysis patients likewise demonstrated that depending on cut-off cut-off values do not consider determinants of phase angle, lower
values, the phase angle failed to reliably detect clinically relevant phase angle values cannot indiscriminately be attributed to
 K. Norman et al. / Clinical Nutrition 31 (2012) 854e861 857

Table 2
Studies on prognostic impact of phase angle.

Study population N Cut-off value BIA device Clinical outcome of patients below cut-off value
HIV/AIDS
HIV67 75 5.6 101, RJL Systems Decreased survival: parameter estimate in LR test: 0.799, P < 0.0001
HIV61 469 5.3 2000-1, Data Input Decreased survival: 463 days (95% CI: 397e528) vs. 697 (95% CI: 690e705),
P < 0.0001
Increased progression of disease: 406 days (95% CI: 330e483) vs. 670 days
(95% CI: 652e688), P > 0.0001
Tumour disease
Lung cancer60 63 4.5 101, RJL Systems Decreased survival: OR ¼ 1.25
(95% CI: 1.01e1.55), P ¼ 0.04 - Stage IIIB: 3.7 vs. 12.1 months
- Stage IV: 1.4 vs. 5.0 months
Colorectal cancer57 52 5.57 101Q, RJL Systems Decreased survival: 8.6 months (95% CI: 4.8e12.4) vs. 40.4 months
(95% CI: 21.9e58.8), P ¼ 0.0001
Increased mortality: RR ¼ 10.75 (95% CI: 1.92e60.24; P ¼ 0.007)
Pancreatic cancer56 58 5.08 101Q, RJL Systems Decreased survival: 6.3 months (95% CI: 3.5e9.2) vs. 10.2 months
(95% CI: 9.6e10.8), P ¼ 0.02
Reduction of RR 0.75 (95% CI: 0.58e0.96, P ¼ 0.02) with every 1
increase in phase angle
Breast cancer58 259 5.6 101Q, RJL Systems Decreased survival: 23.1 months (95% CI: 14.2e31.9) vs. 49.9 months
(95% CI: 35.6e77.8), P ¼ 0.031
Reduction of RR 0.82 (95% CI: 0.68e0.99, P ¼ 0.041) with every 1
increase in phase angle
Lung cancer59 165 5.3 101Q, RJL Systems Decreased survival: 7.6 months (95% CI: 4.7e9.5) vs. 12.4 months
(95% CI: 10.5e18.7), P ¼ 0.02
Reduction of RR 0.79 (95% CI: 0.64e0.97, P ¼ 0.02) with every 1
increase in phase angle
Mixed tumours93 195 1.65 SPA 101Q, RJL Systems Increased 3 years mortality: RR ¼ 2.35 (95% CI: 1.41e3.90, P ¼ 0.001)
Mixed tumours43 399 5th percentile Nutriguard M, Data Input Increased six month mortality OR ¼ 4.0 (95% CI: 2.4e6.8; P < 0.001)
of reference
values11
Dialysis
Haemodialysis29 131 _ 4.5 101, RJL Systems/Akern Decreased 2 year survival rate (59.3% vs. 91.3%), P < 0.01
\ 4.2 Increased mortality: RR ¼ 2.6 (95% CI: 1.6e4.2), P < 0.0001
Haemodialysis94 3009 3.0 Quantum, RJL Systems Increased mortality: RR ¼ 2.2 (95% CI: 1.6e3.2, P < 0.05)
3e4.0 RR ¼ 1.3 (95% CI: 1.0e1.7, P < 0.05)
Peritoneal dialysis35 45 6.0 101, RJL Systems Decreased 1 year survival (P ¼ 0.01)
Peritoneal dialysis62 48 6.0 101, RJL Systems Decreased 2.5 year survival (P ¼ 0.008); RR ¼ 0.39, P ¼ 0.027
Peritoneal dialysis92 53 6.0 101, RJL Systems Decreased 5 year survival (P ¼ 0.004); RR ¼ 0.536, P ¼ 0.01
Haemodialysis53 149 6.0 101A, RJL Systems Increased mortality RR ¼ 4.12 (95% CI: 1.09e15.53; P ¼ 0.036)
Other
Liver cirrhosis15 305 5.4 101, RJL Systems/Akern Decreased 4.5 year survival, P < 0.01
Surgical patients66 225 0.8 SPA 101Q, RJL Systems/Akern 4.3 fold increased risk of postoperative complication
RR ¼ 4.3 (95% CI: 1.6e11.8), P ¼ 0.02
ALS63 168 2.5 Analycor 3, Spengler Decreased survival: 384 vs. 572 days, P ¼ 0.017,
HR ¼ 0.80 (95% CI: 0.65e0.98), P ¼ 0.03
Geriatric patients 95
1071 3.5 Nutriguard M, Data Input 4-fold increased hospital mortality of 20% (95% CI: 15e24%)
Heart failure51 41 Absolute 101, RJL Systems Decreased survival (AUC ¼ 0.86; 95%CI 0.72e1.0; P ¼ 0.01)
Systemic sclerosis64 124 3.9 Nutriguard M, Data Input Decreased survival (61% vs. 98.8%), P < 0.05

LR ¼ likelihood ratio; CI ¼ confidence interval; OR ¼ odds ratio; RR ¼ relative risk.

SPA ¼ standardized phase angle ¼ (observed phase angle  mean of reference value/standard deviation of reference value).
ALS ¼ Amyotrophic lateral sclerosis; HR ¼ hazard ratio; AUC ¼ area under the curve.

impairment of nutritional status as they can also be due to age or percentile therefore allows identification of patients who are in
female sex. By contrast, reference values from a healthy population particular need of intensified medical and nutritional attention.
offer the possibility of assessing individual deviations of a patient in A new approach to the interpretation of the phase angle is its
relation to the population average and percentiles of reference standardization according to reference values, i.e., creating
values might be used as cut-offs in the general clinical setting for a Z-score as follows: standardized phase angle ¼ (observed phase
the identification of patients at risk of impaired functional and angle  mean phase angle)/SD of the phase angle, where mean and
nutritional status and increased mortality. Whereas several refer- SD are from reference values. This allows comparing values among
ence values have been published11,33 only the reference values patients differing in age, sex, BMI and disease. Also, since stan-
generated in a healthy German population (n ¼ 214,732 adults)11 dardized phase angle values indicate individual deviations from the
are stratified according to sex, age, and BMI, which are estab- population, more complex and immediate information is gained
lished major determinants of the phase angle. than with a dichotomous cut-off variable (e.g. below or above the
We recently demonstrated that the 5th phase angle percentile of 5th reference percentile). Since the standardized phase angle is
sex-, age-, and BMI-stratified reference values is a simple and prog- adjusted for age, sex and BMI, lower values indicate a true
nostically relevant cut-off in cancer patients suitable for the clinical derangement of nutritional status or health status more reliably
setting.43 Values below the 5th reference percentile clearly indicate than absolute phase angle values.
impaired functional and nutritional status, decreased quality of life We found that the standardized phase angle was a significant
and increased 6-month mortality. The use of the 5th reference predictor for malnutrition and impaired functional status as well as
858 K. Norman et al. / Clinical Nutrition 31 (2012) 854e861

a superior indicator of 6-month survival than malnutrition and age-, and BMI-stratified reference values from a German pop-
disease severity in cancer patients.43 ulation80 are available as well as US reference values stratified
according to sex, age and ethnicity.42
3. Bioelectrical impedance vector analysis for assessment and
monitoring of hydration and nutritional status 3.1. BIVA in comparison to phase angle

The bioelectrical impedance vector analysis (BIVA) approach BIVA thus enables a more detailed understanding of hydration
developed by Piccoli et al.68 uses the plot of the impedance status and cell mass compared to phase angle alone. Since phase
parameters resistance (R) and reactance (Xc) normalized per height angle is calculated from reactance and resistance, different posi-
as a bivariate vector in the RXc graph. The normalization for height tions of the vector in the RXc Graph can theoretically produce
allows for the length of the conductor and thus provides a qualita- identical phase angles (see Fig. 1). Differentiation between obese
tive measure of soft tissue that does not depend on body size. The (high phase angle, short vector) and athletic subjects (high phase
position and length of the vector provides information about angle and long vector) is consequently possible with BIVA just as
hydration status, body cell mass and cell integrity. A migration discrimination between cachectic (low phase angle and long
sideways of the vector due to low or high reactance indicates vector) and lean subjects (normal phase angle and long vector)
decrease or increase of dielectric mass (membranes and tissue Longitudinal changes in hydration and cell mass can therefore be
interfaces) of soft tissues. The length of the vector indicates interpreted more reliably by BIVA than phase angle, which makes
hydration status from fluid overload (decreased resistance, short BIVA a valuable tool for assessment and monitoring of patients. For
vector) to exsiccosis (increased resistance, longer vector)68,69 (see monitoring, high reproducibility is required. Within-day variability
Fig. 1). If groups of patients are portrayed in the RXc graph as mean of whole-body impedance has been shown to be small in healthy
vector, the vector distribution is described by its associated 95% populations.18,81 When BIA measurements are carried out accord-
confidence interval (confidence ellipse). Significant vector ing to standardized protocols in sick patients,82 inter-rater vari-
displacement is seen with increasing disease severity,70,71 in ability is very low as shown by low coefficients of variance 3% for
obesity,37 disease-related malnutrition16,17 and fluid removal R and Xc.43,83 Since minimal detectable changes depend on the
during dialysis.8,72,73 precision of the method, BIVA qualifies as monitoring tool.
The BIVA approach has gained attention as a tool to assess and
monitor patients’ hydration and nutrition status in patients on 3.2. BIVA as measure of hydration
haemodialysis8,72e74 or continuous ambulatory peritoneal dial-
ysis,75 liver cirrhosis,76 critically ill patients77 and obese patients BIVA has been well investigated for the evaluation of hydration
with stable and changing weight78 since it is independent of status. In patients with renal insufficiency in particular, BIVA has
disputable regression equations for calculation of lean body mass shown to be an effective method to assess hydration status84 and
and fat mass as well as independent of measurement of body identify patients with a critical fluid overload73 which is associated
weight. with increased risk of mortality. Pillon et al. demonstrated that
Comparison to reference values is moreover possible in BIVA; shorter vector length as a measure of inadequate ultrafiltration was
individual vectors can immediately be ranked in regard to tolerance associated with increased mortality, and that the increase in rela-
ellipses representing 50%, 75% and 95% of reference values, which tive risk with shorter vector length was independent of age, sex,
allows a detailed classification of vector position. Healthy subjects ethnicity, diabetes, length of time on dialysis, albumin, creatinine,
are usually positioned within the 75th tolerance ellipse.79 Sex-, haemoglobin, ferritin, and even phase angle.85 In critically ill,

exsiccosis

95%
athletic
75%
Xc/H [Ohm/m]

50% lean
lean patient within 75% tollerance elipse

ideal patient
obese

obese patient outside the 95% tollerance elipse

cachectic
severely hyperhydrated patient
hyperhydratation
α
R/H [Ohm/m]
Fig. 1. Different positions of the vector in the RXc graph indicating different body composition can theoretically produce comparable phase angles (a). Longitudinal changes in
hydration and cell mass are therefore interpreted more reliably by BIVA than phase angle alone.
 K. Norman et al. / Clinical Nutrition 31 (2012) 854e861 859

central venous pressure correlated more closely with direct Mini Nutritional Assessment. Also, vector migration due to both
impedance measurements than with calculated total body water. increasing Xc/H and decreasing R/H, indicating greater cell mass,
Shorter impedance vectors reliably indicated overhydration in was reflected by increased hand grip strength.91
patients with central venous pressure values >12 mmHg.77 Also, In conclusion, bioelectrical phase angle and BIVA represent
fluid removal during haemodialysis can be monitored in the RXc a clinically feasible approach to body composition, free from
graph.8 In obese uraemic patients, weight loss due to fluid loss was equation inherent errors and necessary assumptions, although
reflected by a lengthening of the impedance vector, which was not quantities of body compartments are not measured. While this
seen when weight loss was due to calorie restriction in the obese might make BIA less attractive for some research purposes, the
control group.78 During pregnancy and post partum, BIVA has been evaluation and identification of patients by BIVA (Fig. 1) might be
shown to be a valid method to monitor changes in total body water used as screening tool for the development of inclusion and
and identify women with excessive fluid gain when compared to exclusion criteria for clinical studies. Phase angle has been shown
isotope dilution.86 to be a superior indicator of survival and outcome and should
In peritoneal dialysis, it has been proposed that segmental therefore be used as screening tool for identification of patients at
bioimpedance measurements of the thoracic region provide more risk because of impaired nutritional or functional status. BIVA
relevant information of the fluid changes than whole-body provides more detailed information on hydration and cell mass
impedance.73 Indeed, overhydrated patients with an increased risk integrity and should therefore be considered as an assessment and
for cardiovascular disease were more accurately identified.73 monitoring tool.
Similarly, overhydration in hypertensive haemodialysis patients
was identified with segmental thoracic measurements.87 Funding

3.3. BIVA as measure of nutritional status There was no funding source.

In healthy elderly, impedance vectors clearly indicate the age Statement of authorship
associated reduction of soft tissue, particularly after the age of 80.
Xc/H and phase angle decrease with age in both men and women.88 The authors’ responsibilities were as follows e KN, NS: literature
In patients with Alzheimer disease, mean vector position was research and writing of the manuscript; and AB-W and MP: critical
significantly different in the patients with mild-moderate Alz- input and revision of the manuscript.
heimer disease with respect to controls, indicating lower soft
tissue.89 Women with severe Alzheimer disease also showed both Conflict of interest
reduced tissue mass and dehydration when compared with
patients with mildemoderate disease severity.89 The authors declare that they have no financial conflict of
The mean impedance vectors from patients with heart failure in interest.
the NYHA IIIeIV group were significantly shorter and more
downsloping than those in the NYHA IeII group, indicating Acknowledgement
a progressive increase in soft tissue hydration with increasing
disease severity.71 No writing assistance was needed.
Ongoing weight loss common in malnutrition has a distinct
effect on electric properties of the tissue which is not seen in References
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